Dr Jack Henderson
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Transition strengths for decays from low-lying states in A ∼ 70 nuclei have been deduced from lifetime measurements using the recoil distance Doppler shift technique. The results confirm the collectivity previously reported for the 21+→0gs+ decay in 68Se and reveal a relative decrease in collectivity in 70Br. This trend is reproduced by shell model calculations using the GXPF1A interaction in an fp model space including the Coulomb, spin-orbit and isospin non-conserving interactions. The 31+→21+ decay in 70Br is found to have a very small B(M1) value, which is consistent with the configuration of the state being dominated by the coupling of f52 protons and neutrons. The results suggest that the g92 orbit does not play an important role at low spin in these nuclei. The B(E2) values for the decays of the (T = 1) 21+ states in 70Br and 70Se are almost identical, suggesting there is no major shape change between the two nuclei at low spin.
The half-lives of isotopes around the \(N=82\) shell closure are an important ingredient in astrophysical simulations and strongly influence the magnitude of the second \(r\)-process abundance peak in the \(A\sim130\) region. The most neutron-rich \(N=82\) nuclei are not accessible to the current generation of radioactive beam facilities and \(r\)-process simulations must therefore rely on calculations of the half-lives of the isotopes involved. Half-life measurements of the experimentally accessible nuclei in this region are important in order to benchmark these calculations. The half-life of \(^{130}\)Cd is particularly important as it is used to tune the Gamow-Teller quenching in shell-model calculations for the \(\beta\) decay of other nuclei in this region. In this work, the GRIFFIN \(\gamma\)-ray spectrometer at the TRIUMF-ISAC facility was used to measure the half-life of \(^{130}_{~48}\)Cd\(_{82}\) to be \(T_{1/2}= 126(4)\) ms. In addition, the half-lives of the three \(\beta\) decaying states of \(^{131}_{~49}\)In\(_{82}\) were measured to be \(T_{1/2}(1/2^-)=328(15)\) ms, \(T_{1/2}(9/2^+)=265(8)\) ms, and \(T_{1/2}(21/2^+)=323(50)\) ms, respectively, providing an important benchmark for half-life calculations in this region.
The Tz = -$\frac{3}{2}$ nucleus 21Mg has been studied by Coulomb excitation on 196Pt and 110
With the recent emergence of fast nuclear reactors, there has been a corresponding increasing interest in 238U-related nuclear data. However, while existing literature data span much of the energy ranges of interest for the prompt fission neutron spectrum (PFNS) for neutron-induced fission of 238U, most literature data sets are highly correlated, and thus new, independent measurements of this quantity are needed. In this work, we report the results of a new measurement of the 238U PFNS at the Los Alamos Neutron Science Center for incident neutron energies from 1.5–20.0 MeV, and outgoing neutron energies of 0.01–10.0 MeV. With some notable exceptions, the present results generally agree with existing literature data, especially with regard to features relating to multichance fission and pre-equilibrium features in the PFNS, thus adding confidence to existing nuclear data evaluations and filling in gaps of knowledge at previously unmeasured incident neutron energies. This result is the third in a series of PFNS measurements by the Chi-Nu collaboration now spanning all three major actinides, 239Pu, 235U, and 238U. Thus, for the first time, we report reliable experimental PFNS ratios and average PFNS energy comparisons for measurements of all three of these isotopes including accurate correlations between the different, but correlated experiments.
A high precision lifetime measurement of the 2(1)(+) state in 94Sr was performed at TRIUMF's ISAC- II facility by coupling the recoil distance method implemented via the TIGRESS integrated plunger with unsafe Coulomb excitation in inverse kinematics. Due to limited statistics imposed by the use of a radioactive 94Sr beam, a likelihood ratio chi(2) method was derived and used to compare experimental data to Geant4 simulations. The B(E2; 2(1)(+)-> 0(1)(+)) value extracted from the lifetime measurement of 7.80(- 0.40)(+0.50)(stat.)+/- 0.07(sys.) ps is approximately 25% larger than previously reported while the relative error has been reduced by a factor of approximately 8. A baseline deformation has been established for Sr isotopes with N
Background: Two-photon emission, while well known in atomic physics, is a rare second-order process in nuclear physics with only three cases where a two-photon branch is measured. The limited knowledge stems from the experimental difficulty in resolving two-photon emission from dominant single-photon emission, restricting practical cases for study to 0(+) -> 0(+) (E0) transitions, since single-photon emission is forbidden. In practical terms, this limits the range of easily accessible cases to even-even nuclei with the unusual property of a first excited state with spin/parity of 0+. Purpose: Two-photon branches are measured for the closed-shell nuclei, O-16, Ca-40, and Zr-90. The intention of the present work was to obtain data for a case which was not a closed-shell nucleus. Of the possible nuclei relevant to such a study, Mo-98 was chosen as its first-excited state is 0(+) and lies below 1 MeV, meaning that internal pair transitions are not allowed. Method: The first excited state (J(pi) = 0(+)) in Mo-98 was excited in resonant inelastic proton scattering using a 6.7-MeV proton beam. The population of the state was selected using an annular double-sided silicon strip detector (DSSD). The decay of the state by conversion electrons was observed using the same DSSD, while Gammasphere was used to detect possible two-photon events. Results: An upper limit on the two-photon branch obtained was 1 x 10(-4) at the 95% confidence level (CL). Conclusions The upper limit obtained is smaller than any other previously obtained two-photon branch. Phase space considerations suggest that the actual value of the branching ratio in this case may be significantly smaller than the upper limit obtained.
Fine structure in the a decay of high-spin isomers in Lu-155( 25/2(-)) and Hf-156(8(+))has been studied for the first time using alpha gamma- coincidence analysis. Three new a decays from Lu-155(25/2(-)) and two from Hf-156(8(+)) have been identified, populating seniority s > 1 states in the N = 82 nuclei Tm-151 and Yb-152, respectively. The reduced hindrance factors of the a decays support the previous configuration assignments of the populated states. This is the first observation of states with excitation energy greater than 1.5 MeV being populated following a decay in nuclei outside of the Pb-208 region.
An extensive, model-independent analysis of the nature of triaxial deformation in Ge-76, a candidate for neutrinoless double-beta (0 nu beta beta) decay, was carried out following multistep Coulomb excitation. Shape parameters deduced on the basis of a rotational-invariant sum-rule analysis provided considerable insight into the underlying collectivity of the ground-state and gamma bands. Both sequences were determined to be characterized by the same beta and gamma deformation parameter values. In addition, compelling evidence for low-spin, rigid triaxial deformation in Ge-76 was obtained for the first time from the analysis of the statistical fluctuations of the quadrupole asymmetry deduced from the measured E2 matrix elements. These newly determined shape parameters are important input and constraints for calculations aimed at providing, with suitable accuracy, the nuclear matrix elements relevant to 0 nu beta beta.
The TRIUMF Fast Ion Counter (TRIFIC) is a tilted-grid gas counter detector for identifying the impurities in radioactive ion beams (RIBs). Placed at zero degrees downstream of a primary reaction target at the TRIUMF ISAC-II facility, TRIFIC provides accurate Z identification of beam species on an event-by-event basis at a rate of up to 5×105 particles per second and functions as a beam composition monitor at rates up to 5×107 particles per second. With this additional level of diagnostics, RIB experiments at TRIUMF are able to reach levels of sensitivity not previously possible. Details regarding the construction and instrumentation of TRIFIC as well as results from the commissioning of the detector are presented.
The low-lying structure of semi-magic $^{118}$Sn has been investigated through the $\beta$-decay of $^{118}$In ($T_{1/2}=4.45$ min) to study shape coexistence via the reduced transition probabilities of states in the 2p-2h proton intruder band. This high-statistics study was carried out at TRIUMF-ISAC with the GRIFFIN spectrometer. In total, 99 transitions have been placed in the level scheme with 43 being newly observed. Three low-lying $\gamma$-ray transitions with energies near 285 keV have been resolved from which the 2$^+_{\mathrm{intr.}} \rightarrow 0^+_{\mathrm{intr.}}$ 284.52-keV transition was determined to have half of the previous branching fraction leading to a $B(E2;2^+_2\rightarrow 0^+_2)$ of 21(4) W.u. compared to 39(7) W.u. from the previous measurement. Calculations using $sd$ IBM-2 with mixing have also been made to compare the experimental $B(E2)$ values to the theoretical values and to make comparisons to the $^{114,116}$Sn isotopes previously studied using the same theoretical model.
Proton inelastic scattering off a neutron halo nucleus, (11) Li, has been studied in inverse kinematics at the IRIS facility at TRIUMF. The aim was to establish a soft dipole resonance and to obtain its dipole strength. Using a high quality 66 MeV (11) Li beam, a strongly populated excited state in (11) Li was observed at E-x= 0.80 +/- 0.02 MeV with a width of r = 1.15 +/- 0.06 MeV. A DWBA (distorted-wave Born approximation) analysis of the measured differential cross section with isoscalar macroscopic form factors leads us to conclude that this observed state is excited in an electric dipole (El) transition. Under the assumption of isoscalar El transitions, the strength is evaluated to be extremely large amounting to 30 similar to 296 Weisskopf units, exhausting 2.2% similar to 21% of the isoscalar El energy-weighted sum rule (EWSR) value. The large observed strength originates from the halo and is consistent with the simple di-neutron model of (11) Li halo. (c) 2017 The Authors. Published by Elsevier B.V.
In this Letter, the observation of two previously unknown isotopes is presented for the first time: Rb-72 with 14 observed events and Zr-77 with one observed event. From the nonobservation of the less proton-rich nucleus Rb-73, we derive an upper limit for the ground-state half-life of 81 ns, consistent with the previous upper limit of 30 ns. For Rb-72, we have measured a half-life of 103(22) ns. This observation of a relatively long-lived odd-odd nucleus, Rb-72, with a less exotic odd-even neighbor, Rb-73, being unbound shows the diffuseness of the proton drip line and the possibility of sandbanks to exist beyond it. The Rb-72 half-life is consistent with a 5(+) -> 5/2(-) proton decay with an energy of 800-900 keV, in agreement with the atomic mass evaluation proton-separation energy as well as results from the finite-range droplet model and shell model calculations using the GXPF1A interaction. However, we cannot explicitly exclude the possibility of a proton transition between 9(+)(Rb-72) -> 9/2(+)(Kr-71) isomeric states with a broken mirror symmetry. These results imply that Kr-72 is a strong waiting point in x-ray burst rp-process scenarios.
The low-spin structure of the semimagic ^{64}Ni nucleus has been considerably expanded: combining four experiments, several 0^{+} and 2^{+} excited states were identified below 4.5 MeV, and their properties established. The Monte Carlo shell model accounts for the results and unveils an unexpectedly complex landscape of coexisting shapes: a prolate 0^{+} excitation is located at a surprisingly high energy (3463 keV), with a collective 2^{+} state 286 keV above it, the first such observation in Ni isotopes. The evolution in excitation energy of the prolate minimum across the neutron N=40 subshell gap highlights the impact of the monopole interaction and its variation in strength with N.
Lifetimes of low-lying excited states have been measured in Ge-66, Se-69, and Ga-65 using a gamma-ray lineshape method. The results confirm the previously reported 7(1)(-) state lifetime in Ge-66. The lifetime of the yrast 5/2(-) state in Ga-65 is measured for the first time. Lifetime measurements of two excited 3/2(-) states in Se-69 are also reported. Two previously unobserved. rays have been identified in Se-69. gamma - gamma coincidence measurements have been used to place one of these in the level scheme gamma Se-69 excited state populations are compared to shell-model calculations using the GXPF1A interaction in the fp model space. Theoretical spectroscopic factors to excited states in Se-69 have identified three candidate levels for the origin of one of the new transitions.
The island of inversion for neutron-rich nuclei in the vicinity of N=20 has become the testing ground par excellence for our understanding and modeling of shell evolution with isospin. In this context, the structure of the transitional nucleus ²⁹Mg is critical. The first quantitative measurements of the single-particle structure of ²⁹Mg are reported, using data from the d(²⁸Mg, p γ)²⁹Mg reaction. Two key states carrying significant ℓ=3 (f-wave) strength were identified at 2.40±0.10 (Jπ=5/2¯) and 4.28±0.04 MeV (7/2¯). New state-of-the-art shell-model calculations have been performed and the predictions are compared in detail with the experimental results. While the two lowest 7/2¯ levels are well described, the sharing of single-particle strength disagrees with experiment for both the 3/2¯ and 5/2¯ levels and there appear to be general problems with configurations involving the p3/2 neutron orbital and core-excited components. These conclusions are supported by an analysis of the neutron occupancies in the shell-model calculations.
An in-beam study has been performed to further investigate the known isomeric decays and to identify T = 1 excited states in the medium-heavy N = Z = 33 nucleus As-66. The fusion-evaporation reaction Ca-40(Si-28,pn)As-66 was employed at beam energies of 75 and 83 MeV. The half-lives and ordering of two known isomeric states in As-66 have been determined with improved accuracy. In addition, several prompt gamma-ray transitions from excited states, both bypassing and decaying to the isomeric states in As-66, have been observed. Most importantly, candidates for the 4(+) -> 2(+) and 6(+) -> 4(+) transitions in the T = 1 band have been identified. The results are compared with shell-model calculations using the modern JUN45 interaction in the pf(5/2)g(9/2) model space.
The recoil-β tagging technique was used to identify transitions associated with the decay of the 2 + and, tentatively, the 4 + excited states in 74 Sr. Combining these results with published data for the A = 74 isobars, triplet energy differences (TEDs) have been extracted, the heaviest case for which these values have been evaluated. State-of-the-art shell-model calculations using the JUN45 interaction and incorporating a J = 0 isospin nonconserving (INC) interaction with an isotensor strength of 100 keV can reproduce the trend in the TED data, with particularly good agreement for the 2 + state. This agreement for the TED data taken together with the fact that agreement has also been shown between shell-model calculations with the same strength of INC interaction in the f 7/2 shell and recently for A = 66 strongly suggests that such an interaction exists throughout the nuclear chart and cannot have a strong dependence on details of nuclear structure such as which nuclear orbitals are occupied. It also supports the hypothesis that only a J = 0 component of the INC interaction need be included to explain the observed TEDs.
The deformation properties of the low-lying states in 76Ge have been investigated following a safe-energy Coulomb excitation measurement with the GRETINA tracking array and CHICO2 heavy-ion counter at the ATLAS accelerator facility at Argonne National Laboratory. A comprehensive set of transition and static E2 matrix elements were extracted from the measured differential Coulomb cross sections and compared with results of configuration-interaction shell-model calculations and computations carried out within the framework of the generalized triaxial rotor model. The remarkable agreement between the calculated and experimental data supports a near-maximum triaxial deformation for the ground state of 76Ge. In addition, the degree of softness of the asymmetry in 76Ge and 76Se was investigated using rotational invariants generated from configuration-interaction shell-model wave functions computed with the jj44b and JUN45 effective interactions. The resulting invariants are shown to be consistent with a stiff triaxial deformation in 76Ge and a predominantly soft triaxial potential for 76Se, in agreement with the conclusions of recent works by this collaboration.
Background: Detailed spectroscopy of neutron-rich, heavy, deformed nuclei is of broad interest for nuclear astrophysics and nuclear structure. Nuclei in the r-process path and following freeze-out region impact the resulting r-process abundance distribution, and the structure of nuclei midshell in both proton and neutron number helps to understand the evolution of subshell gaps and large deformation in these nuclei. Purpose: We aim to improve the understanding of the nuclear structure of Gd160, specifically the Kπ=4+ bands, as well as study the β decay of Eu160 into Gd160. Methods: High-statistics decay spectroscopy of Gd160 resulting from the β-decay of Eu160 was collected using the GRIFFIN spectrometer at the TRIUMF-ISAC facility. Results: Two new excited states and ten new transitions were observed in Gd160. The β-decaying half-lives of the low- and high-spin isomers in Eu160 were determined, and the low-spin state's half-life was measured to be t1/2=26.0(8) s, ≈16% shorter than previous measurements. Lifetimes of the two Kπ=4+ bandheads in Gd160 were measured for the first time, as well as γ-γ angular correlations and mixing ratios of intense transitions out of those bandheads. Conclusions: Lifetimes and mixing ratios suggest that the hexadecapole phonon model of the Kπ=4+ bandheads in Gd160 is preferred over a simple two-state strong mixing scenario, although further theoretical calculations are needed to fully understand these states. Additionally, the 1999.0-keV state in Gd160 heavily populated in β decay is shown to have positive parity, which raises questions regarding the structure of the high-spin β-decaying state in Eu160.
We report the current results of a large effort to accurately measure the Prompt Fission Neutron Spectra (PFNS) for neutron-induced fission of ²³⁵U and ²³⁹Pu for incident neutrons with energies from 1 to 20 MeV. The Chi-Nu experiment at the Los Alamos Neutron Science Center used an unmoderated, white spectrum of neutrons to induce fission in actinide samples that were placed inside a parallel plate avalanche counter to provide a fast fission signal. A double time-of-flight technique was used to determine the incoming and outgoing neutron energies. Two neutron detector arrays, one with 54 liquid scintillators and another with 22 lithium glass detectors, were used to detect the outgoing neutrons and measure the PFNS distributions over a wide range in outgoing neutron energy, from below 100 keV to 10 MeV. Extensive Monte Carlo modeling was used to understand the experiment response and extract the PFNS. Systematic errors and uncertainties in the method have been examined and quantified. A summary of these results for incoming energies from 1 to 5 MeV is presented here.
Spectroscopic data, such as precise γ-ray branching and E2/M1 multipole-mixing ratios, provide vital constraints when performing multi-dimensional Coulomb-excitation analyses. Consequently, as part of our new Coulomb-excitation campaign aimed at investigating the role of exotic non-axial (triaxial) deformations in the unstable refractory Ru-Mo isotopes, additional beta-decay data was obtained. These measurements make use of ANL's CARIBU facility, which provides intense beams of radioactive refractory isotopes along with the excellent efficiency and angular resolution of the GRETINA γ-ray tracking array. In this article, we report on the analysis of the A = 110 decay chain, focussing on the identification of previously unreported states in 110Ru following the decay of 110Tc.
Although the prompt fission neutron spectrum (PFNS) is an essential component of neutron-driven systems that has been measured for decades, there are still multiple glaring unknowns regarding the PFNS of major actinides in the fission neutron incident energy range, specifically with regard to multichance fission and preequilibrium neutron emission processes. The only impactful experimental Pu-239 PFNS measurements included in recent nuclear data evaluations were measured over a limited outgoing neutron energy range at thermal and 1.5-MeV average incident neutron energy, while other potentially impactful measurements have been shown to contain errors that resulted in either large uncertainty increases or in complete exclusion from nuclear data evaluation. We report here a measurement of the Pu-239 PFNS over a wide range of incident neutron energy (1-20 MeV) and three orders of magnitude in outgoing neutron energy (0.01-10 MeV) resulting from the Chi-Nu experiment at the Los Alamos Neutron Science Center. These results are the combination of separate PFNS measurements in the same experimental area, one using a Li-glass and the other a liquid scintillator detector array. Covariances between all PFNS data points from each detector and within each incident energy range were generated between all other data in both detector arrays and within all other incident neutron energy bins, yielding a single covariance matrix for all 1300 PFNS data points reported here. These covariances are based on a thorough assessment of systematic bias and uncertainties associated with the measurement, PFNS extraction technique, combination of data from each detector type, and other aspects of the analysis. The existence of covariances between PFNS data points in different incident neutron energy ranges yielded covariances between average PFNS energy values at each incident energy to be reported here as well, which allowed for firm statements to be made regarding a shape of a purely experimental mean PFNS energy trend for the first time. Although minor PFNS shape differences exist between the results reported here and recent nuclear data evaluations, the ENDF/B-VIII.0 and JEFF-3.3 PFNS evaluations agree reasonably well with the present results from 1-to 10-MeV incident neutron energy, which spans the well-measured 1.5-MeV incident neutron energy PFNS from Lestone and Shores as well as the onset of second-chance fission. However, while the pre-equilibrium component of the PFNS above 12-MeV incident neutron energy roughly agrees in position and magnitude with ENDF/B-VIIL0 and JEFF-3.3, clear differences relating to the relative magnitude of third-chance fission PFNS features are present in the PFNS shape and in the mean PFNS energy trends.
The level structure of Na-25 and Ne-22 in the high excitation energy/high spin regime was studied using gamma-ray spectroscopy and Doppler shift lifetime measurements following C-12(O-18 - alpha p)Na-25 and C-12(O-18, 2 alpha)Ne-22 reactions. Multiple new levels and transitions were identified in both nuclides based on y-y coincidence data. In Na-25, evidence was found for higher-spin negative-parity states up to I-pi = 13/2(-) resulting from neutron excitation into the pf shell, in good agreement with shell model calculations using the SDPF-MU and FSU interactions. Candidates for the yrast 7(+) and 8(+) levels in Ne-22, consistent with calculations using the USDB interaction, were also identified.
A recent study by Hoff and collaborators [Nature (London) 580, 52 (2020)] presented evidence that the ground-state spin of [Math Processing Error] is different from that of its mirror, [Math Processing Error], likely due to an inversion of the ground- and first-excited states. Here, we assess the likelihood of such an inversion arising from normal isospin-symmetry breaking and, more broadly, whether such phenomena challenge our understanding of charge-symmetry-breaking forces in atomic nuclei. By placing the result within the context of previous experimental and theoretical work we demonstrate that this inversion lies entirely within the bounds of normal isospin-symmetry-breaking behavior. We further note that, in the context of isospin, neither level inversions nor the nuclear ground state hold any special significance.
The beta decay of neutron-rich In-129 into Sn-129 was studied using the GRIFFIN spectrometer at the ISAC facility at TRIUMF. The study observed the half-lives of the ground state and each of the beta-decaying isomers. The level scheme of Sn-129 has been expanded with thirty-one new gamma-ray transitions and nine new excited levels, leading to a reevaluation of the beta branching ratios and level spin assignments. The observation of the beta decay of the (29/2(+)) 1911-keV isomeric state in In-129 is reported for the first time, with a branching ratio of 2.0(5)%.
The TIGRESS Integrated Plunger device (TIP) has been developed for recoil distance method (RDM) lifetime measurements using the TIGRESS array of HPGe γ-ray detectors at TRIUMF’s ISAC-II facility. A commissioning experiment was conducted utilizing a 250 MeV 84Kr beam at ≈2×108 particles per second. The 84Kr beam was Coulomb excited to the 21+ state on a movable 27Al target. A thin Cu foil fixed downstream from the target was used as a degrader. Excited nuclei emerged from the target and decayed by γ-ray emission at a distance determined by their velocity and the lifetime of the 21+ state. The ratio of decays which occur between the target and degrader to those occurring after traversing the degrader changes as a function of the target–degrader separation distance. Gamma-ray spectra at 13 target–degrader separation distances were measured and compared to simulated lineshapes to extract the lifetime. The result of τ=5.541±0.013(stat.)±0.063(sys.) ps is shorter than the literature value of 5.84±0.18 ps with a reduction in uncertainty by a factor of approximately two. The TIP plunger device, experimental technique, analysis tools, and result are discussed.
The neutron -rich strontium, zirconium, and molybdenum nuclei have been observed to undergo a dramatic evolution, becoming strongly deformed around N = 60, sometimes interpreted as a quantum phase transition between "normal" and intruder configurations. Key to understanding this evolution is to understand the configurations in isolation, in regions where interference can be neglected. A deformed coexisting configuration is inferred from the presence of a 0 2 state which decreases in excitation energy with increasing neutron number, becoming the first -excited state at 98Mo. We present here the results of a low -energy Coulomb -excitation measurement of the nucleus 96Mo, extracting B(E2) values and quadrupole moments. It is found that, while the B(E2) values agree with those found in the literature, there is a significant disagreement with literature spectroscopic quadrupole moments. The results are compared with shell -model calculations using a 88Sr core with good agreement found, likely indicating that intruder structures do not significantly impact the ground -state structure, in contrast with the heavier molybdenum isotopes.
Across the physics disciplines, the Pb-186 nucleus is the only known system, where the two first excited states, together with the ground state, form a triplet of zero-spin states assigned with prolate, oblate and spherical shapes. Here we report on a precision measurement where the properties of collective transitions in Pb-186 were determined in a simultaneous in-beam gamma-ray and electron spectroscopy experiment employing the recoil-decay tagging technique. The feeding of the 0(2)(+) state and the interband 2(2)(+) -> 2(1)(+) transition have been observed. We also present direct measurement of the energies of the electric monopole transitions from the excited 0(+) states to the 0(+) ground state. In contrast to the earlier understanding, the obtained reduced transition probability B(E2; 2(1)(+) -> 0(2)(+)) value of 190(80) W.u., the transitional quadrupole moment vertical bar Q(t)(2(1)(+) -> 0(2)(+))vertical bar = 7.7(33) eb and intensity balance arguments provide evidence to reassign the 0(2)(+) and 0(3)(+) states with predominantly prolate and oblate shape, respectively. Our work demonstrates a step-up in experimental sensitivity and paves the way for systematic studies of electric monopole transitions in this region. These electric monopole transitions probe the nuclear volume in a unique manner and provide unexploited input for development of the next-generation energy density functional models.
This paper reports on the performance of the inorganic scintillator caesium hafnium chloride (CHC) under exposure to the mixed radiation field of an AmBe neutron source and coupled to a silicon photomultiplier (SiPM). The neutron response is determined using the pulse shape discrimination charge comparison technique which can clearly identify both the (n,α) and (c) reactions in the material. Figures of merit for the pulse shape discrimination are presented and the quenching of the different channels is assessed through comparison to Monte Carlo simulations.
Background: Nuclei exhibit both single-particle and collective degrees of freedom, with the latter often subdivided into vibrational and rotational motions. Experimentally identifying the relative roles of these collective modes is extremely challenging, particularly in the face of possible shape coexistence. Model-independent, invariant quantities describing the deformation of a nucleus in the intrinsic frame have long been known but their determination potentially requires a large quantity of experimental data to achieve convergence. Purpose: Through comparison with the nuclear shell model, the question of convergence is addressed. Methods: Shell-model calculations performed in the sd- and pf -shell model spaces are used to determine electric-quadrupole matrix elements for a multitude of low-lying states using the first 40 states of the relevant spins. Relative contributions to the rotationally invariant quantities from multiple states can therefore be determined. Results: It is found that, on average, the inclusion of four intermediate states results in the leading-order invariant, , converging to within 10% of its true value and the triaxiality term, cos (38), converging to its true value, though some variance remains. Higher-order quantities relating to the softness of the nuclear shape are found to converge more slowly. Conclusions: The convergence of quadrupole rotationally invariant sum rules was quantified in the sd- and pf-shell model spaces and indicates the challenge inherent in a full determination of nuclear shape. The present study is limited to relatively small valence spaces. Larger spaces, such as the rare-earth region, potentially offer faster convergence.
The extremely neutron-deficient isotope Au179 has been studied by a combination of in-beam γ-ray and isomeric-decay spectroscopy. For in-beam spectroscopy, the recoil-isomer tagging technique was employed, using the known 3/2-, T1/2=328 ns isomer. A new rotational band, associated with the unfavored signature band of the 1h9/2?2f7/2 proton-intruder configuration, was revealed. A previously unknown, high-spin isomeric state with an excitation energy of 1743(17) keV and T1/2=2.16(8)μs was discovered. Five decay paths were identified, some of them feeding previously unknown non-yrast excited states, associated with the 1i13/2 proton-intruder configuration. Calculations based on the particle-plus-triaxial-rotor model were performed to interpret the data. On the basis of these calculations, the new 1h9/2?2f7/2 rotational band is interpreted as due to triaxial deformation of the underlying configuration with β2≈0.26 and γ≈27?. Observed non-yrast states of the positive-parity 1i13/2 intruder configuration are interpreted as due to triaxial deformation with β2≈0.26 and γ≈20?. © 2022 American Physical Society.
The response of LaBr3(Ce) and LaCl3(Ce) scintillators to fast neutrons is investigated. Neutron-induced charged-particle reactions are observed in both materials when exposed to the fast neutrons produced by an AmBe source, with pulse-shape discrimination used to separate channels. LaBr3(Ce) is found to have the best separation between reaction channels, while LaCl3(Ce) has a significantly higher efficiency.
The neutron-rich barium nuclei have been the subject of intense interest due to the enhanced octupole correlations they are predicted to exhibit. The observation of enhanced octupole collectivity in Ba-144,Ba-146 as measured in sub-barrier Coulomb excitation, consistent with static octupole deformation, has further heightened this interest. In the present work, these studies are extended to the neighboring odd-mass Ba-143 to investigate the interplay between single-particle and collective octupole degrees of freedom. A new measurement of the first 9(-)/2-state lifetime is also presented. Reflection-Asymmetric Triaxial Particle Rotor Model calculations indicate that the negative-parity bands in Ba-143 can be understood as a decoupled structure of nu h(9/2) parentage, while the positive-parity bands are built on a decoupled octupole phonon. No evidence for E3 excitation is observed in this work, but an upper limit is placed on the E3 matrix element to the lowest octupole band.
Prompt fission neutron spectrum (PFNS) evaluations use provide nuclear data for the PFNS across a wide range of incident and outgoing neutron energies. However, experimental data underlying the evaluation are sparse, inconsistent, and incomplete with respect to the desired incident and outgoing energy coverage. As such, evaluations sometimes predict features of the PFNS, such those relating to multi-chance fission and pre-equilibrium pre-fission neutron emission, without any experimental validation. The Chi-Nu experiment at Los Alamos National Laboratory has recently obtained high-precision results for the ²³⁹Pu and ²³⁵U PFNS which, for the first time in both cases, have shed light on multi-chance fission and pre-equilibrium contributions to the observed fission neutron spectrum. In addition to providing the first experimental data on some of these fission properties, the angular coverage of the Chi-Nu experiment allows for the extraction of angular distributions of pre-equilibrium pre-fission neutrons. PFNS results of multi-chance fission and pre-equilibrium pre-fission neutron emission are discussed in this proceedings in terms of the observed neutron spectrum and the average PFNS energies.
Spectroscopy of doubly magic Sn-132(50)82 has been performed with the GRIFFIN spectrometer at TRIUMF-ISAC following the beta decay of In-132(49)83. The analysis has allowed for the placement of a total of 70 transitions and 29 excited states in Sn-132. Detailed spectroscopy has also been performed on Sb-131, resulting from the beta decay of Sn-131, produced from the beta-delayed neutron decay of In-132. Measurement of gamma rays in both Sn-131 and Sb-131 has led to the determination of the beta-delayed neutron emission probability P-n from In-132. This is the first time that P-n has been measured for this nucleus by using y spectroscopy, and the new value of 12.3(4)% is consistent with the most recent beta-n counting experiment. Additionally, y-y angular correlations have been performed in Sn-132, supporting the spin assignments of several excited states. Novel ab initio calculations are presented which describe several of the excited states, and these are compared with the experimental spectrum.
The extremely neutron-deficient isotopes Au-177,Au-179 were studied by means of in-beam gamma-ray spectroscopy. Specific tagging techniques, alpha-decay tagging in Au-177 and isomer tagging in Au-179, were used for these studies. Feeding of positive-parity, nearly spherical states, which are associated with 2d(3/2) and 3s(1/2) proton-hole configurations, from the 1i(13/2) proton-intruder configuration was observed in Au-177. Such a decay path has no precedent in odd-Au isotopes and it is explained by the effect of mixing of wave functions of the initial state. (C) 2020 Published by Elsevier B.V.
The characteristics of 126, 128Xe were investigated in Coulomb excitation measurements performed at the National Superconducting Cyclotron Laboratory (NSCL) Re-accelerator facility, ReA3, Michigan State University (MSU). The Xe nuclei were accelerated to sub-barrier energies and were impinged on 196Pt and 208Pb targets in separate experimental runs. The scattered nuclei and the de-excitation γ-rays were detected using the JANUS setup. Electromagnetic matrix elements were extracted from the experimental data with the help of the GOSIA/GOSIA2 codes. The results were compared to schematic Davydov-Filippov γ-rigid rotor theoretical calculations and large-scale calculations within a newly-established microscopic shell model (called PMMU model). The experimental results agree well with the theoretical predictions, except for the quadrupole moments of the second 2+ states in both nuclei, therefore challenging the interpretation of the γ-bands structure.
The low-lying structure of semimagic Sn-118 has been investigated through the beta decay of In-118 (T-1/2 = 4.45 min) to study shape coexistence via the reduced transition probabilities of states in the 2p-2h proton intruder band. This high-statistics study was carried out at TRIUMF-ISAC with the GRIFFIN spectrometer. In total, 99 transitions have been placed in the level scheme with 43 being newly observed. Three low-lying gamma-ray transitions with energies near 285 keV have been resolved from which the 2(intr.)(+) -> 0(intr.)(+) 284.52-keV transition was determined to have half of the previous branching fraction leading to a B (E2; 2(2)(+)-> 0(2)(+)) of 21(4) W.u. compared to 39(7) W.u. from the previous measurement. Calculations using sd IBM-2 with mixing have also been made to compare the experimental B(E2) values to the theoretical values and to make comparisons to the Sn-114,Sn-116 isotopes previously studied using the same theoretical model.
Background: Detailed spectroscopy of neutron-rich, heavy, deformed nuclei is of broad interest for nuclear astrophysics and nuclear structure. Nuclei in the r-process path and following freeze-out region impact the resulting r-process abundance distribution, and the structure of nuclei midshell in both proton and neutron number helps to understand the evolution of subshell gaps and large deformation in these nuclei. Purpose: We aim to improve the understanding of the nuclear structure of 160Gd, specifically the Kπ = 4+ bands, as well as study the β decay of 160Eu into 160Gd. Methods: High-statistics decay spectroscopy of 160Gd resulting from the β-decay of 160Eu was collected using the GRIFFIN spectrometer at the TRIUMF-ISAC facility. Results: Two new excited states and ten new transitions were observed in 160Gd. The β-decaying half-lives of the low- and high-spin isomers in 160Eu were determined, and the low-spin state’s half-life was measured to be t1/2 = 26.0(8) s, ≈16% shorter than previous measurements. Lifetimes of the two Kπ = 4+ bandheads in 160Gd were measured for the first time, as well as γ -γ angular correlations and mixing ratios of intense transitions out of those bandheads. Conclusions: Lifetimes and mixing ratios suggest that the hexadecapole phonon model of the Kπ = 4+ bandheads in 160Gd is preferred over a simple two-state strong mixing scenario, although further theoretical calculations are needed to fully understand these states. Additionally, the 1999.0-keV state in 160Gd heavily populated in β decay is shown to have positive parity, which raises questions regarding the structure of the high-spin β-decaying state in 160Eu.
High-spin states in the N = 128 nucleus 218Th have been investigated following fusion–evaporation reactions, using the recoil-decay tagging technique. Due to the short-lived nature of the ground state of 218Th prompt γ rays have been correlated with the α decay of the daughter nucleus 214Ra. The level scheme representing the decay of excited states has been extended to (16+) with the observation of six previously unreported transitions. The observations are compared with the results of shell model calculations and within the context of the systematics of neighbouring nuclei.
Prompt fission neutron spectrum (PFNS) evaluations use provide nuclear data for the PFNS across a wide range of incident and outgoing neutron energies. However, experimental data underlying the evaluation are sparse, inconsistent, and incomplete with respect to the desired incident and outgoing energy coverage. As such, evaluations sometimes predict features of the PFNS, such those relating to multi-chance fission and pre-equilibrium pre-fission neutron emission, without any experimental validation. The Chi-Nu experiment at Los Alamos National Laboratory has recently obtained high-precision results for the ²³⁹Pu and ²³⁵U PFNS which, for the first time in both cases, have shed light on multi-chance fission and pre-equilibrium contributions to the observed fission neutron spectrum. In addition to providing the first experimental data on some of these fission properties, the angular coverage of the Chi-Nu experiment allows for the extraction of angular distributions of pre-equilibrium pre-fission neutrons. PFNS results of multi-chance fission and pre-equilibrium pre-fission neutron emission are discussed in this proceedings in terms of the observed neutron spectrum and the average PFNS energies.
The Chi-Nu project to measure prompt fission neutron energy spectra for the major actinides has now completed two measurements, for neutron-induced fission of 239Pu and 235U for incident neutron energies from 1 to 20 MeV, and has almost completed the prompt fission neutron spectrum measurement for 238U. In addition, similar data have been taken for spontaneous fission in some other isotopes, and a measurement of fission neutron spectra from 240Pu(n, f ) is in progress. These measurements are done at the same facility, with the same equipment and analyzed in a similar way. A useful way to look at such data is to examine ratios of the prompt fission neutron spectra among actinides, as some of the experimental uncertainties involved in these data are the same from isotope to isotope, making the ratios of fission neutron spectra less dependent on the experimental and analysis details. We discuss here the ratios of prompt fission neutron spectra between 239Pu(n, f ) and 235U(n, f ) , as well as the evolution of the mean fission neutron energy for 239Pu(n, f ) and 235,238U(n, f ) with increasing incident neutron energy.
The response of the recently developed Tl2LiYCl6(Ce) scintillator to neutrons was measured and the 35Cl(n,p) channel observed in the material for the first time. The scintillator was exposed to a 252Cf source mounted within a parallel-plate avalanche counter for fission-fragment coincident measurements. Proton- and t+α-like events are selected with the pulse-shape discrimination technique. Comparison of energy-deposition and time-of-flight allows the quenching of the different channels to be assessed. The 6Li(n,t)α channel is found to have its energy quenched to 37(1)% relative to γ-ray detection, while the proton energy deposition spectrum is quenched to 60(1)%.
Tagging with β-particles at the focal plane of a recoil separator has been shown to be an effective technique for the study of exotic proton-rich nuclei. This article describes three new pieces of apparatus used to greatly improve the sensitivity of the recoil-beta tagging technique. These include a highly-pixelated double-sided silicon strip detector, a plastic phoswich detector for discriminating high-energy β-particles, and a charged-particle veto box. The performance of these new detectors is described and characterised, and the resulting improvements are discussed.© 2013 IOP Publishing Ltd and Sissa Medialab srl.
The 25 Al(p, γ) reaction has long been highlighted as a possible means to bypass the production of 26 Al cosmic γ rays in classical nova explosions. However, uncertainties in the properties of key resonant states in 26 Si have hindered our ability to accurately model the influence of this reaction in such environments. We report on a detailed γ-ray spectroscopy study of 26 Si and present evidence for the existence of a new, likely ℓ = 1, resonance in the 25 Al + p system at Er = 153.9(15) keV. This state is now expected to provide the dominant contribution to the 25 Al(p, γ) stellar reaction rate over the temperature range, T ∼ 0.1 − 0.2 GK. Despite a significant increase in the rate at low temperatures, we find that the final ejected abundance of 26 Al from classical novae remains largely unaffected even if the reaction rate is artificially increased by a factor of 10. Based on new, Galactic chemical evolution calculations, we estimate that the maximum contribution of novae to the observed Galactic abundance of 26 Al is ∼0.2 M⊙. Finally, we briefly highlight the important role that Super-AGB stars may play in the production of 26 Al.
Internal conversion coefficients have been measured for transitions in Pd-110 to allow the determination of electric monopole (E0) transition strengths. Excited states were populated through inelastic alpha scattering and the SPICE+FTIGRESS detectors at TRIUMF-ISAC-II were used to detect the emitted gamma rays and internal conversion electrons. The E0 transition strengths are found to be small and are consistent with the expectation for transitions from a K = 2 band to the K = 0 ground-state band in the axial rotor model.
Background: Nuclei approaching N = Z = 40 are known to exhibit strongly deformed structures and are thought to be candidates for shape coexistence. In the krypton isotopes, 80 , 82 Kr are poorly characterized, preventing an understanding of evolving deformation approaching N = 40 . Purpose: The present work aims to determine electric quadrupole transition strengths and quadrupole moments of 80 , 82 Kr in order to better characterize their deformation. Methods: Sub-barrier Coulomb excitation was employed, impinging the isotopes of krypton on 196 Pt and 208 Pb targets. Utilizing a semiclassical description of the safe Coulomb-excitation process E 2 matrix elements could then be determined. Results: Eleven new or improved matrix elements are determined in 80 Kr and seven in 82 Kr . The new B ( E 2 ; 0 + 1 → 2 + 1 ) value in 82 Kr disagrees with the evaluated value by 3 σ , which can be explained in terms of deficiencies in a previous Coulomb-excitation analysis. Conclusions: Comparison of measured Q s ( 2 + 1 ) and B ( E 2 ; 0 + 1 → 2 + 1 ) values indicates that neutron-deficient ( N ≤ 42 ) isotopes of krypton are closer to axial deformation than other isotopic chains in the mass region. A continuation of this trend to higher Z may result in Sr and Zr isotopes exhibiting near-axial prolate deformation.
The low-lying structure of 13 Be has remained an enigma for decades. Despite numerous experimental and theoretical studies, large inconsistencies remain. Being both unbound and one neutron away from 14 Be , the heaviest bound beryllium nucleus, 13 Be is difficult to study through simple reactions with weak radioactive-ion beams or more complex reactions with stable-ion beams. Here, we present the results of a study using the 12 Be (𝑑,𝑝) 13 Be reaction in inverse kinematics using a 9.5 MeV per nucleon 12 Be beam from the ISAC-II facility. The solid deuteron target of IRIS was used to achieve an increased areal thickness compared to conventional deuterated polyethylene targets. The 𝑄-value spectrum below −4.4 MeV was analyzed using a Bayesian method with geant4 simulations. A three-point angular distribution with the same 𝑄-value gate was fit with a mixture of 𝑠- and 𝑝-wave, 𝑠- and 𝑑-wave, or pure 𝑝-wave transfer. The 𝑄-value spectrum was also compared with geant simulations obtained using the energies and widths of states reported in four previous works. It was found that our results are incompatible with works that revealed a wide 5/2+ resonance but shows better agreement with ones that reported a narrower width.
Excited states of 129In populated following the beta decay of Cd-129 were experimentally studied with the GRIFFIN spectrometer at the ISAC facility of TRIUMF, Canada. A 480-MeV proton beam was impinged on a uranium carbide target and Cd-129 was extracted using the Ion Guide Laser Ion Source (IG-LIS). beta and. rays following the decay of 129Cd were detected with the GRIFFIN spectrometer comprising the plastic scintillator SCEPTAR and 16 high-purity germanium (HPGe) clover-type detectors. From the beta-gamma-gamma coincidence analysis, 32 new transitions and seven new excited states were established, expanding the previously known level scheme of In-129. The log ft values deduced from the beta-feeding intensities suggest that some of the high-lying states were populated by the nu 0g7(/2) -> pi 0g(9/2) allowed Gamow-Teller (GT) transition, which indicates that the allowed GT transition is more dominant in the Cd-129 decay than previously reported. Observation of fragmented Gamow-Teller strengths is consistent with theoretical calculations.
Candidates for three excited states in the 66^Se have been identified using the recoil-{_beta} tagging method together with a veto detector for charged-particle evaporation channels. These results allow a comparison of mirror and triplet energy differences between analogue states across the A = 66 triplet as a function of angular momentum. The extracted triplet energy differences follow the negative trend observed in the f_7/2 shell. Shell-model calculations indicate a continued need for an additional isospin non-conserving interaction in addition to the Coulomb isotensor part as a function of mass.
The iThemba Laboratory for Accelerator Based Sciences (iThemba LABS) is a centre of expertise and innovation in the field of nuclear-structure physics and is a leader in several high-impact studies. One of the highlights of these nuclear-structure experiments is the study of the broad structure of the IsoVector Giant Dipole Resonance (IVGDR) in the rare-earth region. Proton inelastic scattering experiments with Ep = 200 MeV were performed on the even-even Nd isotope chain and 152Sm at very forward scattering angles including zero degrees with the K600 magnetic spectrometer. The evolution of the shape of the IVGDR in the transition from spherical to deformed nuclei was investigated. One of the goals of this highlighted study was to confirm the K-splitting observed in previous photo-absorption measurements from Saclay. Significant discrepancies were found between the direct (γ, xn) data obtained at Saclay and the equivalent photo-absorption cross sections obtained using (p, p') data from the K600. Furthermore, discrepancies exist for several nuclei between photo-absorption data taken at the Saclay and Livermore laboratories. These discrepancies, possible reasons for them and future investigations will be presented and discussed.
Motivated primarily by the large uncertainties in the thermonuclear rate of the 30P(p,γ)31S reaction that limit our understanding of classical novae, we carried out lifetime measurements of 31S excited states using the Doppler Shift Lifetimes (DSL2) facility at the TRIUMF Isotope Separator and Accelerator (ISAC-II) facility. The 31S excited states were populated by the 3He(32S,α)31S reaction. The deexcitation γ rays were detected by a clover-type high-purity germanium detector in coincidence with the α particles detected by a silicon detector telescope. We have applied modern Markov chain Monte Carlo-based Bayesian statistical techniques to perform lineshape analyses of Doppler-shift attenuation method γ-ray data for the first time. We have determined the lifetimes of the two lowest-lying 31S excited states. First experimental upper limits on the lifetimes of four higher-lying states have been obtained. The experimental results were compared to shell-model calculations using five universal sd-shell Hamiltonians. Evidence for γ rays originating from the astrophysically important Jπ=3/2+, 260-keV 30P(p,γ)31S resonance with an excitation energy of Ex=6390.2(7) keV in 31S has also been observed, although strong constraints on the lifetime will require better statistics.
The recoil-β tagging technique has been used in conjunction with the 40 Ca(32 S ,2n) reaction at a beam energy of 88 MeV to identify transitions associated with the decay of the 2 + and, tentatively, 4 + states in the nucleus 70 Kr. These data are used, along with previously published data, to examine the triplet energy differences (TED) for the mass 70 isobars. The experimental TED values are compared with shell model calculations, performed with the JUN45 interaction in the fpg model space, that include a J = 0 isospin nonconserving (INC) interaction with an isotensor strength of 100 keV. The agreement is found to be very good up to spin 4 and supports the expectation for analog states that all three nuclei have the same oblate shape at low-spin. The A = 70 results are compared with the experimental and shell model predicted TED and mirror energy differences (MED) for the mass 66 and 74 systems. The comparisons clearly demonstrate the importance of the isotensor INC interaction in replicating the TED data in this region. Issues related to the observed MED values and their interpretation within the shell model are discussed.
The low-spin structure of the semimagic Ni-64 nucleus has been considerably expanded: combining four experiments, several 0(+) and 2(+) excited states were identified below 4.5 MeV, and their properties established. The Monte Carlo shell model accounts for the results and unveils an unexpectedly complex landscape of coexisting shapes: a prolate 0(+) excitation is located at a surprisingly high energy (3463 keV), with a collective 2(+) state 286 keV above it, the first such observation in Ni isotopes. The evolution in excitation energy of the prolate minimum across the neutron N = 40 subshell gap highlights the impact of the monopole interaction and its variation in strength with N.
The structure of 64Ni, the heaviest stable Ni isotope, has been investigated via high-statistics, multistep safe Coulomb excitation to search for shape coexistence, a phenomenon recently observed in neutron-rich 66Ni and 70Ni as well as in doubly magic, N = 40, 68Ni. The study was motivated by recent, state-of-the-art Monte Carlo shell-model calculations (MCSM), where a Hamiltonian with effective interactions incorporating the monopole tensor force predicts the existence of shape coexistence, also in the lower-mass 62,64Ni isotopes. A set of transition and static E2 matrix elements for both yrast and near-yrast structures was extracted from the differential Coulomb excitation cross sections. From comparisons between the new results and MCSM as well as other shell-model calculations, a clearer picture of the structure of 64Ni emerges. Specifically, the low-spin states are shown to be dominated by proton and neutron excitations mainly within the fp shell, with minimal contribution from the g9/2 shape-driving neutron orbital. The agreement between experimental data and MCSM results indicates a small oblate deformation for the 0+2 level and a spherical shape for the 0+3 state. In addition, the small upper limit determined for the B(E2) probability of a transition associated with the decay of the recently observed 3463-keV, 0+4 state agrees with its proposed assignment to a prolate shape, herewith providing first evidence for triple shape coexistence in a stable Ni isotope.
Discovering unexplored high-spin states in neutron-rich nuclei can open up a new direction to study band structure and the associated shell structure in isospin-asymmetric many-body systems. However, experimental reach has so far been limited to neutron-deficient or stable nuclei which are preferentially produced in fusion reactions used in such studies. Here in this paper, we report the first γ-ray spectroscopy with fusion reactions using a reaccelerated rare-isotope beam of 45K performed at the ReA3 facility of the National Superconducting Cyclotron Laboratory. Using particle and γ-ray coincidence techniques, three new higher-lying states around 6 MeV and five new γ-ray transitions were identified for 46Ca, suggesting three independent band structures formed from different particle-hole configurations. The rotational-like band built on the 0$^+_2$ state is established up to the tentatively assigned 6$^+_2$ state. New results are compared to large-scale shell model calculations, confirming the validity of the effective interaction describing particle-hole excitations across the Z=20 and N=28 shell gaps in the vicinity of doubly-magic 48Ca.
Lying between 16O and 40Ca, the $sd$ shell is well described by robust phenomenological and ab initio nuclear theories. In this work, however, we highlight an unexplained reduction in electric-quadrupole strength in the rare isotope 32Si, studied through sub-barrier Coulomb excitation. It is found that the oblate nature of the deformation is well reproduced, while the absolute scale of quadrupole deformation, however, is inhibited by approximately a factor of 2 compared to theoretical predictions. Through comparison with shell-model and ab initio calculations, we present a number of possible explanations for this inhibited $E2$ strength. By comparing the results of these calculations to multiple observables, we conclude that there is a reduced role for out-of-space excitations in 32Si, resulting in a reduction in the corrections normally applied to both models.
A measurement of proton inelastic scattering of 8He at 8.25A MeV at TRIUMF shows a resonance at 3.54(6) MeV with a width of 0.89(11) MeV. The energy of the state is in good agreement with coupled cluster and no-core shell model with continuum calculations, with the latter successfully describing the measured resonance width as well. Its differential cross section analyzed with phenomenological collective excitation form factor and microscopic coupled reaction channels framework consistently reveals a large deformation parameter β2 = 0.40(3), consistent with no-core shell model predictions of a large neutron deformation. This deformed double-closed shell at the neutron drip-line opens a new paradigm.
This paper describes results that combine Shan-Chen two-phase (liquid-vapour) and partial-bounce back (semi-permeable membrane) methods of Lattice Boltzmann numerical modelling of fluid dynamics in order to understand how bordered pits influence the drying and rewetting of wood. In addition, preliminary results that introduce pressure induced distortion are included.
We present new experimental measurements of resonance strengths in the astrophysical 23Al(p,γ)24Si reaction, constraining the pathway of nucleosynthesis beyond 22Mg in X-ray burster scenarios. Specifically, we have performed the first measurement of the (d,p) reaction using a radioactive beam of 23Ne to explore levels in 24Ne, the mirror analog of 24Si. Four strong single-particle states were observed and corresponding neutron spectroscopic factors were extracted with a precision of ∼20%. Using these spectroscopic factors, together with mirror state identifications, we have reduced uncertainties in the strength of the key ℓ = 0 resonance at Er = 157 keV, in the astrophysical 23Al(p,γ) reaction, by a factor of 4. Our results show that the 22Mg(p,γ)23Al(p,γ) pathway dominates over the competing 22Mg(α,p) reaction in all but the most energetic X-ray burster events (T>0.85 GK), significantly affecting energy production and the preservation of hydrogen fuel.
Background: Electric-quadrupole (E2) strengths relate to the underlying quadrupole deformation of a nucleus and present a challenge for many nuclear theories. Mirror nuclei in the vicinity of the line of N = Z represent a convenient laboratory for testing deficiencies in such models, making use of the isospin-symmetry of the systems. Purpose: Uncertainties associated with literature E2 strengths in 23 Mg are some of the largest in Tz = 1 2 nuclei in the sd-shell. The purpose of the present work is to improve the precision with which these values are known, to enable better comparison with theoretical models. Methods: Coulomb-excitation measurements of 23 Mg and 23 Na were performed at the TRIUMF-ISAC facility using the TIGRESS spectrometer. They were used to determine the E2 matrix elements of mixed E2/M 1 transitions. Results: Reduced E2 transition strengths, B(E2), were extracted for 23 Mg and 23 Na. Their precision was improved by factors of approximately six for both isotopes, while agreeing within uncertainties with previous measurements. Conclusions: A comparison was made with both shell-model and ab initio valence-space in-medium similarity renormalization group calculations. Valence-space in-medium similarity-renormalization-group calculations were found to underpredict the absolute E2 strength-in agreement with previous studies.
Background: Recent developments in {\it ab initio} nuclear theory demonstrate promising results in medium- to heavy-mass nuclei. A particular challenge for many of the many-body methodologies, however, is an accurate treatment of the electric-quadrupole, $E2$, strength associated with collectivity. Purpose: The valence-space in-medium similarity renormalization group (VS-IMSRG) is a particularly powerful method for accessing medium- and high-mass nuclei but has been found to underpredict $E2$ strengths. The purpose of this work is to evaluate the isospin dependence of this underprediction. Methods: We perform a systematic comparison of valence-space in-medium similarity renormalization group (VS-IMSRG) calculations with available literature. We make use of isoscalar and isovector contributions to the $E2$ matrix elements to assess isoscalar and isovector contributions to the missing strength. Results: It is found that the $E2$ strength is consistent throughout $T_z=\left|\frac{1}{2}\right|$, $T_z=\left|1\right|$, $T_z=\left|\frac{3}{2}\right|$ and $T_z=2$ pairs within the $sd$-shell. Furthermore, no isovector contribution to the deficiency is identified. Conclusions: A comparison with toy-models and coupled-cluster calculations is used to discuss potential origins of the missing strength, which arises from missing many-particle, many-hole excitations out of the model space. The absence of any significant isovector contribution to the missing $E2$ strength indicates that the $E2$ strength discrepancy, and therefore any correction, is largely independent of the isospin of the nuclei in question.
The island of inversion for neutron-rich nuclei in the vicinity of N=20 has become the testing ground par excellence for our understanding and modeling of shell evolution with isospin. In this context, the structure of the transitional nucleus ²⁹Mg is critical. The first quantitative measurements of the single-particle structure of ²⁹Mg are reported, using data from the d(²⁸Mg, p γ)²⁹Mg reaction. Two key states carrying significant ℓ=3 (f-wave) strength were identified at 2.40±0.10 (Jπ=5/2¯) and 4.28±0.04 MeV (7/2¯). New state-of-the-art shell-model calculations have been performed and the predictions are compared in detail with the experimental results. While the two lowest 7/2¯ levels are well described, the sharing of single-particle strength disagrees with experiment for both the 3/2¯ and 5/2¯ levels and there appear to be general problems with configurations involving the p3/2 neutron orbital and core-excited components. These conclusions are supported by an analysis of the neutron occupancies in the shell-model calculations.