Professor Jeffrey Tostevin

Bound states of the neutron-deficient, near-dripline nucleus 36Ca were populated in two-neutron removal from the ground state of 38Ca, a direct reaction sensitive to the single-particle configurations and couplings of the removed neutrons in the projectile wave function. Final-state exclusive cross sections for the formation of 36Ca and the corresponding longitudinal momentum distributions, both determined through the combination of particle and γ-ray spectroscopy, are compared to predictions combining eikonal reaction theory and shell-model two-nucleon amplitudes from the USDB, USDC, and ZBM2 effective interactions. The final-state cross-section ratio σ($2$$^{+}_{1}$) / σ (0+) shows particular sensitivity and is approximately reproduced only with the two-nucleon amplitudes from the ZBM2 effective interaction that includes proton cross-shell excitations into the pf shell. Characterizing the proton pf-shell occupancy locally and schematically, an increase of the sd – pf shell gap by 250 keV yields an improved description of this cross-section ratio and simultaneously enables a reproduction of the B(E2;$0$$^{+}_{1}$ → $2$$^{+}_{1}$) excitation strength of 36Ca. Furthermore, this highlights an important aspect if a new shell-model effective interaction for the region was to be developed on the quest to model the neutron-deficient Ca isotopes and surrounding nuclei whose structure is impacted by proton cross-shell excitations.

A sequence of excited states has been established for the first time in the proton-rich nucleus 48Fe (Z=26, N=22). The technique of mirrored (i.e. analogue) one-nucleon knockout reactions was applied, in which the Tz= ±2 mirror pair, 48Fe/48Ti were populated via one-neutron/one-proton knockout from the secondary beams 49Fe/49V, respectively. The analogue properties of the reactions were used to help establish the new level scheme of 48Fe. The inclusive and exclusive cross sections were determined for the populated states. Large differences between the cross sections for the two mirrored reactions were observed and have been interpreted in terms of different degrees of binding of the mirror nuclei and in the context of the recent observations of suppression of spectroscopic strength as a function of nuclear binding, for knockout reactions on light solid targets. Mirror energy differences (MED) have been determined between the analogue T=2 states and compared with the shell model predictions. MED for this mirror pair, due to their location in the shell, are especially sensitive to excitations out of the f7/2 shell, and present a stringent test of the shell-model prescription.

International audience; The region of neutron-rich Cr isotopes has garnered much attention in recent years due to a rapid onset of collectivity near neutron number N = 40. We report here on the first γ-ray spectroscopy beyond the (4 + 1) state in 62,64 Cr, using nucleon removal reactions from several projectiles within a rare-isotope beam cocktail. A candidate for the 6 + state in 64 Cr is presented as well as one for, possibly, the second excited 0 + state in 62 Cr. The results are discussed in comparison to the LNPS shell-model predictions that allow for neutron excitations across the N = 40 harmonic oscillator gap into the g 9/2 and d 5/2 orbitals. The calculated level schemes for 62,64 Cr reveal intriguing collective structures. From the predicted neutron particle-hole character of the low-lying states in these Cr isotopes, 62 Cr emerges as a transitional system on the path to the center of the N = 40 island of inversion.

Situated in the so-called “island of inversion,” the nucleus $^{32}$Mg is considered as an archetypal example of the disappearance of magicity at N=20. We report on high statistics in-beam spectroscopy of $^{32}$Mg with a unique approach, in that two direct reaction probes with different sensitivities to the underlying nuclear structure are employed at the same time. More specifically, states in $^{32}$Mg were populated by knockout reactions starting from $^{33}$Mg and $^{34}$Si, lying inside and outside the island of inversion, respectively. The momentum distributions of the reaction residues and the cross sections leading to the individual final states were confronted with eikonal-based reaction calculations, yielding a significantly updated level scheme for $^{32}$Mg and spin-parity assignments. By fully exploiting observables obtained in this measurement, a variety of structures coexisting in $^{32}$Mg was unraveled. Comparisons with theoretical predictions based on shell-model overlaps allowed for clear discrimination between different structural models, revealing that the complete theoretical description of this key nucleus is yet to be achieved.

The nucleus Ne29 is situated at the border of the island of inversion. Despite significant efforts, no bound low-lying intruder f7/2 state, which would place Ne29 firmly inside the island of inversion, has yet been observed. Here, the first investigation of unbound states of Ne29 is reported. The states were populated in Ne30(p,pn) and Na30(p,2p) reactions at a beam energy of around 230 MeV/nucleon, and analyzed in terms of their resonance properties, partial cross sections, and momentum distributions. The momentum distributions are compared to calculations using the eikonal, direct reaction model, allowing ℓ assignments for the observed states. The lowest-lying resonance at an excitation energy of 1.48(4) MeV shows clear signs of a significant ℓ=3 component, giving first evidence for f7/2 single particle strength in Ne29. The excitation energies and strengths of the observed states are compared to shell-model calculations using the sdpf-u-mix interaction.

The nucleus $^{29}$Ne is situated at the border of the island of inversion. Despite significant efforts, no bound low-lying intruder $f_{7/2}$-state, which would place $^{29}$Ne firmly inside the island of inversion, has yet been observed. Here, the first investigation of unbound states of $^{29}$Ne is reported. The states were populated in $^{30}\mathrm{Ne}(p,pn)$ and $^{30}\mathrm{Na}(p,2p)$ reactions at a beam energy of around $230$ MeV/nucleon, and analyzed in terms of their resonance properties, partial cross sections and momentum distributions. The momentum distributions are compared to calculations using the eikonal, direct reaction model, allowing $\ell$-assignments for the observed states. The lowest-lying resonance at an excitation energy of 1.48(4) MeV shows clear signs of a significant $\ell$=3-component, giving first evidence for $f_{7/2}$ single particle strength in $^{29}$Ne. The excitation energies and strengths of the observed states are compared to shell-model calculations using the sdpf-u-mix interaction Comment: 12 pages, 14 figures, accepted for publication in Physical Review C

In recent decades, rare-isotope facilities have enabled the study of short-lived, neutron-rich nuclei. Their measured properties indicate that shell structure changes in the regime of unbalanced neutron-to-proton ratios compared with that of stable nuclei. In the so-called islands of inversion in the nuclear chart—around the neutron-rich nuclei 32Mg, 42Si and 64Cr, for example—the textbook shell model predicts spherical shapes due to the respective magic neutron numbers of 20, 28 and 40 of these nuclei. However, nuclei in these regions turn out to be deformed in their ground states. Another hallmark of these islands is shape coexistence, where a nucleus assumes different shapes with excitation energy. Here we present evidence for this phenomenon from the observation of an excited 0+ state in 62Cr, two neutrons away from the heart of the island of inversion around neutron number N = 40. We use large-scale shell-model calculations to interpret the results, and we report extrapolations for the doubly magic nucleus 60Ca.

A novel pathway for the formation of multi-particle-multi-hole (np − mh) excited states in rare isotopes is reported from highly energy-and momentum-dissipative inelastic-scattering events measured in reactions of an intermediate-energy beam of 38 Ca on a Be target. The negative-parity, complex-structure final states in 38 Ca were observed following the in-beam γ-ray spectroscopy of events in the 9 Be(38 Ca, 38 Ca + γ)X reaction in which the scattered projectile lost longitudinal momentum of order ∆p || = 700 MeV/c. The characteristics of the observed final states are discussed and found to be consistent with the formation of excited states involving the rearrangement of multiple nucleons in a single, highly-energetic projectile-target collision. Unlike the far-less dissipative, surface-grazing reactions usually exploited for the in-beam γ-ray spectroscopy of rare isotopes, these more energetic collisions appear to offer a practical pathway to nuclear-structure studies of more complex multi-particle configurations in rare isotopes – final states conventionally thought to be out of reach with high-luminosity fast-beam-induced reactions.

Background: Very neutron-rich isotopes, including Ne30-32, in the vicinity of N=20 are known to exhibit ground states dominated by fp-shell intruder configurations: the “island of inversion.” Systematics for the Ne-isotopic chain suggest that such configurations may be in strong competition with normal shell-model configurations in the ground state of Ne29. Purpose: A determination of the structure of Ne29 is thus important to delineate the extent of the island of inversion and better understand structural evolution in neutron-rich Ne isotopes. This is accomplished here through a combined investigation of nuclear and Coulomb-induced one-neutron removal reactions. Method: Cross sections for one-neutron removal on carbon and lead targets and the parallel momentum distribution of the Ne28 residues from the carbon target are measured at around 240 MeV/nucleon. The measurements are compared with reaction calculations combined with spectroscopic information from SDPF-M shell-model wave functions. Results: The deduced width of the inclusive parallel momentum distribution, 98(12) MeV/c (FWHM), suggests that the ground state of Ne29 has a spin parity of 3/2-. Detailed comparisons of the measured inclusive and partial cross sections of the two targets and the parallel momentum distribution of the carbon target with reaction calculations, combined with spectroscopic information from large-scale shell-model calculations, are all consistent with a 3/2- spin-parity assignment. Conclusions: The results indicate that Ne29 lies within the island of inversion and that the ground state of Ne29 is dominated by a Ne28(0+1)xp3/2 neutron configuration. Combined with recently measured interaction cross sections, it is concluded that Ne29 may exhibit a moderately developed halo-like distribution

Spectroscopic factors of neutron-hole and proton-hole states in Sn-131 and In-131, respectively, were measured using one-nucleon removal reactions from doubly magic Sn-132 at relativistic energies. For In-131, a 2910(50)-keV gamma ray was observed for the first time and tentatively assigned to a decay from a 5/2(-) state at 3275(50) keV to the known 1/2(-) level at 365 keV. The spectroscopic factors determined for this new excited state and three other single-hole states provide first evidence for a strong fragmentation of single-hole strength in Sn-131 and In-131. The experimental results are compared to theoretical calculations based on the relativistic particle-vibration coupling model and to experimental information for single-hole states in the stable doubly magic nucleus Pb-208.

The structure of 19,20,22C has been investigated using high-energy (around 240 MeV/nucleon) one- and two-neutron removal reactions on a carbon target. Measurements were made of the inclusive cross sections and momentum distributions for the charged residues. Narrow momentum distributions were observed for one-neutron removal from 19C and 20C and two-neutron removal from 22C. Two-neutron removal from 20C resulted in a relatively broad momentum distribution. The results are compared with eikonal-model calculations combined with shell-model structure information. The neutron removal cross sections and associated momentum distributions are calculated for transitions to both the particle-bound and particle-unbound final states. The calculations take into account the population of the mass A−1 reaction residues A−1C and, following one-neutron emission after one-neutron removal, the mass A−2 two-neutron removal residues A−2C. The smaller contributions of direct two-neutron removal, that populate the A−2C residues in a single step, are also computed. The data and calculations are shown to be in good overall agreement and consistent with the predicted shell-model ground-state configurations and one-neutron overlaps with low-lying states in 18−21C. These suggest significant νs1/22 valence neutron configurations in both 20C and 22C. The results for 22C strongly support the picture of 22C as a two-neutron halo nucleus with a dominant νs1/22 ground-state configuration.

The momentum distributions of the residual nuclei after one-neutron removal, measured in coincidence with gamma rays, identify the excited levels of these residues. The resulting differential partial cross sections map the momentum content and structure of the removed-nucleon wave function and provide an exacting test of theory. Data for population of the C-14 and Be-10 ground states show an asymmetry that is incompatible with the currently used eikonal descriptions. A fully dynamical description of the elastic break-up mechanism provides an understanding of the new observation, which will be most pronounced for nuclear halo states, This interpretation is clarified by an analysis of the angular distribution of the heavy residues.

There is now a large and increasing body of experimental data and theoretical analyses for reactions that remove a single nucleon from an intermediate-energy beam of neutron- or proton-rich nuclei. In each such measurement, one obtains the inclusive cross section for the population of all bound final states of the mass A−1 reaction residue. These data, from different regions of the nuclear chart, and that involve weakly and strongly bound nucleons, are compared with theoretical expectations. These calculations include an approximate treatment of the reaction dynamics and shell-model descriptions of the projectile initial state, the bound final states of the residues, and the single-particle strengths computed from their overlap functions. The results are discussed in the light of recent data, more exclusive tests of the eikonal dynamical description, and calculations that take input from more microscopic nuclear structure models.

The contributions to the cross sections of intermediate energy two-nucleon knockout reactions from events in which one nucleon is removed by the stripping (inelastic breakup) mechanism and a second by the diffraction (elastic breakup) mechanism are discussed. The small additional contributions from two-nucleon diffraction events are also estimated. The approach used combines the eikonal reaction and shell model structure theory frameworks. For reactions involving the removal of two well-bound like nucleons, at incident energies of order 100 MeV per nucleon, the additional cross sections are shown to be of approximately the same size as those from events in which both nucleons are stripped in inelastic interactions. These more complete dynamical calculations now permit a quantitative comparison of the theoretical cross sections with recent partial cross-section measurements of the two-neutron (two-proton) removal reactions from neutron-deficient (neutron-rich) nuclei. As has been observed in both nuclear- and electron-induced single-nucleon knockout reaction analyses, the theoretical two-nucleon knockout cross sections overestimate the measured values, requiring a suppression of the two nucleon shell-model transition strengths. The deduced two-nucleon suppression factors, Rs(2N), are consistent with a value of 0.5 for each of the five reactions considered.

Single-nucleon knockout cross sections from fast secondary beams of the proton-drip-line nuclei 9C, 13O, and 17Ne on a 9Be target have been studied with emphasis on the production of resonance states. These states were identified by their invariant mass, and resonances with two-, three-, and five-body exit channels were examined. The measured cross sections for these states were compared with eikonal-model predictions using shell-model or variational Monte Carlo spectroscopic factors. The experimental yields were found to be suppressed relative to the model predictions, especially when a well-bound neutron or proton is removed. This suppression exceeds that found systematically in measured inclusive cross sections to particle-bound final states. In neutron knockout from 9C and 13O projectiles, this suppression of the unbound ground-state residuals yield is a factor of two to three times larger than that found in the bound final-state studies. Modifications to the structure of these systems due to coupling of the shell-model configurations to the continuum is expected to contribute to this extra suppression, especially when the final state is a near-threshold resonance. Furthermore, other considerations including the role of nuclear dynamics may be required to explain all the observed trends.

The calcium isotopes have emerged as a critical testing ground for new microscopically derived shell-model interactions, and a great deal of experimental and theoretical focus has been directed toward this region. We investigate the relative spectroscopic strengths associated with 1f7=2 neutron hole states in 47;49Ca following one-neutron knockout reactions from 48;50Ca. The observed reduction of strength populating the 7/2

Theoretical models of low-energy (d,p) single-neutron transfer reactions are a crucial link between experimentation, nuclear structure and nuclear astrophysical studies. Whereas reaction models that use local optical potentials are insensitive to short-range physics in the deuteron, we show that including the inherent nonlocality of the nucleon-target interactions and realistic deuteron wave functions generates significant sensitivity to high n-p relative momenta and to the underlying nucleon-nucleon interaction. We quantify this effect upon the deuteron channel distorting potentials within the framework of the adiabatic deuteron breakup model. The implications for calculated (d,p) cross sections and spectroscopic information deduced from experiments are discussed.

Detailed spectroscopy of the neutron-unbound nucleus 28 F has been performed for the first time following proton/neutron removal from 29 Ne / 29 F beams at energies around 230 MeV / nucleon . The invariant-mass spectra were reconstructed for both the 27 F ( * ) + n and 26 F ( * ) + 2 n coincidences and revealed a series of well-defined resonances. A near-threshold state was observed in both reactions and is identified as the 28 F ground state, with S n ( 28 F ) = − 199 ( 6 )     keV , while analysis of the 2 n decay channel allowed a considerably improved S n ( 27 F ) = 1620 ( 60 )     keV to be deduced. Comparison with shell-model predictions and eikonal-model reaction calculations have allowed spin-parity assignments to be proposed for some of the lower-lying levels of 28 F . Importantly, in the case of the ground state, the reconstructed 27 F + n momentum distribution following neutron removal from 29 F indicates that it arises mainly from the 1 p 3 / 2 neutron intruder configuration. This demonstrates that the island of inversion around N = 20 includes 28 F , and most probably 29 F , and suggests that 28 O is not doubly magic.

The region of neutron-rich Cr isotopes has garnered much attention in recent years due to a rapid onset of collectivity near neutron number N=40. We report here on the first γ-ray spectroscopy beyond the (41+) state in Cr62,64, using nucleon removal reactions from several projectiles within a rare-isotope beam cocktail. A candidate for the 6+ state in Cr64 is presented as well as one for, possibly, the second excited 0+ state in Cr62. The results are discussed in comparison to the LNPS shell-model predictions that allow for neutron excitations across the N=40 harmonic oscillator gap into the g9/2 and d5/2 orbitals. The calculated level schemes for Cr62,64 reveal intriguing collective structures. From the predicted neutron particle-hole character of the low-lying states in these Cr isotopes, Cr62 emerges as a transitional system on the path to the center of the N=40 island of inversion.

In the island of inversion, ground states of neutron-rich sd-shell nuclei exhibit strong admixtures of intruder configurations from the fp shell. The nucleus 30Mg, located at the boundary of the island of inversion, serves as a cornerstone to track the structural evolution as one approaches this region. Spin-parity assignments for excited states in 30Mg, especially negative-parity levels, have yet to be established. In the present work, the nuclear structure of 30Mg was investigated by in-beam gamma-ray spectroscopy mainly focusing on firm spin-parity determinations. High-intensity rare-isotope beams of 31Mg, 32Mg, 34Si, and 35P bombarded a Be target to induce nucleon removal reactions populating states in 30Mg. Gamma rays were detected by the state-of-the-art gamma-ray tracking array GRETINA. For the direct one-neutron removal reaction, final-state exclusive cross sections and parallel momentum distributions were deduced. Multi-nucleon removal reactions from different projectiles were exploited to gain complementary information. With the aid of the parallel momentum distributions, an updated level scheme with revised spin-parity assignments was constructed. Spectroscopic factors associated with each state were also deduced. Results were confronted with large-scale shell-model calculations using two different effective interactions, showing excellent agreement with the present level scheme. Furthermore, a marked difference in the spectroscopic factors indicates that the full delineation of the transition into the island of inversion remains a challenge for theoretical models.

A classical dynamical model that treats breakup stochastically is presented for low energy reactions of weakly bound nuclei. The three-dimensional model allows a consistent calculation of breakup, incomplete, and complete fusion cross sections. The model is assessed by comparing the breakup observables with continuum discretized coupled-channel quantum mechanical predictions, which are found to be in reasonable agreement. Through the model, it is demonstrated that the breakup probability of the projectile as a function of its distance from the target is of primary importance for understanding complete and incomplete fusion at energies near the Coulomb barrier.

We report final-state-exclusive measurements of the light charged fragments in coincidence with 26Ne residual nuclei following the direct two-proton removal from a neutron-rich 28Mg secondary beam. A Dalitz-plot analysis and comparisons with simulations show that a majority of the triple-coincidence events with two protons display phase-space correlations consistent with the (two-body) kinematics of a spatially correlated pair-removal mechanism. The fraction of such correlated events, 56(12)%, is consistent with the fraction of the calculated cross section, 64%, arising from spin S=0 two-proton configurations in the entrance-channel (shell-model) 28Mg ground state wave function. This result promises access to an additional and more specific probe of the spin and spatial correlations of valence nucleon pairs in exotic nuclei produced as fast secondary beams.

The role of continuum states in nuclei is considered, along with use of breakup theories of stripping and diffractive dissociation in probing nuclear structure. In the breakup of three-body nuclei such as Borromean halo systems, both three-body and two-body continuum final states need to be modelled and measured.

Excited states in the neutron-rich 70Fe nucleus were populated in a one-proton removal reaction from 71Co projectiles at 87 MeV/nucleon. A new transition was observed with the γ-ray tracking array GRETINA and shown to feed the previously assigned 4+ 1 state. In comparison to reaction theory calculations with shell-model spectroscopic factors, it is argued that the new γ ray possibly originates from the 6+ 1 state. It is further shown that the Doppler-reconstructed γ-ray spectra are sensitive to the very different lifetimes of the 2+ and 4+ states, enabling their approximate measurement. The emerging structure of 70Fe is discussed in comparison to LNPS-new large-scale shell-model calculations.

A systematic investigation of the inclusive cross sections for single-nucleon knockout reactions from p-shell nuclei has been performed. A total of seven reactions were studied for projectiles with masses between A = 7 and 10, having a wide range of nucleon separation energies. Results were obtained for a range of incident beam energies and targets. These differences were found to have a minimal impact on the deduced cross sections. Experimental results were compared to theoretical predictions based on variational Monte Carlo (VMC) nuclear structure calculations, whose radial overlap functions and neutron and proton densities were included in the reaction description. These results are compared with the conventional model, developed for heavier nuclei, that uses shell-model and Hartree-Fock structure inputs. The VMC-based calculations agreed with the experimental data for several reactions where deeply bound nucleons are removed but does not describe some of the more weakly bound nucleon removal cases with comparable accuracy.

We report on the measurement of new low-lying states in the neutron-rich 81,82,83,84Zn nuclei via in-beam γ -ray spectroscopy. These include the View the MathML source41+→21+ transition in 82Zn, the View the MathML source21+→0g.s.+ and View the MathML source41+→21+ transitions in 84Zn, and low-lying states in 81,83Zn were observed for the first time. The reduced View the MathML sourceE(21+) energies and increased View the MathML sourceE(41+)/E(2+1) ratios at N=52N=52, 54 compared to those in 80Zn attest that the magicity is confined to the neutron number N=50N=50 only. The deduced level schemes are compared to three state-of-the-art shell model calculations and a good agreement is observed with all three calculations. The newly observed 2+2+ and 4+4+ levels in 84Zn suggest the onset of deformation towards heavier Zn isotopes, which has been incorporated by taking into account the upper sdg orbitals in the Ni78-II and the PFSDG-U models.

The highly selective, intermediate-energy heavy-ion-induced neutron-pickup reaction, in combination with γ-ray spectroscopy using the γ-ray energy-tracking in-beam nuclear array (GRETINA), is shown to provide reliable relative spectroscopic strengths for high-ℓ orbitals in nuclei more neutron rich than the projectile. The reaction mechanism gives a significant final-state-spin alignment that is validated through γ-ray angular-distribution measurements enabled by the position sensitivity of GRETINA. This is the first time that γ-ray angular distributions could be extracted from a high-luminosity, fast-beam reaction other than inelastic scattering. This holds great promise for the restriction and assignment of Jπ quantum numbers in exotic nuclei. We advance this approach to study the crucial N=28 shell closure and extract the ratio g9/2:f5/2 of bound neutron single-particle strengths in Ca49, a benchmark for emerging multi-shell ab initio and configuration-interaction theories that are applicable along the Ca isotopic chain.

The interaction cross sections (σI ) of the very neutron-rich carbon isotopes 19C, 20C and 22C have been measured on a carbon target at 307, 280, and 235 MeV/nucleon, respectively. A σI of 1.280±0.023 b was obtained for 22C, significantly larger than for 19,20C, supporting the halo character of 22C. A 22C root-mean-squared matter radius of 3.44 ± 0.08 fm was deduced using a four-body Glauber reaction model. This value is smaller than an earlier estimate (of 5.4 ± 0.9 fm) derived from a σI measurement on a hydrogen target at 40 MeV/nucleon. These new, higher-precision σI data provide stronger constraints for assessing the consistency of theories describing weakly bound nuclei.

Low lying states of neutron-rich 32Ne were populated by means of one- and two-proton knock- out reactions at the RIKEN Radioactive Isotope Beam Factory. A new transition is observed at 1410(15) keV and assigned to the 4+ 1 → 2+ 1 decay. With this energy the R4/2 ratio is calculated to be 2.99(6), lying close to the rigid rotor limit and suggests a high degree of collectivity and strongest deformation among neutron-rich Neon isotopes. Comparisons of experimental inclusive and exclu- sive reaction cross sections with shell model and eikonal reaction dynamical calculations reveals considerable quenching for this highly asymmetric system and contributes to systematic trends.

An experiment with the aim to obtain information on the excited states of neutron-rich nuclei with N~82 was performed at RIBF/RIKEN as part of the HiCARI campaign. The method to identify nuclei on ion-by-ion basis, including charge-state identification, is presented. The Doppler correction technique was validated using the test case of ¹³¹In, based on the prompt πp3/2 → πp1/2 transition at 988 keV. Preliminary analysis of the ¹³⁰Cd spectrum is also presented.

Theories of (d,p) reactions frequently use a formalism based on a transition amplitude that is dominated by the components of the total three-body scattering wave function where the spatial separation between the incoming neutron and proton is confined by the range of the n-p interaction, Vnp. By comparison with calculations based on the continuum discretized coupled channels method we show that the (d,p) transition amplitude is dominated by the first term of the expansion of the three-body wave function in a complete set of Weinberg states. We use the 132Sn(d,p)133Sn reaction at 30 and 100 MeV as examples of contemporary interest. The generality of this observed dominance and its implications for future theoretical developments are discussed.

The possibility of studying particle-like states near the Fermi surfaces of exotic nuclei by using measurements of heavy-ion-induced single-nucleon pickup reactions with in-flight separated rare-isotope beams is discussed. The analysis of an exploratory data set for the intermediate-energy Be-9(Ne-20,Na-21)X proton pickup reaction measured using a Ne-20 beam at 63 MeV per nucleon is reported. The data are compared with expectations based on model calculations of the transfer reaction cross sections and the Na-21 residue spectroscopy prediction by the sd-shell model. The measured cross sections are broadly consistent with these expectations.

The breakdown of the N=20 magic number in the so-called island of inversion around Mg-32 is well established. Recently developed large-scale shell-model calculations suggest a transitional region between normal- and intruder-dominated nuclear ground states, thus modifying the boundary of the island of inversion. In particular, a dramatic change in single-particle structure is predicted between the ground states of Mg-30 and Mg-32, with the latter consisting nearly purely of 2p-2h N=20 cross-shell configurations. Single-neutron knockout experiments on Mg-30,Mg-32 projectiles have been performed. We report on a first direct observation of intruder configurations in the ground states of these very neutron-rich nuclei. Spectroscopic factors to low-lying negative-parity states in the knockout residues are deduced and compare well with shell-model predictions.

A more detailed test of the implementation of nuclear forces that drive shell evolution in the pivotal nucleus 42Si – going beyond earlier comparisons of excited-state energies – is important. The two leading shell-model effective interactions, SDPF-MU and SDPF-U-Si, both of which reproduce the low-lying 42Si(2+ 1) energy, but whose predictions for other observables differ significantly, are interrogated by the population of states in neutron-rich 42Si with a one-proton removal reaction from 43P projectiles at 81 MeV/nucleon. The measured cross sections to the individual 42Si final states are compared to calculations that combine eikonal reaction dynamics with these shell-model nuclear structure overlaps. The differences in the two shell-model descriptions are examined and linked to predicted low-lying excited 0+ states and shape coexistence. Based on the present data, which are in better agreement with the SDPF-MU calculations, the state observed at 2150(13) keV in 42Si is proposed to be the (0+ 2) level.

Newly published 42Si γ-ray spectra and a final-state-inclusive 42Si production cross section value, obtained in a higher-statistics intermediate-energy two-proton removal experiment from 44S, are considered in terms of the final-state-exclusive cross sections computed using proposed shell-model effective interactions for nuclei near N=28. Specifically, we give cross section predictions when using the two nucleon amplitudes of the two-proton overlaps 〈42Si(Jπ) | 44S〉 computed using the newly proposed sdpf-mu shell-model Hamiltonian. We show that these partial cross sections or their longitudinal momentum distributions should enable a less-tentative interpretation of the measured gamma-ray spectra and provide a more quantitative assessment of proposed shell-model Hamiltonians in this interesting and challenging region of the chart of nuclides.

Data have been obtained on exclusive single neutron knockout cross sections from 12Be to study its ground state structure. The cross sections for the production of 11Be in its ground state (1/2 +) and first excited state (0.32 MeV, 1/2 -) have previously been measured, indicating a strong (2s 1 2) 2 component to the 12Be ground state. In the present experiment, performed at the GANIL laboratory, cross sections for the first (0.32 MeV, 1/2 -) and second (1.78 MeV, 5/2 +, unbound) excited states in 11Be were measured, which gives information on the admixture of (1p 1 2) 2 and (1d 5 2) 2 components in the ground state of 12Be. A fragmentation beam of 12Be of ∼10000 pps (95% pure) was incident on a carbon target at 41 MeV/u. The beam particles were tracked onto the target, and their energies were measured event-by-event. The beam-like residues were measured in a position sensitive telescope mounted at zero degrees, and neutrons were measured in the DéMoN array. The 1/2 - state of 11Be was identified by measuring coincident 320 keV γ-rays, using four NaI detectors. Full kinematic reconstruction of unbound states in 11Be was performed using coincident neutrons and 10Be ions. Detailed simulations were performed in order to interpret the data, and spectroscopic factors were calculated, using preliminary single particle removal cross sections calculated using a Glauber model. © 2005 American Institute of Physics.

A sequence of excited states has been established for the first time in the proton-rich nucleus 48Fe (Z=26, N=22). The technique of mirrored (i.e. analogue) one-nucleon knockout reactions was applied, in which the Tz= ±2 mirror pair, 48Fe/48Ti were populated via one-neutron/one-proton knockout from the secondary beams 49Fe/49V, respectively. The analogue properties of the reactions were used to help establish the new level scheme of 48Fe. The inclusive and exclusive cross sections were determined for the populated states. Large differences between the cross sections for the two mirrored reactions were observed and have been interpreted in terms of different degrees of binding of the mirror nuclei and in the context of the recent observations of suppression of spectroscopic strength as a function of nuclear binding, for knockout reactions on light solid targets. Mirror energy differences (MED) have been determined between the analogue T=2 states and compared with the shell model predictions. MED for this mirror pair, due to their location in the shell, are especially sensitive to excitations out of the f7/2 shell, and present a stringent test of the shell-model prescription.

The astrophysical s-process is one of the two main processes forming elements heavier than iron. A key outstanding uncertainty surrounding s-process nucleosynthesis is the neutron flux generated by the Ne-22(alpha, n)Mg-25 reaction during the He-core and C-shell burning phases of massive stars. This reaction, as well as the competing Ne-22(alpha, gamma)Mg-26 reaction, is not well constrained in the important temperature regime from similar to 0.2-0.4 GK, owing to uncertainties in the nuclear properties of resonances lying within the Gamow window. To address these uncertainties, we have performed a new measurement of the Ne-22(Li-6, d)Mg-26 reaction in inverse kinematics, detecting the outgoing deuterons and Mg-25,Mg-26 recoils in coincidence. We have established a new n/gamma decay branching ratio of 1.14(26) for the key E-x = 11.32 MeV resonance in Mg-26, which results in a new (alpha, n) strength for this resonance of 42(11) mu eV when combined with the well-established (alpha, gamma) strength of this resonance. We have also determined new upper limits on the alpha partial widths of neutron-unbound resonances at E-x = 11.112, 11.163, 11.169, and 11.171 MeV. Monte-Carlo calculations of the stellar Ne-22(alpha, n)Mg-25 and Ne-22(alpha, gamma)Mg-26 rates, which incorporate these results, indicate that both rates are substantially lower than previously thought in the temperature range from similar to 0.2-0.4 GK. (C) 2020 The Authors. Published by Elsevier B.V.

The most exotic neutron-proton asymmetric nuclei are produced most efficiently in high-energy fragmentation reactions. These exotic nuclei are produced as fast secondary beams with energies of order 100 MeV per nucleon or greater, and hence v/c > 0.3, often with relatively low intensities. For such beams one can exploit both the detection efficiency and the cross sections for fast one- and two-nucleon removal reactions. The reaction dynamics of these channels is relatively simple due to the fast (sudden), surface grazing, and forward travelling (eikonal) nature of the reaction mechanism. More recent work now interfaces these sudden, eikonal reaction models with more microscopic nuclear structure model input. The possible roles of nucleon and pair correlations and their observation using such reaction data are discussed.

Direct reaction techniques are powerful tools to study the single-particle nature of nuclei. Performing direct reactions on short-lived nuclei requires radioactive ion beams produced either via fragmentation or the Isotope Separation OnLine (ISOL) method. Some of the most interesting regions to study with direct reactions are close to the magic numbers where changes in shell structure can be tracked. These changes can impact the final abundances of explosive nucleosynthesis. The structure of the chain of tin isotopes is strongly influenced by the Z = 50 proton shell closure, as well as the neutron shell closures lying in the neutron-rich, N = 82, and neutron-deficient, N = 50, regions. Here, we present two examples of direct reactions on exotic tin isotopes. The first uses a one-neutron transfer reaction and a low-energy reaccelerated ISOL beam to study states in 131Sn from across the N = 82 shell closure. The second example utilizes a one-neutron knockout reaction on fragmentation beams of neutron-deficient 106,108Sn. In both cases, measurements of γ rays in coincidence with charged particles proved to be invaluable.

The structure of the neutron-rich silicon isotopes Si36,38,40 was studied by one-neutron and one-proton knockout reactions at intermediate beam energies. We construct level schemes for the knockout residues Si35,37,39 and Al35,37,39 and compare knockout cross sections to the predictions of an eikonal model in conjunction with large-scale shell-model calculations. The agreement of these calculations with the present experiment lends support to the microscopic explanation of the enhanced collectivity in the region of Si42. We also present an empirical method for reproducing the observed low-momentum tails in the parallel momentum distributions of knockout residues.

The nucleosynthesis of elements beyond iron is dominated by neutron captures in the s and r processes. However, 32 stable, proton-rich isotopes cannot be formed during those processes, because they are shielded from the s-process flow and r-process, β-decay chains. These nuclei are attributed to the p and rp process. For all those processes, current research in nuclear astrophysics addresses the need for more precise reaction data involving radioactive isotopes. Depending on the particular reaction, direct or inverse kinematics, forward or time-reversed direction are investigated to determine or at least to constrain the desired reaction cross sections. The Facility for Antiproton and Ion Research (FAIR) will offer unique, unprecedented opportunities to investigate many of the important reactions. The high yield of radioactive isotopes, even far away from the valley of stability, allows the investigation of isotopes involved in processes as exotic as the r or rp processes.

The (10Be,10B [1.74 MeV]) charge-exchange reaction at 100 AMeV is presented as a new probe for isolating the isovector (ΔT = 1) non-spin-transfer (ΔS = 0) response of nuclei, with 28Si being the rst nucleus studied. By using a secondary 10Be beam produced by fast fragmentation of 18O nuclei at the NSCL Coupled Cyclotron Facility, applying the dispersion-matching technique with the S800 magnetic spectrometer to determine the excitation energy in 28Al, and performing high- resolution -ray tracking with the Gamma-Ray Energy Tracking Array (GRETINA) to identify the 1022-keV ray associated with the decay from the 1.74-MeV T = 1 isobaric analog state in 10B, a ΔS = 0 excitation-energy spectrum in 28Al was extracted. Monopole and dipole contributions were determined through a multipole-decomposition analysis, and the isovector giant dipole (IVGDR) and monopole (IVGMR) resonances were identi ed. The results show that this probe is a powerful tool for studying the elusive IVGMR, which is of interest for performing stringent tests of modern density functional theories at high excitation energies and for constraining the bulk properties of nuclei and nuclear matter. The extracted distributions were compared with theoretical calculations based on the normal-modes formalism and the proton-neutron relativistic time-blocking approximation. Calculated cross sections based on these strengths underestimate the data by about a factor of two, which likely indicates de ciencies in the reaction calculations based on the distorted wave Born approximation.

We present quantitative reaction cross section calculations for halo nuclei at high energy which retain the essential few-body correlations in such structures. Inclusion of these correlations leads to larger deduced halo radii. For three-body (n+n+core) projectiles, cross section measurements determine the rms hyperradius of the system and, given the core nucleus size, the projectile matter radius. The importance of finite range effects are estimated in the case of the lightest two-neutron-halo system He-6. Outstanding uncertainties are discussed.

The development of advanced γ-ray tracking arrays allows for a sensitive new technique to investigate elusive states of exotic nuclei with fast rare-isotope beams. By taking advantage of the excellent energy and position resolution of the Gamma-Ray Energy Tracking In-beam Nuclear Array, we developed a novel technique to identify in-flight isomeric decays of the 0⁺₂ state in 32Mg populated in a two-proton removal reaction. We confirm the 0⁺₂→2+1γ-ray transition of ³²Mg and constrain the 0⁺₂ decay lifetime, suggesting a large collectivity. The small partial cross section populating the 0⁺₂ state in this reaction provides experimental evidence for the reduced occupancy of the normal configuration of the 0⁺₂ state, indicating the intruder dominance of this state.

Shell evolution is studied in the neutron-rich silicon isotopes Si36,38,40 using neutron single-particle strengths deduced from one-neutron knockout reactions. Configurations involving neutron excitations across the N=20 and N=28 shell gaps are quantified experimentally in these rare isotopes. Comparisons with shell model calculations show that the tensor force, understood to drive the collective behavior in Si42 with N=28, is already important in determining the structure of Si40 with N=26. New data relating to cross-shell excitations provide the first quantitative support for repulsive contributions to the cross-shell T=1 interaction arising from three-nucleon forces.

The Be9(Mg28,Na27) one-proton removal reaction with a large proton separation energy of Sp(Mg28)=16.79 MeV is studied at intermediate beam energy. Coincidences of the bound Na27 residues with protons and other light charged particles are measured. These data are analyzed to determine the percentage contributions to the proton removal cross section from the elastic and inelastic nucleon removal mechanisms. These deduced contributions are compared with the eikonal reaction model predictions and with the previously measured data for reactions involving the removal of more weakly bound protons from lighter nuclei. The role of transitions of the proton between different bound single-particle configurations upon the elastic breakup cross section is also quantified in this well-bound case. The measured and calculated elastic breakup fractions are found to be in good agreement.

Coulomb dissociation of one neutron halo nuclei ‘IBe and 15,1QC has been studied at energies around 70A MeV by kinematically complete measurements of breakup particles at RIKEN. The experiment of l1Be breakup on a Pb target aims at studying the characteristic features of low-lying El strength. In particular the effects of higher order excitation and nuclear contribution have been closely studies by using the impact parameter analysis. We have found that these contributions are very small, and the selection of the large impact parameter is useful to extract the spectroscopic information of the halo ground state. We have then successfully applied this analysis method to l”C and “C, where we have extracted the spectroscopic factor for the s-wave halo configuration. We also show preliminary results on the breakup of “Be on a carbon target in order to study the nuclear breakup mechanism and low-lying discrete states of the halo nucleus.

Background: Measurements of neutron-unbound states can test nuclear models in very neutron-rich nuclei that in some cases cannot be probed with other methods. Purpose: Search for highly excited neutron-unbound states of 25O above the three neutron separation energy. Method: The decay energy of 25O was reconstructed using the invariant mass spectroscopy method. A 101.3 MeV/u 27Ne beam collided with a liquid deuterium target. Two-proton removal reactions populated excited 25O that decayed into three neutrons and an 22O fragment. The neutrons were detected by arrays of plastic scintillator bars, while a 4 Tm dipole magnet placed directly after the target redirected charged fragments to a series of charged-particle detectors. The data were compared with detailed Monte Carlo simulations of the reaction process and subsequent decay. Results: The data show evidence of neutron-unbound level(s) in 25O at an excitation energy of about 9 MeV which decay sequentially by the emission of three neutrons to 22O. Conclusion: The observation of resonance strength in 25O at about 9 MeV is consistent with shell-model/eikonal calculations for the two-proton removal reaction from 27Ne.

The reaction cross section (σ R σR ) of the very neutron-rich carbon isotope 22 22 C has been measured on a carbon target at 235 MeV/nucleon. A σ R σR of 1.280±0.023 1.280±0.023 b was obtained for 22 22 C, significantly larger than those for the neighboring isotopes, supporting the halo character of 22 22 C. A 22 22 C root-mean-squared matter radius of 3.44±0.08 3.44±0.08 fm was deduced using a four-body Glauber reaction model. This value is smaller than an earlier estimate of 5.4±0.9 5.4±0.9 fm derived from a σ R σR measurement on a hydrogen target at 40 MeV/nucleon.

Both one-proton and one-neutron knockout reactions were performed with fast beams of two asymmetric, neutron-deficient rare isotopes produced by projectile fragmentation. The reactions are used to probe the nucleon spectroscopic strengths at both the weakly and strongly bound nucleon Fermi surfaces. The one-proton knockout reactions 9Be(28S,27P)X and 9Be(24Si,23Al)X probe the weakly bound valence proton states and the one-neutron knockout reactions and 9Be(28S, 27S)X and 9Be(24Si, 23Si)X the strongly bound neutron states in the two systems. The spectroscopic strengths are extracted from the measured cross sections by comparisons with an eikonal reaction theory. The reduction of the experimentally deduced spectroscopic strengths, relative to the predictions of shell-model calculations, is of order 0.8–0.9 in the removal of weakly bound protons and 0.3–0.4 in the knockout of the strongly bound neutrons. These results support previous studies at the extremes of nuclear binding and provide further evidence that in asymmetric nuclear systems the nucleons of the deficient species, at the more-bound Fermi surface are more strongly correlated than those of the more weakly bound excess species.

A classical dynamical model that treats breakup stochastically is presented for low energy reactions of weakly bound nuclei. The three-dimensional model allows a consistent calculation of breakup, incomplete, and complete fusion cross sections. The model is assessed by comparing the breakup observables with continuum discretized coupled-channel quantum mechanical predictions, which are found to be in reasonable agreement. Through the model, it is demonstrated that the breakup probability of the projectile as a function of its distance from the target is of primary importance for understanding complete and incomplete fusion at energies near the Coulomb barrier.

The distribution of single-particle strength in 67,69Ni was characterized with one-neutron knockout reactions from intermediate-energy 68,70Ni secondary beams, selectively populating neutron-hole configurations at N = 39 and 41, respectively. The spectroscopic strengths deduced from the measured partial cross sections to the individual final states, as tagged by their γ-ray decays, are used to identify and quantify neutron configurations in the wave functions. While 69Ni compares well to shell-model predictions, the results for 67Ni challenge the validity of current effective shell-model Hamiltonians by revealing discrepancies that cannot be explained so far. These results suggest that our understanding of the low-lying states in the neutron-rich, semi-magic Ni isotopes may be incomplete and requires further investigation on both the experimental and theoretical sides.

The contributions to the cross sections of intermediate energy two-nucleon knockout reactions from events in which one nucleon is removed by the stripping (inelastic breakup) mechanism and a second by the diffraction (elastic breakup) mechanism are discussed. The small additional contributions from two-nucleon diffraction events are also estimated. The approach used combines the eikonal reaction and shell model structure theory frameworks. For reactions involving the removal of two well-bound like nucleons, at incident energies of order 100 MeV per nucleon, the additional cross sections are shown to be of approximately the same size as those from events in which both nucleons are stripped in inelastic interactions. These more complete dynamical calculations now permit a quantitative comparison of the theoretical cross sections with recent partial cross-section measurements of the two-neutron (two-proton) removal reactions from neutron-deficient (neutron-rich) nuclei. As has been observed in both nuclear- and electron-induced single-nucleon knockout reaction analyses, the theoretical two-nucleon knockout cross sections overestimate the measured values, requiring a suppression of the two nucleon shell-model transition strengths. The deduced two-nucleon suppression factors, R-s(2N), are consistent with a value of 0.5 for each of the five reactions considered.

Neutron-rich isotopes are an important source of new information on nuclear physics. Specifically, the spin-isospin components in the nucleon-nucleon (NN) interaction, e.g., the proton-neutron tensor force, are expected to modify shell structure in exotic nuclei. These potential changes in the intrinsic shell structure are of fundamental interest. The study of the excitation energy of states corresponding to specific configurations in even-even isotopes, together with the single-particle character of the first excited states of odd-A, neutron-rich Ni isotopes, probes the evolution of the neutron orbitals around the Fermi surface as a function of the neutron number a step forward in the understanding of the region and the nature of the NN interaction at large N/Z ratios. In an experiment carried out at the National Superconducting Cyclotron Laboratory, new spectroscopic information was obtained for 68Ni and the distribution of single-particle strengths in 67 , 69Ni was characterized by means of single-neutron knockout from 68 , 70Ni secondary beams. The spectroscopic strengths, deduced from the measured partial cross sections to the individual states tagged by their de-exciting gamma rays, is used to identify and quantify configurations that involve neutron excitations across the N = 40 harmonic oscillator shell closure. The de- excitation rays were measured with the GRETINA tracking array. The results challenge the validity of the most current shell-model Hamiltonians and effective interactions, highlighting shortcomings that cannot yet be explained. These results suggest that our understanding of the low-energy states in such nuclei is not complete and requires further investigation.

The cross sections for single-nucleon knockout from Ca-36 on a Be-9 target at 70 MeV/nucleon were measured to be sigma(exp)(-p) = 51.1 +/- 2.6 mb for proton knockout and sigma(exp)(-n) = 5.03 +/- 0.46 mb for neutron knockout. The spectroscopic factors and orbital angular momenta of the neutrons and protons removed from Ca-36, leading to bound A = 35 residues, were deduced by comparison of the experimental cross sections and longitudinal- momentum distributions to those calculated in an eikonal reaction theory, and found to be S(p, 1d(3/2)) = 0.79 +/- 0.04 and S(n, 2s(1/2)) = 0.23 +/- 0.02 (relative to independent-particle-model values and only including experimental contributions to the uncertainties). As found in previous knockout studies, the spectroscopic factor deduced for the deeply bound neutron was significantly reduced relative to shell-model calculations, a result at variance with dispersive optical model (DOM) extrapolations, which suggest a spectroscopic factor closer to 60% of the independent-particle-model value.

One-neutron knockout reactions from the deeply bound N=16 isotones with Z=16,17, and 18 have been studied in inverse kinematics with intermediate-energy beams. gamma-ray spectroscopy in coincidence with the detection of knockout residues allowed for an investigation of the one-neutron removal leading to individual excited states. Spectroscopic factors are deduced in the framework of the sudden and eikonal approximations and are compared to USD shell-model predictions. The momentum distributions observed in the experiment are used to identify the angular momentum l carried by the knockedout neutron by comparing with calculations based on a black-disk reaction model. The systematics of reduced single-particle occupancies attributed to the effect of short-range correlations, observed so far for stable and near-magic nuclei in (e,e(')p) and (d,He-3) reactions and in one-nucleon knockout on light deeply bound systems, are extended in this work.

The breakdown of the N=20 magic number in the so-called island of inversion around 32Mg is well established. Recently developed large-scale shell-model calculations suggest a transitional region between normal- and intruder-dominated nuclear ground states, thus modifying the boundary of the island of inversion. In particular, a dramatic change in single-particle structure is predicted between the ground states of 30Mg and 32Mg, with the latter consisting nearly purely of 2p-2h N=20 cross-shell configurations. Single-neutron knockout experiments on 30Mg and 32Mg projectiles have been performed. We report on a first direct observation of intruder configurations in the ground states of these very neutron-rich nuclei. Spectroscopic factors to low-lying negative-parity states in the knockout residues are deduced and compare well with shell-model predictions.

One-neutron knockout reactions have been performed on a beam of radioactive 53Co in a high-spin isomeric state. The analysis is shown to yield highly-selective population of high-spin states in an exotic nucleus with a significant cross section, and hence represents a technique that is applicable to the planned new generation of fragmentation-based radioactive beam facilities. Additionally, the relative cross sections among the excited states can be predicted to a high level of accuracy when reliable shell-model input is available. The work has resulted in a new level scheme, up to the 11+ band-termination state, of the proton-rich nucleus 52Co (Z = 27, N = 25). This has in turn enabled a study of mirror energy differences in the A = 52 odd-odd mirror nuclei, interpreted in terms of isospin-non-conserving (INC) forces in nuclei. The analysis demonstrates the importance of using a full set of J-dependent INC terms to explain the experimental observations.

Preliminary results of one-neutron removal reactions on the nuclei 34,35Si and 37S at the intersection of the 1s0d and 1p0f neutron shells are reported. Momentum distributions of the projectile residues have been measured at the National Superconducting Cyclotron Laboratory at Michigan State University. The populated final states in the neutron removal reactions could be resolved by measuring the γ rays from the de-excitation of bound states using a position-sensitive NaI(T1) array. Partial cross sections to specific final states have been deduced. A comparison of the cross sections in the reactions (35Si,34Si) and (37S,36S) shows differences that point to the influence of intruder configurations in the vicinity of 34Si.

The recently developed four-body continuum-discretized coupled-channels (CDCC) method, making use of the binning procedure [1], is applied to the reaction 6He+64Zn at 13.6 MeV (around the Coulomb barrier). Excellent agreement with available elastic data [2] is found.

In-beam gamma-ray spectroscopy of Fe-66,Fe-68 following intermediate-energy one- and two-proton knockout from cobalt and nickel secondary beams has been performed at the National Superconducting Cyclotron Laboratory. New transitions have been observed in Fe-66 and Fe-68. This is the first observation of gamma-ray transitions in Fe-68. In addition, Cr-64 was produced using the Be-9(Fe-66,Cr-64)X two-proton knockout reaction. An unexpectedly low inclusive cross section is observed for this reaction, an order of magnitude smaller than for the Be-9(Ni-68,Fe-66)X reaction. This observation is discussed in terms of a significant structural difference between Fe-66 and Cr-64 and considerable admixtures of nu(pf)(n-2) (g(9/2))(+2) configurations in the ground and excited states of Cr-64 at N = 40.

Inclusive one- and multi-nucleon removal cross sections have been measured for several Sn, Sb and Te isotopes just beyond the N=82 neutron shell closure. The beams were produced in the projectile fission of a 238U beam at the Radioactive Isotope Beam Factory at RIKEN. The experimental cross sections are compared to predictions from the most recent version of the Liege intranuclear cascade model. Although the overall agreement is good, severe discrepancies are observed for the cases of one- and two-neutron removal from 134Sn and 135Sb projectiles and one-proton knockout from all measured N=84 isotones. These discrepancies, as well as the relevance of quasi-elastic reaction channels to the one-neutron removal cross sections, are discussed. In addition, the measured inclusive one-proton knockout cross section for the semi-magic 134Sn projectile is compared to eikonal direct reaction theory calculations to assess if the suppression factors to these calculated cross sections, deduced from data on reactions of lighter projectile nuclei, are also applicable to heavy nuclei.

The first spectroscopy of excited states in 52Ni (Tz=-2) and 51Co (Tz=-3/2) has been obtained using the highly selective two-neutron knockout reaction. Mirror energy differences between isobaric analogue states in these nuclei and their mirror partners are interpreted in terms of isospin nonconserving effects. A comparison between large-scale shell-model calculations and data provides the most compelling evidence to date that both electromagnetic and an additional isospin nonconserving interactions for J=2 couplings, of unknown origin, are required to obtain good agreement.

The halo structure of Ne31 is studied using 1n-removal reactions on C and Pb targets at 230  MeV/nucleon. A combined analysis of the cross sections of these nuclear and Coulomb dominated reactions that feed directly the Ne30 ground-state reveals Ne31 to have a small neutron separation energy, 0.15+0.16−0.10  MeV, and spin-parity 3/2−. Consistency of the data with reaction and large-scale shell-model calculations identifies Ne31 as deformed and having a significant p-wave halo component, suggesting that halos are more frequent occurrences at the neutron drip line.

Detailed γ-ray spectroscopy of the exotic neon isotope ²⁸Ne has been performed for the first time using the one-neutron removal reaction from ²⁹Ne on a liquid hydrogen target at 240 MeV/nucleon. Based on an analysis of parallel momentum distributions, a level scheme with spin-parity assignments has been constructed for 28Ne and the negative-parity states are identified for the first time. The measured partial cross sections and momentum distributions reveal a significant intruder p-wave strength providing evidence of the breakdown of the N=20 and N=28 shell gaps. Only a weak, possible f-wave strength was observed to bound final states. Large-scale shell-model calculations with different effective interactions do not reproduce the large p-wave and small f-wave strength observed experimentally, indicating an ongoing challenge for a complete theoretical description of the transition into the island of inversion along the Ne isotopic chain.

Background: The Island of Inversion near the N = 20 shell gap is home to nuclei with reordered single-particle energy levels compared to the spherical shell model. Studies of 31 Ne have revealed that its ground state has a halo component, characterized by a valence neutron orbiting a deformed 30 Ne core. This lightly-bound nucleus with a separation energy of only Sn = 170 keV is expected to have excited states that are neutron unbound. Purpose: The purpose of this experiment was to investigate the low-lying excited states in 31 Ne that decay by the emission of a single neutron. Methods: An 89 MeV/nucleon 33 Mg beam impinged on a segmented Be reaction target. Neutron-unbound states in 31 Ne were populated via a two-proton knockout reaction. The 30 Ne fragment and associated neutron from the decay of 31 Ne were detected by the MoNA-LISA-Sweeper experimental setup at the National Superconducting Cyclotron Laboratory. Invariant mass spec-troscopy was used to reconstruct the two-body decay energy (30 Ne + n). Results: The two-body decay energy spectrum exhibits two features: a low-lying peak at 0.30 ± 0.17 MeV and a broad enhancement at 1.50 ± 0.33 MeV, each fit with an energy-dependent asymmetric Breit-Wigner line shape representing a resonance in the continuum. Accompanying shell model calculations using the FSU interaction within NuShellX, combined with cross-section calculations using the eikonal reaction theory, indicate that these peaks in the decay energy spectrum are caused by multiple resonant states in 31 Ne. Conclusions: Excited states in 31 Ne were observed for the first time. Transitions from calculated shell model final states in 31 Ne to bound states in 30 Ne are in good agreement with the measured decay energy spectrum. Cross-section calculations for the two-proton knockout populating 31 Ne states as well as spectroscopic factors pertaining to the decay of 31 Ne into 30 Ne are used to examine the results within the context of the shell model expectations.

Cross sections of 1n-removal reactions from the neutron-rich nucleus Mg37 on C and Pb targets and the parallel momentum distributions of the Mg37 residues from the C target have been measured at 240  MeV/nucleon. A combined analysis of these distinct nuclear- and Coulomb-dominated reaction data shows that the Mg37 ground state has a small 1n separation energy of 0.22+0.12−0.09  MeV and an appreciable p-wave neutron single-particle strength. These results confirm that Mg37 lies near the edge of the “island of inversion” and has a sizable p-wave neutron halo component, the heaviest such system identified to date.

Reactions that involve the direct and sudden removal of a pair of like or unlike nucleons from a fast projectile beam by a light target nucleus are considered. Specifically, we study the three two-nucleon removal channels from 12C that populate final states in the 10Be, 10B, and 10C reaction residues. The calculated two-nucleon removal cross sections and the residue momentum distributions are compared with available high-energy data at 250, 1050, and 2010 MeV per nucleon, i.e., data that are inclusive with respect to the bound final states of the residues. The measured np removal cross sections only are significantly greater than the values calculated, suggesting that the reaction mechanism observes enhanced np spatial correlations compared to those present in the shell-model wave functions.

Two-proton removal reaction cross sections, from 208Pb at 1 GeV/nucleon, are estimated as an example of the direct population of (high-spin) seniority-2 isomeric states, here in 206Hg. Nucleon removal by both the stripping and diffractive mechanisms is considered. The cross sections in this specific (test) case are significant and can provide direct two-nucleon removal predictions of isomeric ratios

In atomic nuclei, the spin-orbit interaction originates from the coupling of the orbital motion of a nucleon with its intrinsic spin. Recent experimental and theoretical works have suggested a weakening of the spin-orbit interaction in neutron-rich nuclei far from stability. To study this phenomenon, we have investigated the spin-orbit energy splittings of single-hole and single-particle valence neutron orbits of 132Sn. The spectroscopic strength of single-hole states in 131Sn was determined from the measured differential cross sections of the tritons from the neutron-removing 132Sn(d,t)131Sn reaction, which was studied in inverse kinematics at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. The spectroscopic factors of the lowest 3=2+, 1=2+ and 5=2+states were found to be (2 j+1), confirming the robust N = 82 shell closure at 132Sn. We compared the spin-orbit splitting of neutron single-hole states in 131Sn to those of single-particle states in 133Sn determined in a recent measurement of the 132Sn(d,p)133Sn reaction. We found a significant reduction of the energy splitting of the weakly bound 3p orbits compared to the well-bound 2d orbits, and that all the observed energy splittings can be reproduced remarkably well by calculations using a onebody spin-orbit interaction and a Woods-Saxon potential of standard radius and diffuseness. The observed reduction of spin-orbit splitting can be explained by the extended radial wavefunctions of the weakly bound orbits, without invoking a weakening of the spin-orbit strength.

The single-particle structure of the N = 27 isotones provides insights into the shell evolution of neutron-rich nuclei from the doubly-magic 48Ca toward the drip line. 43S was studied employing the one-neutron knockout reaction from a radioactive 44S beam. Using a combination of prompt and delayed γ-ray spectroscopy the level structure of 43S was clarified. Momentum distributions were analyzed and allowed for spin and parity assignments. The deduced spectroscopic factors show that the 44S ground-state configuration has a strong intruder component. The results were confronted with shell model calculations using two effective interactions. General agreement was found between the calculations, but strong population of states originating from the removal of neutrons from the 2p3/2 orbital in the experiment indicates that the breakdown of the N = 28 magic number is more rapid than the theoretical calculations suggest.

The decay of excited states of the nucleus 135Sn, with three neutrons outside the doubly-magic 132Sn core, was studied in an experiment performed at the Radioactive Isotope Beam Factory at RIKEN. Several γ rays emitted from excited 135Sn ions were observed following one-neutron and one-neutron-one-proton removal from 136Sn and 137Sb beams, respectively, on a beryllium target at relativistic energies. Based on the analogy to 133Sn populated via one-neutron removal from 134Sn, an excitation energy of 695(15) keV is assigned to the 3/2− state with strongest single-particle character in 135Sn. This result provides the first direct information about the evolution of the neutron shell structure beyond N=82 and thus allows for a crucial test of shell-model calculations in this region. The experimental findings are in full agreement with calculations performed employing microscopic effective two-body interactions derived from CD-Bonn and N3LO nucleon-nucleon potentials, which do not predict a pronounced subshell gap at neutron number N=90. The occurrence of such a gap in 140Sn, i.e., when the 1 orbital is completely filled, had been proposed in the past, in analogy to the magicity of 48Ca, featuring a completely filled 0 orbital one harmonic oscillator shell below.

The ground state two-proton decay lifetime of Mg19, populated by the one-neutron knockout of an intermediate-energy Mg20 radioactive beam, was measured utilizing a new experimental technique. A thin silicon detector positioned at varying distances (0.0–1.0 mm) downstream of the reaction target measured the energy loss of Mg19 and the two-proton decay product Ne17. The lifetime was deduced from fits to the measured energy-loss line shapes and depended upon the contribution of prompt reaction processes to the yield of Ne17. For relative Ne17 prompt contributions from 82% to 92%, the extracted lifetimes ranged from 1.75+0.43−0.42 to 6.4+2.4−2.7 ps. The results are consistent with the previously reported Mg19 lifetime measurement and serve as both an important complementary study and a validation of this new technique, which can provide lifetime information for short-lived states beyond the proton drip line.

Intruder configurations in the ground state of 30Ne were studied using the one neutron knockout reaction 12C(30Ne,29Ne + γ)X at 228 MeV/nucleon at the Radioactive Isotope Beam Factory. Individual parallel momentum distributions and partial cross sections were measured by tagging γ rays. A significant population of the p-wave intruder state in 29Ne is observed, providing further experimental evidence for the vanishing of the N = 20 shell closure.

We present a high-resolution in-beam γ-ray spectroscopy study of excited states in the mirror nuclei 55Co and 56Ni following one-nucleon knockout from a projectile beam of 56Ni. The newly determined partial cross sections and the γ-decay properties of excited states provide a test of state-of-the-art nuclear structure models and probe mirror symmetry in unique ways. The new experimental data are compared to large-scale shell-model calculations in the full pf space and including charge-dependent contributions. A mirror asymmetry for the partial cross sections leading to the two lowest 3/2− states in the A = 55 mirror pair was identified as well as a significant difference in the E1 decays from the 1/21+ state to the same two 3/2− states. The mirror asymmetry in the partial cross sections cannot be reconciled with the present shell-model picture or small mixing introduced in a two-state model. The observed mirror asymmetry in the E decay pattern, however, points at stronger mixing between the two lowest 3/2− states in 55Co than in its mirror 55Ni.

We report on the first detailed study of intruder configurations in the ground state of 30Ne by means of the 12C(30Ne,29Ne+γ)X one-neutron knockout reaction at 228 MeV/nucleon. Using a combined analysis of individual parallel momentum distributions and partial cross sections we find: (a) comparable p- and d-wave removal strength to 29Ne final states with excitation energies below 200 keV, and (b) significant p-wave removal strength to the 620 keV state of 29Ne, and (c) no evidence for f-wave intruder strength leading to bound 29Ne final states. The SDPF-U-MIX shell model calculation in the sd-pf model space provides a better overall agreement with the measured energy levels of 29Ne and the fp-intruder amplitudes in 30Ne than the SDPF-M prediction, suggesting that the refinement of the sd-pf cross shell interaction and extension of the model space to include the 2p1/2 and 1f5/2 levels are important for understanding the island of inversion.

The body of experimental measurements of intermediate-energy reactions that remove a single nucleon from a secondary beam of neutron-or proton-rich nuclei continues to grow. These data have been analysed consistently using an approximate, eikonal-model treatment of the reaction dynamics combined with appropriate shell-model descriptions of the projectile initial state, the bound final states spectrum of the reaction residue and single-particle removal strengths computed from their wave-function overlaps. The systematics of the ratio Rs of the measured inclusive cross-section to all bound final states and the calculated cross-section to bound shell-model states – in different regions of the nuclear chart and involving both very weakly-bound and strongly-bound valence nucleons – is important in relating the empirically deduced orbital occupancies to those from the best available shell-model predictions. Importantly, several new higher-energy measurements, for which the sudden-approximation aspect of the dynamical description is placed on an even stronger footing, now supplement the previously-analysed measurements. These additional data sets are discussed. Their Rs values are shown to conform to and reinforce the earlier-observed systematics, with no indication that the approximately linear reduction in Rs with increasing nucleon separation energy is a consequence of a breakdown of the sudden approximation.

Simpson and Tostevin proposed that the width and shape of exclusive parallel momentum distributions of the A

Background: The study of excited states in mirror nuclei allows us to extract information on charge-dependent (i.e., isospin-nonconserving) interactions in nuclei. Purpose: To extend previous studies of mirror nuclei in the f7/2 region, investigating charge symmetry breaking of the strong nuclear force. Methods: γ-ray spectroscopy has been performed for the mirror (Tz= ±3/2) pair 53Ni and 53Mn, produced via mirrored one-nucleon knockout reactions. Results: Several new transitions have been identified in 53Ni from which a new level scheme has been constructed. Cross sections for knockout have been analyzed and compared with reaction model calculations where evidence is found for knockout from high-spin isomeric states. Mirror energy differences between isobaric analog states have been computed, compared to large scale shell-model calculations, and interpreted in terms of isospin-nonconserving effects. In addition, lifetimes for the long-lived J π = 5 /2 − 1 analog states in both 53Mn and 53Ni have been extracted through lineshape analysis, giving half-lives of t1/2 = 120 (14) ps and t1/2 = 198 (12) ps,respectively. Conclusions: The inclusion of a set of isovector isospin-nonconserving matrix elements to the shell-model calculations gave the best agreement with the experimental data.

Both one-proton and one-neutron knockout reactions were performed with fast beams of two asymmetric, neutron-deficient rare isotopes produced by projectile fragmentation. The reactions are used to probe the nucleon spectroscopic strengths at both the weakly and strongly bound nucleon Fermi surfaces. The one-proton knockout reactions Be-9(S-28, P-27)X and Be-9(Si-24, Al-23)X probe the weakly bound valence proton states and the one-neutron knockout reactions and Be-9(S-28, S-27)X and Be-9(Si-24, Si-23)X the strongly bound neutron states in the two systems. The spectroscopic strengths are extracted from the measured cross sections by comparisons with an eikonal reaction theory. The reduction of the experimentally deduced spectroscopic strengths, relative to the predictions of shell-model calculations, is of order 0.8-0.9 in the removal of weakly bound protons and 0.3-0.4 in the knockout of the strongly bound neutrons. These results support previous studies at the extremes of nuclear binding and provide further evidence that in asymmetric nuclear systems the nucleons of the deficient species, at the more-bound Fermi surface are more strongly correlated than those of the more weakly bound excess species.

Fragmentation reactions with intermediate-energy heavy-ion beams exhibit a wide range of reaction mechanisms, ranging from direct reactions to statistical processes. We examine this transition by measuring the relative population of excited states in several sd-shell nuclei produced by fragmentation with the number of removed nucleons ranging from two to sixteen. The two-nucleon removal is consistent with a non-dissipative process, whereas the removal of more than five nucleons appears to be mainly statistical.

New experimental data obtained from γ-ray tagged one-neutron and one-proton knockout from 55Co is presented. A candidate for the sought-after T = 1, T z = 0, Jπ = 6+ state in 54Co is proposed based on a comparison to the new data on 54Fe, the corresponding observables predicted by large-scale-shell-model (LSSM) calculations in the full fp-model space employing charge-dependent contributions, and isospin-symmetry arguments. Furthermore, possible isospin-symmetry breaking in the A = 54, T = 1 triplet is studied by calculating the experimental c coefficients of the isobaric mass multiplet equation (IMME) up to the maximum possible spin J = 6 expected for the (1f7/2)-2 two-hole configuration relative to the doubly-magic nucleus 56Ni. The experimental quantities are compared to the theoretically predicted c coefficients from LSSM calculations using two-body matrix elements obtained from a realistic chiral effective field theory potential at next-to-next-to-next-to leading order (N3LO).

Two-nucleon removal (or knock-out) reactions at intermediate energies are a developing tool for both nuclear spectroscopy and for the study of certain nucleon correlations in very exotic and some stable nuclei. We present an overview of these reactions with specific emphasis on the nature of the two-nucleon correlations that can be probed. We outline future possibilities and tests needed to fully establish these sensitivities.

Many properties of the atomic nucleus, such as vibra- 21 tions, rotations and incompressibility can be interpreted 22 as due to a two-component quantum liquid of protons and 23 neutrons. Electron scattering measurements on stable nu- 24 clei demonstrate that their central densities are saturated, 25 as for liquid drops. In exotic nuclei near the limits of mass 26 and charge, with large imbalances in their proton and neu- 27 tron numbers, the possibility of a depleted central density, 28 or a “bubble” structure, was discussed in a recurrent man- 29 ner since the seventies. Here we report first experimental 30 evidence that points to a depletion of the central density of protons in the short-lived nucleus 34 31 Si. The proton-toneutron density asymmetry in 34 32 Si offers the possibility to 33 place constraints on the density and isospin dependence 34 of the spin-orbit force - on which nuclear models have dis- 35 agreed for decades- and on its stabilizing effect towards 36 limits of nuclear existence

The astrophysical s-process is one of the two main processes forming elements heavier than iron. A key outstanding uncertainty surrounding s-process nucleosynthesis is the neutron flux generated by the 22Ne(; n)25Mg reaction during the He-core and C-shell burning phases of massive stars. This reaction, as well as the competing 22Ne(;)26Mg reaction, is not well constrained in the important temperature regime from 0:2–0:4 GK, owing to uncertainties in the nuclear properties of resonances lying within the Gamow window. To address these uncertainties, we have performed a new measurement of the 22Ne(6Li; d)26Mg reaction in inverse kinematics, detecting the outgoing deuterons and 25;26Mg recoils in coincidence. We have established a new n= decay branching ratio of 1:14(26) for the key Ex = 11:32 MeV resonance in 26Mg, which results in a new (; n) strength for this resonance of 42(11) eV when combined with the well-established (; ) strength of this resonance. We have also determined new upper limits on the partial widths of neutron-unbound resonances at Ex = 11:112; 11:163, 11:169, and 11:171 MeV. Monte-Carlo calculations of the stellar 22Ne(; n)25Mg and 22Ne(; )26Mg rates, which incorporate these results, indicate that both rates are substantially lower than previously thought in the temperature range from 0:2–0:4 GK.

Situated in the so-called “island of inversion,” the nucleus 32Mg is considered as an archetypal example of the disappearance of magicity at . We report on high statistics in-beam spectroscopy of 32Mg with a unique approach, in that two direct reaction probes with different sensitivities to the underlying nuclear structure are employed at the same time. More specifically, states in 32Mg were populated by knockout reactions starting from 33Mg and 34Si, lying inside and outside the island of inversion, respectively. The momentum distributions of the reaction residues and the cross sections leading to the individual final states were confronted with eikonal-based reaction calculations, yielding a significantly updated level scheme for 32Mg and spin-parity assignments. By fully exploiting observables obtained in this measurement, a variety of structures coexisting in 32Mg was unraveled. Comparisons with theoretical predictions based on shell-model overlaps allowed for clear discrimination between different structural models, revealing that the complete theoretical description of this key nucleus is yet to be achieved.

There exists a class of nuclei that are obtained by adding one nucleon to a loosely-bound nucleon-core system, for example $^{12}$Be, $^9$C, $^{18}$Ne. For such nuclei, one-nucleon overlap integrals that represent single-particle motion can strongly differ from the standard ones due to the correlations between the two nucleons above the core. The possible non-standard overlap behaviour should be included in the interpretation of the experimental data derived from one nucleon removal reactions such as knockout, transfer and breakup, as well as the predictions of low-energy nucleon capture that leads to these nuclei. We investigate the non-standard behaviour within a three-body model and discuss the challenges associated with this problem.

In Wolf-Rayet and asymptotic giant branch (AGB) stars, the Al26g(p,γ)Si27 reaction is expected to govern the destruction of the cosmic γ-ray emitting nucleus Al26. The rate of this reaction, however, is highly uncertain due to the unknown properties of key resonances in the temperature regime of hydrogen burning. We present a high-resolution inverse kinematic study of the Al26g(d,p)Al27 reaction as a method for constraining the strengths of key astrophysical resonances in the Al26g(p,γ)Si27 reaction. In particular, the results indicate that the resonance at Er=127  keV in Si27 determines the entire Al26g(p,γ)Si27 reaction rate over almost the complete temperature range of Wolf-Rayet stars and AGB stars.

Comprehensive spectroscopy of the N = 29 nucleus 47Ar is presented, based on two complemen-tary direct reaction mechanisms: one-neutron pickup onto 46Ar projectiles and one-proton removal from the 1− ground state of 48K. The results are compared to shell-model calculations that use the state-of-the-art SDPF-U and SDPF-MU effective interactions. Also, from the 9Be(46Cl,45S+γ)X one-proton removal reaction, we report the first γ-ray transitions observed from 45S. Using compar-isons with shell-model calculations, and from the observed intensities and energy sums, we propose a first tentative level scheme for 45S.

The structure of ³³Si was studied by a one-neutron knockout reaction from a ³⁴Si beam at 98.5 MeV/u incident on a 9 Be target. The prompt γ rays following the de-excitation of ³³Si were detected using the GRETINA γ -ray tracking array while the reaction residues were identified on an event-by-event basis in the focal plane of the S800 spectrometer at the National Superconducting Cyclotron Laboratory. The presently derived spectroscopic factor values, C 2 S , for the 3 / 2 + and 1 / 2 + states, corresponding to a neutron removal from the 0 d 3 / 2 and 1 s 1 / 2 orbitals, agree with shell model calculations and point to a strong N = 20 shell closure. Three states arising from the more bound 0 d 5 / 2 orbital are proposed, one of which is unbound by about 930 keV. The sensitivity of this experiment has also confirmed a weak population of 9 / 2 − and 11 / 2 − 1 , 2 final states, which originate from a higher-order process. This mechanism may also have populated, to some fraction, the 3 / 2 − and 7 / 2 − negative-parity states, which hinders a determination of the C 2 S values for knockout from the normally unoccupied 1 p 3 / 2 and 0 f 7 / 2 orbits.

Bound states of the neutron-deficient, near-dripline nucleus ³⁶Ca were populated in two-neutron removal from the ground state of ³⁸Ca, a direct reaction sensitive to the single-particle configurations and couplings of the removed neutrons in the projectile wave function. Final-state exclusive cross sections for the formation of ³⁶Ca and the corresponding longitudinal momentum distributions, both determined through the combination of particle and γ-ray spectroscopy, are compared to predictions combining eikonal reaction theory and shell-model two-nucleon amplitudes from the USDB, USDC, and ZBM2 effective interactions. The final-state cross-section ratio σ(2 + 1)/σ(0 +) shows particular sensitivity and is approximately reproduced only with the two-nucleon amplitudes from the ZBM2 effective interaction that includes proton cross-shell excitations into the pf shell. Characterizing the proton pf-shell occupancy locally and schematically, an increase of the sd-pf shell gap by 250 keV yields an improved description of this cross-section ratio and simultaneously enables a reproduction of the B(E2; 0 + 1 → 2 + 1) excitation strength of ³⁶Ca. This highlights an important aspect if a new shell-model effective interaction for the region was to be developed on the quest to model the neutron-deficient Ca isotopes and surrounding nuclei whose structure is impacted by proton cross-shell excitations.

The neutron-deficient Ca isotopes continue to attract attention due to their importance for testing isospin symmetry and their relevance in capture reactions of interest for nova nucleosynthesis and the shape of light curves in Type I x-ray bursts. To date, spectroscopic information on 38,39Ca is largely limited to data on lower-spin excited states. Here, we report in-beam γ-ray spectroscopy of complementary higher-spin, complex-structure states in 39Ca populated in fast-beam-induced, momentum-dissipative processes leading to neutron pickup onto excited configurations of the projectile, 9Be(38Ca∗,39Ca+γ)X. Such a dissipative reaction was recently characterized for the case of inelastic scattering of 38Ca off 9Be, 9Be(38Ca, 38Ca+γ)X. Additional data and discussion on the nuclear structure of 38Ca is also presented. An explanation for the more-complex-structure states, populated with small cross sections in one-nucleon knockout reactions, and observed in the tails of their longitudinal momentum distributions, is also offered.

The 9 Be(25 F(5/2 +), 24 O)X proton-removal reaction was studied at the NSCL using the S800 spectrometer. The experimental spectroscopic factor for the ground-state to ground-state transition indicates a substantial depletion of the proton d 5/2 strength compared to shell-model expectations, similar to the findings of an inverse-kinematics (p, 2p) measurement performed at RIBF. The 25 F to 24 O ground-states overlap is considerably less than anticipated if the core nucleons behaved as rigid, doubly-magic 24 O within 25 F. We interpret the new results within the framework of the Particle-Vibration Coupling (PVC) model, of a d 5/2 proton coupled to a quadrupole phonon of an effective core. This approach provides a good description of the experimental data, requiring an effective 24 O* core with a phonon energy of ¯ hω2= 3.2 MeV and a B(E2) ≈ 2.7 W.u. – softer and more collective than a bare 24 O. Both the Nilsson deformed mean field and the PVC models appear to capture the properties of the effective core of 25 F, suggesting that the additional proton polarizes 24 O in such a way that it becomes either slightly deformed or a quadrupole vibrator.

Spectroscopic factors of neutron-hole and proton-hole states in 131Sn and 131In, respectively, were measured using one-nucleon removal reactions from doubly magic 132Sn at relativistic energies. For 131In, a 2910(50)-keV γ ray was observed for the first time and tentatively assigned to a decay from a 5=2− state at 3275(50) keV to the known 1=2− level at 365 keV. The spectroscopic factors determined for this new excited state and three other single-hole states provide first evidence for a strong fragmentation of singlehole strength in 131Sn and 131In. The experimental results are compared to theoretical calculations based on the relativistic particle-vibration coupling

The r-process is supposed to be a primary process which assembles heavy nuclei from a photo-dissociated nucleon gas. Hence, the reaction flow through light elements can be important as a constraint on the conditions for the r-process. We have studied the impact of di-neutron capture and the neutron-capture of light (Z

We discuss intermediate-energy single-nucleon removal reactions from deformed projectile nuclei. The removed nucleon is assumed to originate from a given Nilsson model single-particle state and the inclusive cross sections, to all rotational states of the residual nucleus, are calculated. We investigate the sensitivity of both the stripping cross sections and their momentum distributions to the assumed size of the model space in the Nilsson model calculations and to the shape of the projectile and residue. We show that the cross sections for small deformations follow the decomposition of the Nilsson state in a spherical basis. In the case of large and prolate projectile deformations the removal cross sections from prolate-like Nilsson states, having large values for the asymptotic quantum number nz, are reduced. For oblate-like Nilsson states, with small nz, the removal cross sections are increased. Whatever the deformation, the residue momentum distributions are found to remain robustly characteristic of the orbital angular momentum decomposition of the initial state of the nucleon in the projectile.

Background The recent discovery and spectroscopic measurements of 27O and 28O suggests the disappearance of the N = 20 shell structure in these neutron-rich oxygen isotopes. Purpose We measured one- and two-proton removal cross sections from 27F and 29Ne, respectively, extracting spectroscopic factors and comparing them to shell model overlap functions coupled with eikonal reaction model calculations. Method The invariant mass technique was used to reconstruct the two-body (24O + n) and three-body (24O + 2n) decay energies from knockout reactions of 27F (106.2 MeV/u) and 29Ne (112.8 MeV/u) beams impinging on a 9Be target. Results The one-proton removal from 27F strongly populated the ground state of 26O and the extracted cross section of 3.4+0.3−1.5 mb agrees with eikonal model calculations that are normalized by the shell model spectroscopic factors and account for the systematic reduction factor observed for single nucleon removal reactions within the models used. For the two-proton removal reaction from 29 Ne an upper limit of 0.08 mb was extracted for populating states in 27O decaying though the ground state of 26O. Conclusions The measured upper limit for the population of the ground state of 26O in the two-proton removal reaction from 29Ne indicates a significant difference in the underlying nuclear structure of 27F and 29Ne.

A novel quantum dynamical model based on the dissipative quantum dynamics of open quantum systems is presented. It allows the treatment of both deep-inelastic processes and quantum tunneling (fusion) within a fully quantum mechanical coupled-channels approach. Model calculations show the transition from pure state (coherent) to mixed state (decoherent and dissipative) dynamics during a near-barrier nuclear collision. Energy dissipation, due to irreversible decay of giant-dipole excitations of the interacting nuclei, results in hindrance of quantum tunneling

The heaviest particle-bound carbon isotope, C22, is thought to have a Borromean three-body structure. We discuss and compare four-body, i.e. three-body projectile plus target, reaction model calculations of reaction cross sections for such systems that use the fast adiabatic approximation. These methods are efficient and well-suited for quantitative analyses of reactions of neutron-rich nuclei with light target nuclei at secondary beam energies of ≈300 MeV/nucleon, as are now becoming available. We compare the predictions of the adiabatic model of the reaction both without and when including the additional eikonal approximation that has been used extensively. We show that the reaction cross section calculations have only limited sensitivity to the size and structure of C22 and that the differences arising from use of the eikonal approximation of the reaction mechanism are of a similar magnitude.

Results are presented from a one-neutron knockout experiment at relativistic energies of $ ____approx 420 A$ MeV on 51-55Sc using the GSI Fragment Separator as a two-stage magnetic spectrometer and the MINIBALL array for gamma-ray detection. Inclusive longitudinal momentum distributions and cross-sections were measured enabling the determination of the contributions corresponding to knockout from the $ ____nu p_{1/2}$ , $ ____nu p_{3/2}$ , (L = 1 and $ ____nu f_{7/2}$ , $ ____nu f_{5/2}$ (L = 3 neutron orbitals. The observed L = 1 and L = 3 contributions are compared with theoretical cross-sections using eikonal knockout theory and spectroscopic factors from shell model calculations using the GXPF1A interaction. The measured inclusive knockout cross-sections generally follow the trends expected theoretically and given by the spectroscopic strength predicted from the shell model calculations. However, the deduced L = 1 cross-sections are generally 30-40% higher while the L = 3 contributions are about a factor of two smaller than predicted. This points to a promotion of neutrons from the $ ____nu f_{7/2}$ to the $ ____nu p_{3/2}$ orbital indicating a weakening of the N = 28 shell gap in these nuclei. While this is not predicted for the phenomenological GXPF1A interaction such a weakening is predicted by recent calculations using realistic low-momentum interactions $ V_{low k}$ obtained by evolving a chiral N3LO nucleon-nucleon potential.

The role of proton shell effects in the structure of the N=28 isotones 45Cl and 44S has been studied via one-proton knockout from 45Cl. We compare measured γ-ray intensities, inclusive and partial knockout cross sections, and the inclusive momentum distribution of outgoing 44S particles with shell-model and reaction-theory predictions. The strong population in this reaction of the recently identified 41+ state in 44S, identified through its subsequent γ-ray decay energy, makes a compelling case for a Jπ=3/2+ ground state in 45Cl.

The first high-resolution in-beam γ -ray spectroscopy is reported for the neutron-rich nucleus 41Si, a tenant of the N = 28 island of inversion. Excited states were populated in the direct one-proton removal reaction from 42P projectiles and pn removal from 43P. Seven γ -ray transitions were observed, only one of which had been reported previously in the literature. This makes 41Si the most neutron-rich odd-even N = 27 isotone with high-resolution excited-state information. For the one-proton removal, the measured partial cross-section distribution to all observed bound final states is contrasted with results from direct one-proton removal calculations that combine eikonal reaction dynamics with SDPF-MU shell-model spectroscopic factors and assume various possible initial states for the poorly known 42P projectile. Rather distinct calculated cross-section distributions emerge that, in comparison to the new data, imply that the initial state in 42P is most likely 3− or 2− rather than 1− or 0−, the predicted shell-model ground state of 42P. It is further shown that the level scheme from the novel VS-IMSRG calculation closely agrees with the one of SDPF-MU, the most successful phenomenological shell-model effective interaction in describing the much discussed neighboring isotope 42Si, perhaps cross-validating these complementary approaches on the quest to model rapid shell evolution away from the valley of β stability.

The (p,t) transfer reaction is being studied for its potential use in surrogate reaction analyses. A theoretical model has been developed to predict spin-parity distributions of final states excited in the reaction. The model, after comparisons with experimental data, may provide a predictive capability to identify candidate isotopes for measurement. Preliminary results are presented for the 92Zr(p,t)90Zr reaction at incident proton energy Ep=28.5MeV. New experimental data for this reaction at a similar energy, and for several other stable Zr isotopes, will soon be available.

We report on the first in-beam -ray spectroscopy of the proton-dripline nucleus 40Sc using two-nucleon pickup onto an intermediate-energy rare-isotope beam of 38Ca. The 9Be(38Ca,40Sc+)X reaction at 60.9 MeV/nucleon mid-target energy selectively populates states in 40Sc for which the transferred proton and neutron couple to high orbital angular momentum. In turn, due to angular-momentum selection rules in proton emission and the nuclear structure and energetics of 39Ca, such states in 40Sc then exhibit-decay branches although they are well above the proton separation energy. This work uniquely complements results from particle spectroscopy following charge-exchange reactions on 40Ca as well as 40Ti EC/B+ decay which both display very different selectivities. The population and-ray decay of the previously known first (5-) state at 892 keV and the observation of a new level at 2744 keV are discussed in comparison to the mirror nucleus and shell-model calculations. On the experimental side, this work shows that high-resolution in-beam -ray spectroscopy is possible with new generation Ge arrays for reactions induced by rare-isotope beams on the level of a few b of cross section.

Excited states in the neutron-rich N=38, 36 nuclei Ti60 and Ti58 were populated in nucleon-removal reactions from V61 projectiles at 90  MeV/nucleon. The γ-ray transitions from such states in these Ti isotopes were detected with the advanced γ-ray tracking array GRETINA and were corrected event by event for large Doppler shifts (v/c∼0.4) using the γ-ray interaction points deduced from online signal decomposition. The new data indicate that a steep decrease in quadrupole collectivity occurs when moving from neutron-rich N=36, 38 Fe and Cr toward the Ti and Ca isotones. In fact, Ti58,60 provide some of the most neutron-rich benchmarks accessible today for calculations attempting to determine the structure of the potentially doubly magic nucleus Ca60.

We studied α cluster states in 26Mg via the 22Ne(6Li,dγ)26Mg reaction in inverse kinematics at an energy of 7 MeV/nucleon. States between Ex = 4–14 MeV in 26Mg were populated and relative α spectroscopic factors were determined. Some of these states correspond to resonances in the Gamow window of the 22Ne(α,n)25Mg reaction, which is one of the main neutron sources in the astrophysical s-process. Using our new 22Ne(α,n)25Mg and 22Ne(α,γ)26Mg reaction rates, we performed new s-process calculations for massive stars and asymptotic giant branch stars and compared the resulting abundances with the abundances obtained using other 22Ne+α rates from the literature. We observe an impact on the s-process abundances up to a factor of three for intermediate-mass AGB stars and up to a factor of ten for massive stars. Additionally, states in 25Mg at Ex < 7.5 MeV are identified via the 22Ne(6Li,t)25Mg reaction for the first time. We present the (6Li, t) spectroscopic factors of these states and note similarities to the (d,p) reaction in terms of reaction selectivity.

Excited states in the nucleus 133Sn, with one neutron outside the doubly-magic 132Sn core, were populated following one-neutron knockout from a 134Sn beam on a carbon target at relativistic energies at the Radioactive Isotope Beam Factory at RIKEN. Besides the rays emitted in the decay of the known neutron single-particle states in 133Sn additional strength in the energy range 3.5-5.5 MeV was observed for the fi rst time. Since the neutron-separation energy of 133Sn is low, Sn=2.402(4) MeV, this observation provides direct evidence for the radiative decay of neutron- unbound states in this nucleus. The ability of electromagnetic decay to compete successfully with neutron emission at energies as high as 3 MeV above threshold is attributed to a mismatch between the wave functions of the initial and final states in the latter case. These fi ndings suggest that in the region south-east of 132Sn nuclear structure effects may play a signifi cant role in the neutron vs. competition in the decay of unbound states. As a consequence, the common neglect of such effects in the evaluation of the neutron-emission probabilities in calculations of global b -decay properties for astrophysical simulations may have to be reconsidered.

The surrogate reaction method may be used to determine the cross section for neutron induced reactions not accessible through standard experimental techniques. This is achieved by creating the same compound nucleus as would be expected in the desired reaction, but through a different incident channel, generally a direct transfer reaction. So far, the surrogate technique has been applied with reasonable success to determine the fission cross section for a number of actinides, but has been less successful when applied to other reactions, e.g. neutron capture, due to a ‘spin-parity mismatch’. This mismatch, between the spin and parity distributions of the excited levels of the compound nucleus populated in the desired and surrogate channels, leads to differing decay probabilities and hence reduces the validity of using the surrogate method to infer the cross section in the desired channel. A greater theoretical understanding of the expected distribution of levels excited in both the desired and surrogate channels is therefore required in order to attempt to address this mismatch and allow the method to be utilised with greater confidence. Two neutron transfer reactions, e.g. (p,t), which allow the technique to be utilised for isotopes further removed from the line of stability, are the subject of this study. Results are presented for the calculated distribution of compound nucleus states populated in 90Zr, via the 90Zr(p,t)90Zr reaction, and are compared against measured data at an incident proton energy of 28.56 MeV.

The lifetimes of the first excited 2+ and 4+ states in Ni72 were measured at the National Superconducting Cyclotron Laboratory with the recoil-distance Doppler-shift method, a model-independent probe to obtain the reduced transition probability. Excited states in Ni72 were populated by the one-proton knockout reaction of an intermediate energy Cu73 beam. γ-ray-recoil coincidences were detected with the γ-ray tracking array GRETINA and the S800 spectrograph. Our results provide evidence of enhanced transition probability B(E2;2+→0+) as compared to Ni68, but do not confirm the trend of large B(E2) values reported in the neighboring isotope Ni70 obtained from Coulomb excitation measurement. The results are compared to shell model calculations. The lifetime obtained for the excited 4+1 state is consistent with models showing decay of a seniority ν=4, 4+ state, which is consistent with the disappearance of the 8+ isomer in Ni72.

Theoretical models of the (d, p) reaction are exploited for both nuclear astrophysics and spectroscopic studies in nuclear physics. Usually, these reaction models use local optical model potentials to describe the nucleon- and deuteron-target interactions. Within such a framework the importance of the deuteron D-state in low-energy reactions is normally associated with spin observables and tensor polarization effects - with very minimal influence on differential cross sections. In contrast, recent work that includes the inherent nonlocality of the nucleon optical model potentials in the Johnson-Tandy adiabatic-model description of the (d, p) transition amplitude, which accounts for deuteron break-up effects, shows sensitivity of the reaction to the large n-p relative momentum content of the deuteron wave function. The dominance of the deuteron D-state component at such high momenta leads to significant sensitivity of calculated (d, p) cross sections and deduced spectroscopic factors to the choice of deuteron wave function [Phys. Rev. Lett. 117, 162502 (2016)]. We present details of the Johnson-Tandy adiabatic model of the (d, p) transfer reaction generalized to include the deuteron D-state in the presence of nonlocal nucleon-target interactions. We present exact calculations in this model and compare these to approximate (leading-order) solutions. The latter, approximate solutions can be interpreted in terms of local optical potentials, but evaluated at a shifted value of the energy in the nucleon-target system. This energy shift is increased when including the D-state contribution. We also study the expected dependence of the D-state effects on the separation energy and orbital angular momentum of the transferred nucleon. Their influence on the spectroscopic information extracted from (d, p) reactions is quantified for a particular case of astrophysical significance.

The neutron-deficient nucleus Sn 107 has been studied by using the one-neutron knockout reaction. By measuring the decay γ rays and momentum distributions of reaction residues, the spins of the ground, 5/2 + , and first-excited, 7/2 + , states of Sn 107 have been assigned by comparisons to eikonal-model reaction calculations. Limits on the inclusive and exclusive cross sections have been measured and transitions due to neutron removals from below the N=50 closed shell have been observed. New excited states up to 5.5 MeV in Sn 107 have been identified.

Background: Thick-target-induced nucleon-adding transfer reactions onto energetic rare-isotope beams are an emerging spectroscopic tool. Their sensitivity to single-particle structure complements one-nucleon removal reaction capabilities in the quest to reveal the evolution of nuclear shell structure in very exotic nuclei. Purpose: Our purpose is to add intermediate-energy, carbon-target-induced one-proton pickup reactions to the arsenal of γ-ray-tagged direct reactions applicable in the regime of low beam intensities and to apply these for the first time to fp-shell nuclei. Methods: Inclusive and partial cross sections were measured for the 12C(48Cr,49Mn+γ)X and 12C(50Fe,51Co+γ)X proton pickup reactions at 56.7 and 61.2 MeV/nucleon, respectively, using coincident particle-γ spectroscopy at the National Superconducting Cyclotron Laboratory. The results are compared to reaction theory calculations using fp-shell-model nuclear structure input. For comparison with our previous work, the same reactions were measured on 9Be targets. Results: The measured partial cross sections confirm the specific population pattern predicted by theory, with pickup into high-ℓ orbitals being strongly favored, driven by linear and angular momentum matching. Conclusion: Carbon-target-induced pickup reactions are well suited, in the regime of modest beam intensity, to study the evolution of nuclear structure, with specific sensitivities that are well described by theory.

The low-lying structure of 55Sc has been investigated using in-beam ____gamma-ray spectroscopy with the 9Be(56Ti,55Sc+____gamma)X one-proton removal and 9Be(55Sc,55Sc+____gamma)X inelastic-scattering reactions at the RIKEN Radioactive Isotope Beam Factory. Transitions with energies of 572(4), 695(5), 1539(10), 1730(20), 1854(27), 2091(19), 2452(26), and 3241(39) keV are reported, and a level scheme has been constructed using ____gamma____gamma coincidence relationships and ____gamma-ray relative intensities. The results are compared to large-scale shell-model calculations in the sd-pf model space, which account for positive-parity states from proton-hole cross-shell excitations, and to {____it ab initio} shell-model calculations from the in-medium similarity renormalization group that includes three-nucleon forces explicitly. The results of proton-removal reaction theory with the eikonal model approach were adopted to aid identification of positive-parity states in the level scheme; experimental counterparts of theoretical 1/2+1 and 3/2+1 states are suggested from measured decay patterns. The energy of the first 3/2- state, which is sensitive to the neutron shell gap at the Fermi surface, was determined. The result indicates a rapid weakening of the N=34 subshell closure in pf-shell nuclei at Z>20, even when only a single proton occupies the ____pi f7/2 orbital.

Level schemes of the proton-rich nuclei, 47 Mn (Z = 25, N = 22) and 45 Cr (Z = 24, N = 21), have been established for the first time. The technique of mirrored one-and two-nucleon knockout reactions was applied to the secondary beams of 48 V/ 48 Mn and 47 V/ 47 Cr to populate states in 47 Ti/ 47 Mn and 45 Sc/ 45 Cr, respectively. Mirror energy differences (MED) have been studied between the mirrored T = 3 2 states for both mirror pairs, and interpreted using both a shell-model approach and a density-functional approach using the No-Core Configuration-Interaction (DFT-NCCI) method. MED in this mass region provide a stringent test of the model prescriptions since both fp-and sd-shell orbitals are active and, in 45 Cr, spherical and well-deformed structures co-exist near the ground-state. The inclusive and exclusive one-nucleon removal cross sections have been determined for the populated states in 47 Ti/ 47 Mn and compared with results from reaction-model calculations.

Fragmentation reactions with intermediate-energy heavy-ion beams exhibit a wide range of reaction mechanisms, ranging from direct reactions to statistical processes. We examine this transition by measuring the relative population of excited states in several sd-shell nuclei produced by fragmentation with the number of removed nucleons ranging from two to sixteen. The two-nucleon removal is consistent with a non-dissipative process, whereas the removal of more than five nucleons appears to be mainly statistical.

The nucleus $^{29}$Ne is situated at the border of the island of inversion. Despite significant efforts, no bound low-lying intruder $f_{7/2}$-state, which would place $^{29}$Ne firmly inside the island of inversion, has yet been observed. Here, the first investigation of unbound states of $^{29}$Ne is reported. The states were populated in $^{30}\mathrm{Ne}(p,pn)$ and $^{30}\mathrm{Na}(p,2p)$ reactions at a beam energy of around $230$ MeV/nucleon, and analyzed in terms of their resonance properties, partial cross sections and momentum distributions. The momentum distributions are compared to calculations using the eikonal, direct reaction model, allowing $\ell$-assignments for the observed states. The lowest-lying resonance at an excitation energy of 1.48(4) MeV shows clear signs of a significant $\ell$=3-component, giving first evidence for $f_{7/2}$ single particle strength in $^{29}$Ne. The excitation energies and strengths of the observed states are compared to shell-model calculations using the sdpf-u-mix interaction

The structure of P35 was studied with a one-proton knockout reaction at 88 MeV/u from a S36 projectile beam at NSCL. The γ rays from the depopulation of excited states in P35 were detected with GRETINA, while the P35 nuclei were identified event-by-event in the focal plane of the S800 spectrograph. The level scheme of P35 was deduced up to 7.5 MeV using γ−γ coincidences. The observed levels were attributed to proton removals from the sd shell and also from the deeply bound p1/2 orbital. The orbital angular momentum of each state was derived from the comparison between experimental and calculated shapes of individual (γ-gated) parallel momentum distributions. Despite the use of different reactions and their associate models, spectroscopic factors, C2S, derived from the S36(−1p) knockout reaction agree with those obtained earlier from S36(d,He3) transfer, if a reduction factor Rs, as deduced from inclusive one-nucleon removal cross sections, is applied to the knockout transitions. In addition to the expected proton-hole configurations, other states were observed with individual cross sections of the order of 0.5 mb. Based on their shifted parallel momentum distributions, their decay modes to negative parity states, their high excitation energy (around 4.7 MeV), and the fact that they were not observed in the (d,He3) reaction, we propose that they may result from a two-step mechanism or a nucleon-exchange reaction with subsequent neutron evaporation. Regardless of the mechanism, that could not yet be clarified, these states likely correspond to neutron core excitations in P35. This newly identified pathway, although weak, offers the possibility to selectively populate certain intruder configurations that are otherwise hard to produce and identify.

Many of the most exotic neutron-proton asymmetric nuclei are produced in relatively small numbers in high-energy fragmentation reactions. They are produced as fast secondary beams with energies of 100 MeV per nucleon or more. Developments made and recent results that both exploit and assess fast one- and two-nucleon removal reactions from such secondary beams are reviewed. This includes very recent work that interfaces the sudden, eikonal reaction models used with more ab-initio nuclear structure inputs. The potential use of neutron pick-up reactions to study particle-like states in exotic nuclei is also outlined.

Background: The nucleus 32Mg (N = 20 and Z = 12) plays a central role in the so-called "island of inversion," where in the ground states sd-shell neutrons are promoted to the fp-shell orbitals across the shell gap, resulting in the disappearance of the canonical neutron magic number N = 20. Purpose: The primary goals of this work are to extend the level scheme of 32Mg, provide spin-parity assignments to excited states, and discuss the microscopic structure of each state through comparisons with theoretical calculations. Method: In-beam gamma -ray spectroscopy of 32Mg was performed using two direct-reaction probes: one-neutron (two-proton) knockout reactions on 33Mg (34Si). Final-state exclusive cross sections and parallel momentum distributions were extracted from the experimental data and compared with eikonal-based reaction model calculations combined with shell-model overlap functions. Results: Owing to the remarkable selectivity of the one-neutron and two-proton knockout reactions, a significantly updated level scheme for 32Mg, which exhibits negative-parity intruder and positive-parity normal states, was constructed. The experimental results were confronted with four different nuclear structure models. Conclusions: In some of these models, different aspects of 32Mg and the transition into the island of inversion are well described. However, unexplained discrepancies remain, and, even with the help of these state-of-the-art theoretical approaches, the structure of this key nucleus is not yet fully captured.

Cross sections of one- and two-neutron removal reactions of 24O, leading to the 23O(½+) ground state and to bound final states of 22O, have been measured at the National Superconducting Cyclotron Laboratory. The experiment was conducted using the S800 spectrograph and a 24O beam energy of 92.3 MeV/u. The measured 23O ground state and 22O inclusive cross section values, of 74(11) mb and 146(33) mb, respectively, are in good agreement with calculations using eikonal reaction dynamics and shell-model nuclear structure overlaps. The widths at half maximum of the associated parallel momentum distributions of these cross sections, deduced from Gaussian fits, are 115(13) MeV/c for 23O and 309(36) MeV/c for 22O in the projectile rest frame. The data and calculations strongly support the shell-model description of 24O as a spherical, doubly-magic structure.

There are now several well-studied instances where very neutron-rich light nuclei at or near the neutron drip-line, such as 6He, 11Li and 14Be, have been found to have a Borromean three-body structure. Such systems are modelled effectively as a well-bound core nucleus plus two weakly-bound valence neutrons, where none of the two-body subsystems forms a bound state. It is now known that the heaviest particle-bound carbon isotope, 22C, shares these properties. We discuss a development of four-body reaction model calculations, using the fast adiabatic approximation, that is particularly well-suited for a quantitative analysis of reactions of such neutron-rich nuclei with a target nucleus at beam energies of order 100-300 MeV per nucleon; energies available at new and future radioactive ion beam (RIB) facilities. The 22C projectile wave function is calculated using the 20C core plus two-valence neutron three-body description.

Nucleon removal reactions have been shown to be an effective tool for studying the single particle structure of nuclei. This work continues efforts to experimentally probe and benchmark the reaction and structure models used to calculate the removal reaction cross sections when using microscopic nuclear structure inputs. Three different single nucleon removal reactions were performed, from p-shell nuclei with masses A = 7, 9, and 10.The residual nuclei from the reactions were detected in coincidence with γ rays to determine partial cross sections to individual final states. The eikonal direct-reaction model is combined with overlap functions and residual nucleus densities from microscopic, variational Monte Carlo calculations to provide consistent nuclear structure input to the partial cross section calculations. Comparisons of measured and calculated cross sections, including for mirror reactions, are presented. The analysis of the partial cross sections leading to the ground states shows a similar behavior to the one observed from analyses of inclusive cross sections using shell model nuclear structure input: the theoretical description of the removal process is in better agreement with the data when removing weakly bound nucleons, than when removing well-bound ones. The two mirror reaction pairs presented here show consistent results between the respective members of the pairs. The results obtained for the population of the excited states, however, show a systematically different trend that appears connected to the structure part of the calculation. Additional cases are needed to better understand the respective roles of structureand dynamical effects in the deviations.

A measurement of the direct two-proton removal from Si42 has provided the first structural information on the N=28 isotone Mg40. The value for the inclusive cross section for two-proton removal from Si42 of 40+27−17 μb is significantly lower than that predicted by structure calculations using the recent SDPF-MU shell-model effective interaction combined with eikonal reaction theory. This observed discrepancy is consistent with the interpretation that only one of the predicted low-lying 0+ states in Mg40 is bound. A two-state mixing analysis describing two-proton knockout cross sections along N=28 provides support for the interpretation of a prolate-deformed Mg40 ground state.

The nuclear structure dependence of direct reactions that remove a pair of like or unlike nucleons from a fast 12C projectile beam are considered. Specifically, we study the differences in the two-nucleon correlations present and the predicted removal cross sections when using p-shell shell-model and multi-ℏω no-core shell-model (NCSM) descriptions of the two-nucleon overlaps for the transitions to the mass A=10 projectile residues. The NCSM calculations use modern chiral two-nucleon and three-nucleon (NN+3N) interactions. The np-removal cross sections to low-lying T=0, 10B final states are enhanced when using the NCSM two-nucleon amplitudes. The calculated absolute and relative partial cross sections to the low-energy 10B final states show a significant sensitivity to the interactions used, suggesting that assessments of the overlap functions for these transitions and confirmations of their structure could be made using final-state-exclusive measurements of the np-removal cross sections and the associated momentum distributions of the forward traveling projectile-like residues.