Professor Philip Walker
About
Biography
Phil Walker joined the Department of Physics in 1987. He obtained his PhD in experimental nuclear physics from the Australian National University in Canberra, went to Michigan State University as a postdoc, and spent seven years at the Daresbury Laboratory in Cheshire, before coming to Guildford to take up a lectureship. He became a professor in 1998. He has worked in various European countries, Australia, Japan, USA and Canada. He focuses his research on the study of nuclei at high angular momentum, with a special interest in nuclear isomers (excited, metastable states of nuclei). In 2019 he received the Ernest Rutherford Medal and Prize from the Institute of Physics, and in 2022 he has been awarded the Lise Meitner Medal and Prize of the European Physical Society. Both prizes are for his work on nuclear isomers.
Nuclear isomers are discussed in, for example, "Energy Traps in Atomic Nuclei", by Philip Walker and George Dracoulis, Nature 399, 35 (1999); "Review of metastable states in heavy nuclei", by G.D. Dracoulis, P.M. Walker and F.G. Kondev, Rep. Prog. Phys. 79, 076301 (2016); and "100 years of nuclear isomers - then and now", by Philip Walker and Zsolt Podolyak, Physica Scripta, 95, 044004 (2020).
ResearchResearch interests
Nuclear-structure physics: study of atomic nuclei with heavy-ion beams to make high-angular-momentum states. Special interest in nuclear isomers as probes and indicators of unusual nuclear structure: pairing effects and superfluidity; tunnelling decay modes; tilted rotation; non-axial shapes; shape coexistence; competition between rotation and vibration.
Experimental investigations through the use of gamma-ray detection: arrays of germanium gamma-ray detectors; Compton suppression; charged-particle coincidences; timing properties; conversion electrons. Current focus on the use of radioactive ion beams, produced by projectile fragmentation and/or the ISOL method. Investigation of the potential for induced emission from isomers, leading to novel energy-storage possibilities.
Nuclear isomers – A primer
Nuclear isomers are excited, metastable states of atomic nuclei. Their ability, in principle, to undergo electromagnetic decay sets isomers apart from nuclear ground states.
The word “isomer” is borrowed from chemistry, with reference to different arrangements of a given set of building blocks – a specific number of neutrons (N) and protons (Z), in the case of atomic nuclei. The existence of isomers was foreseen in 1917 by Frederick Soddy, who referred to them as being “different in their stability and mode of breaking up”. In 1921, Otto Hahn found the first example in Pa-234, but it was not until the mid 1930’s that isomers became firmly established experimentally. Of key importance was the theoretical understanding provided by von Weizsacker, in terms of angular momentum (spin) changes: half-lives are long when the only possible electromagnetic decay transitions involve states with large spin differences.
There is no clear specification as to what half-life is required for an excited nuclear state to be called an isomer. One defining experimental condition is that an isomer should be long enough lived so that its half-life can be determined by electronic means. Alternatively, there should be sufficient time to separate physically an isomeric state from the environment in which it was produced. Either of these conditions leads to the commonly adopted requirement that an isomer should have a half-life of at least one nanosecond (0.000000001 s). While short-lived on a human time scale, a nanosecond is still some thousand times longer than the half-life of a typical nuclear excited state. Thus isomers can be considered to be unusual states of nuclei, with a variety of research opportunities and applications.
Shape isomers
The first type considered here is where shape, rather than spin, confers isomerism. In contrast to the sphericity of atoms, most nuclei are not spherical, and an excited state can have a different shape compared to its respective ground state. The nucleon rearrangements necessary for a shape change can be complex, so that transitions between different shapes are likely to be inhibited, leading to isomerism. Only one shape isomer, Am-242m (the final “m” signifies an isomeric state) has a half-life exceeding 1 ms (here 14 ms).
Spin isomers
Most isomers exist due to spin and energy constraints. The simplest situation is where the first excited state of a nucleus is at low energy, E, and has a large spin difference from the ground state. For gamma-ray decay, the transition multipole order, L, must be greater than or equal to the spin difference. The transition rate has an energy dependence proportional to E to the power (2L+1) but decreases rapidly as L increases. In essence, if L is large and E is small, then the half-life is long and an isomer results. An additional consideration is that for low transition energies there can be large electron-conversion coefficients, thus mitigating to some extent against very long half-lives.
The isomer Ta-180m has an excitation energy of 75 keV and a spin of 9 units, compared to the ground-state spin of 1 unit. The decay of the isomer is so slow that it has never been observed, and only a half-life limit has been determined (much greater than the age of the Universe). This isomer is unique in being naturally occurring – it can be dug out of the ground!
K isomers
While “spin isomers” (discussed above) result from decay transitions requiring large changes in the spin’s magnitude, “K isomers” result from the requirement for large changes in the spin’s direction. For example, the nuclide Hf-180 has a prolate (rugby football) shape, where the long axis is an axis of symmetry. The spin projection, K, on the symmetry axis is a conserved quantity. In this case a broken proton pair can generate 8 units of spin along the symmetry axis, i.e. I=K=8, where I is the total spin. However, the only states available for gamma decay are rotational states where the spin vector is perpendicular to the symmetry axis, with K=0. In practice, an L=1 transition to the I=8, K=0 rotational state is observed, which is called “K forbidden” since the change in K is greater than L. Although it is observed, it is very slow compared to K-allowed transitions, so that the “K-forbidden” designation is useful even though it is not precise, and the associated K=8 state is isomeric, with a half-life of 5.5 hours.
Outlook
Isomers are routinely used for investigating the properties of nuclei. With the recent advent of rare-isotope beams, isomer beams offer new possibilities for research. Outside the province of nuclear physics itself, isomers have well established uses in medical imaging (e.g. Tc-99m, Kr-85m) and can be important in astrophysical (plasma) environments. There has also been much interest in the possibility of controlling the energy release from isomers, but a number of basic problems remain to be solved.
Further reading: See my publications, and the three key articles quoted in my biography.
Research interests
Nuclear-structure physics: study of atomic nuclei with heavy-ion beams to make high-angular-momentum states. Special interest in nuclear isomers as probes and indicators of unusual nuclear structure: pairing effects and superfluidity; tunnelling decay modes; tilted rotation; non-axial shapes; shape coexistence; competition between rotation and vibration.
Experimental investigations through the use of gamma-ray detection: arrays of germanium gamma-ray detectors; Compton suppression; charged-particle coincidences; timing properties; conversion electrons. Current focus on the use of radioactive ion beams, produced by projectile fragmentation and/or the ISOL method. Investigation of the potential for induced emission from isomers, leading to novel energy-storage possibilities.
Nuclear isomers – A primer
Nuclear isomers are excited, metastable states of atomic nuclei. Their ability, in principle, to undergo electromagnetic decay sets isomers apart from nuclear ground states.
The word “isomer” is borrowed from chemistry, with reference to different arrangements of a given set of building blocks – a specific number of neutrons (N) and protons (Z), in the case of atomic nuclei. The existence of isomers was foreseen in 1917 by Frederick Soddy, who referred to them as being “different in their stability and mode of breaking up”. In 1921, Otto Hahn found the first example in Pa-234, but it was not until the mid 1930’s that isomers became firmly established experimentally. Of key importance was the theoretical understanding provided by von Weizsacker, in terms of angular momentum (spin) changes: half-lives are long when the only possible electromagnetic decay transitions involve states with large spin differences.
There is no clear specification as to what half-life is required for an excited nuclear state to be called an isomer. One defining experimental condition is that an isomer should be long enough lived so that its half-life can be determined by electronic means. Alternatively, there should be sufficient time to separate physically an isomeric state from the environment in which it was produced. Either of these conditions leads to the commonly adopted requirement that an isomer should have a half-life of at least one nanosecond (0.000000001 s). While short-lived on a human time scale, a nanosecond is still some thousand times longer than the half-life of a typical nuclear excited state. Thus isomers can be considered to be unusual states of nuclei, with a variety of research opportunities and applications.
Shape isomers
The first type considered here is where shape, rather than spin, confers isomerism. In contrast to the sphericity of atoms, most nuclei are not spherical, and an excited state can have a different shape compared to its respective ground state. The nucleon rearrangements necessary for a shape change can be complex, so that transitions between different shapes are likely to be inhibited, leading to isomerism. Only one shape isomer, Am-242m (the final “m” signifies an isomeric state) has a half-life exceeding 1 ms (here 14 ms).
Spin isomers
Most isomers exist due to spin and energy constraints. The simplest situation is where the first excited state of a nucleus is at low energy, E, and has a large spin difference from the ground state. For gamma-ray decay, the transition multipole order, L, must be greater than or equal to the spin difference. The transition rate has an energy dependence proportional to E to the power (2L+1) but decreases rapidly as L increases. In essence, if L is large and E is small, then the half-life is long and an isomer results. An additional consideration is that for low transition energies there can be large electron-conversion coefficients, thus mitigating to some extent against very long half-lives.
The isomer Ta-180m has an excitation energy of 75 keV and a spin of 9 units, compared to the ground-state spin of 1 unit. The decay of the isomer is so slow that it has never been observed, and only a half-life limit has been determined (much greater than the age of the Universe). This isomer is unique in being naturally occurring – it can be dug out of the ground!
K isomers
While “spin isomers” (discussed above) result from decay transitions requiring large changes in the spin’s magnitude, “K isomers” result from the requirement for large changes in the spin’s direction. For example, the nuclide Hf-180 has a prolate (rugby football) shape, where the long axis is an axis of symmetry. The spin projection, K, on the symmetry axis is a conserved quantity. In this case a broken proton pair can generate 8 units of spin along the symmetry axis, i.e. I=K=8, where I is the total spin. However, the only states available for gamma decay are rotational states where the spin vector is perpendicular to the symmetry axis, with K=0. In practice, an L=1 transition to the I=8, K=0 rotational state is observed, which is called “K forbidden” since the change in K is greater than L. Although it is observed, it is very slow compared to K-allowed transitions, so that the “K-forbidden” designation is useful even though it is not precise, and the associated K=8 state is isomeric, with a half-life of 5.5 hours.
Outlook
Isomers are routinely used for investigating the properties of nuclei. With the recent advent of rare-isotope beams, isomer beams offer new possibilities for research. Outside the province of nuclear physics itself, isomers have well established uses in medical imaging (e.g. Tc-99m, Kr-85m) and can be important in astrophysical (plasma) environments. There has also been much interest in the possibility of controlling the energy release from isomers, but a number of basic problems remain to be solved.
Further reading: See my publications, and the three key articles quoted in my biography.
Publications
This contribution is based on our input to the NuPECC LRP on perspectives of precision experiments at heavy-ion storage rings in the realm of nuclear structure, atomic- and astrophysics. A focus here is on experiments with secondary beams of heavy ions, which can either be stable or long-lived nuclei in specific, high atomic charge states, or unstable nuclides.
Beta-decay spectroscopy of the 187 Ta ground state was performed at KISS. β-delayed γ-rays corresponding to the previously reported in-beam transitions were observed. The β-decay half-life of the 187 Ta ground state was determined to be 283(10) s by analyzing a time spectrum of β-γ coincidence events. The β-decay branching ratio and log(f t) values were evaluated for the first time. Based on the newly evaluated log(f t) values of > 6.0 and a decay scheme, spin-parity values of I π = 7/2 + originating from the odd-proton orbit π7/2[404] were assigned with high confidence, which is consistent with the systematics of neighboring odd-A nuclides.
Isomeric states were observed in nuclei produced in an experiment at the RIKEN Nishina Center Radioactive Isotope Beam Factory following the in-flight fission of a 345 MeV/nucleon 238 U beam. Isomers reported in nuclei spanning a predicted prolate-oblate shape change boundary, 111 Zr ( E = 283.1 keV; τ = 0.326(63) μs), 112Nb (E = 44.2 keV; τ = 0.094(26) μs), 113 Nb (E = 135.4 keV; τ = 0.846(80) μs), and 115Mo (E = 198.6 keV; τ = 63(4) μs), are compared to potential-energy surface calculations which gave a selection of low-lying configurations for each nucleus. Tentative assignments of ground and excited states were made based on energy similarities to the calculations, reduced transition probabilities of the decays, and constraints of transition multipolarities from γ-ray coincidence measurements. These assignments are suggestive of significant deformation being persistent for N > 70 in this region. In addition, isomers in 108Nb, 109Nb, 113Tc, 117Ru, 119Ru, 120Rh, and 122Rh, not spanning the prolate-oblate transition discussed, are presented.
We report on the Facility for Rare Isotope Beams (FRIB) Theory Alliance topical program "Nuclear Isomers in the Era of FRIB". We outline the many ways isomers influence and contribute to nuclear science and technology, especially in the four FRIB pillars: properties of rare isotopes, nuclear astrophysics, fundamental symmetries, and applications for the nation and society. We conclude with a resolution stating our recommendation that the nuclear physics community actively pursue isomer research. A white paper is forthcoming.
Isomers are metastable nuclear excitations with long half-lives, ranging from nanoseconds to years. In general, an isomer’s decay is inhibited by at least one of three physical constraints: spin isomers involve a large change in the magnitude of the angular momentum, often combined with low transition energy; K isomers require a large change in the direction of the angular momentum; and shape isomers arise due to a significant change in the shape of the nucleus. The long half-lives of isomers open up a variety of experimental techniques for studying their properties, which themselves give key information about the nuclear structure. Isomers can also be valuable in providing increased sensitivity for the investigation of exotic nuclei, far from the valley of β-stability. Furthermore, by virtue of their electromagnetic decay, isomers have applications that may differ from those of nuclear ground states, including critical roles in nuclear astrophysics and, more generally, physics at the atomic/nuclear interface. All these topics are discussed in this chapter.
Isomers are metastable nuclear excitations with long half-lives, ranging from nanoseconds to years. In general, an isomer’s decay is inhibited by at least one of three physical constraints: spin isomers involve a large change in the magnitude of the angular momentum, often combined with low transition energy; K isomers require a large change in the direction of the angular momentum; and shape isomers arise due to a significant change in the shape of the nucleus. The long half-lives of isomers open up a variety of experimental techniques for studying their properties, which themselves give key information about the nuclear structure. Isomers can also be valuable in providing increased sensitivity for the investigation of exotic nuclei, far from the valley of β-stability. Furthermore, by virtue of their electromagnetic decay, isomers have applications that may differ from those of nuclear ground states, including critical roles in nuclear astrophysics and, more generally, physics at the atomic/nuclear interface. All these topics are discussed in this chapter.
The combination of the in-flight separator FRS and the storage-ring ESR at GSI offers unique possibilities for high accuracy mass and lifetime measurements of bare and few-electron fragments. Operating the ESR in the isochronous mode allows for measurements of revolution frequencies of stored ions without cooling. Isochronous Mass Spectrometry (IMS) can be applied to fragments with half-lives as short as several tens of microseconds. Newly developed magnetic rigidity tagging increases the resolving power of IMS to about 500000. IMS can be used to measure masses of nuclei with rates even lower than one ion per day, a property also needed for the purpose of the ILIMA project at the future facility FAIR. © 2007 American Institute of Physics.
The properties of high-K isomers have been investigated by measuring γ rays from a source of the 31-year 178Hfm2 isomer and from the decay of implanted 178,179Lu beams. Low-intensity transitions have been identified in the decay of 178Hfm2, demonstrating a consistent extension of K-hindrance systematics to higher multipolarities, and elucidating the spin-dependence of the mixing between the two Kπ = 8- bands. A search is underway for new isomers in 178,179Lu and the preliminary results of the analysis are reported. © 2004 Elsevier B.V. All rights reserved.
One hundred years after "nuclear isomers" were first discovered, Philip Walker and Zsolt Podolyak pick five examples of these long-lived, excited nuclear states to show why they are so important in medical physics and beyond
A cunning addition for the determination of nuclear masses provides world-leading sensitivity for accurate measurements. This is already opening up new physics and applications. It may sound simple to weigh an atomic nucleus, but there is more to it than you might think, especially when the nucleus is very unstable. Finding out the properties of such fleeting nuclei, perhaps surviving for only a few milliseconds, can solve long-standing science problems. Short-lived nuclei are key to our understanding not only of how neutrons and protons bind together in nuclei, but also of the way that explosive events in the cosmos tell us about the life cycle of stars.
The CRIS setup at CERN-ISOLDE is a laser spectroscopy experiment dedicated to the high-resolution study of the spin, hyperfine structure and isotope shift of radioactive nuclei with low production rates (a few per second). It combines the Doppler-free resolution of the in-flight collinear geometry with the high detection efficiency of resonant ionisation. A recent commissioning campaign has demonstrated a 1% experimental efficiency, and as low as a 0.001% non-resonant ionisation. The current status of the experiment and its recent achievements with beams of francium isotopes are reported. The first identified systematic effects are discussed.© 2013 The Authors. Published by Elsevier B.V. All rights reserved.
Relativistic energy projectile fragmentation of Pb-208 has been used to produce a range of exotic nuclei. The nuclei of interest were studied by detecting delayed gamma rays following the decay of isomeric states. Experimental information on the excited states of the neutron-rich N = 126 nucleus, Pt-204, following internal decay of two isomeric states, was obtained for the first time. In addition, decays from the previously reported isomeric I=27h and I=(49/2)h states in Tb-148 and Gd-147, respectively, have been observed. These isomeric decays represent the highest spin discrete states observed to date following a projectile fragmentation reaction, and opens further the possibility of doing 'high-spin physics' using this technique.
In the last two decades a number of nuclear structure and astrophysics experiments were performed at heavy-ion storage rings employing unique experimental conditions offered by such machines. Furthermore, building on the experience gained at the two facilities presently in operation, several new storage ring projects were launched worldwide. This contribution is intended to provide a brief review of the fast growing field of nuclear structure and astrophysics research at storage rings. © 2013 Elsevier B.V. All rights reserved.
Gamow-Teller β decay is forbidden if the number of nodes in the radial wave functions of the initial and final states is different. This Δn=0 requirement plays a major role in the β decay of heavy neutron-rich nuclei, affecting the nucleosynthesis through the increased half-lives of nuclei on the astrophysical r-process pathway below both Z=50 (for N ˃ 82) and Z = 82 (for N ˃ 126). The level of forbiddenness of the Δn=1v1g9/2 → π0g7/2 transition has been investigated from the β decay of the ground state of 207Hg into the single-proton-hole nucleus 207Tl in an experiment at the ISOLDE Decay Station. From statistical observational limits on possible γ-ray transitions depopulating the π0g-17/2 state in 207Tl, an upper limit of 3.9 x 10-3% was obtained for the probability of this decay, corresponding to log ft ˃ 8.8 within a 95% confidence limit. This is the most stringent test of the Δn=0 selection rule to date.
Schottky Mass Spectrometry (SMS) and Isochronous Mass Spectrometry (IMS) have been successfully applied for direct mass measurements of exotic nuclides at FRS-ESR facility at GSI. Both experimental methods are sensitive to single stored ions and can cover very efficiently a large number of nuclides in one run. Presently, more than 1100 masses of different nuclides have been covered by SMS and IMS in several FRS-ESR experiments whereby about 350 of them have been determined for the first time. In this paper, we present the status of our experimental program and use the experimental mass values to test the predictive power of modern mass models.
This paper reports NMR measurements of the magnetic dipole moments of two high-K isomers, the 37/2-, 51.4 m, 2740 keV state in Hf177 and the 8-, 5.5 h, 1142 keV state in Hf180 by the method of on-line nuclear orientation. Also included are results on the angular distributions of γ transitions in the decay of the Hf177 isotope. These yield high precision E2/M1 multipole mixing ratios for transitions in bands built on the 23/2+, 1.1 s, isomer at 1315 keV and on the 9/2+, 0.663 ns, isomer at 321 keV. The new results are discussed in the light of the recently reported finding of systematic dependence of the behavior of the gR parameter upon the quasiproton and quasineutron make up of high-K isomeric states in this region. © 2014 American Physical Society.
The influence of quadrupole-octupole deformations on the energy and magnetic properties of high-K isomeric states in even-even actinide (U, Pu, Cm, Fm, No), rare-earth (Nd, Sm and Gd), and superheavy ( nuclei is examined within a deformed shell model with pairing interaction. The neutron two-quasiparticle (2qp) isomeric energies and magnetic dipole moments are calculated over a wide range in the plane of quadrupole and octupole deformations. In most cases the magnetic moments exhibit a pronounced sensitivity to the octupole deformation. At the same time, the calculations outline three different groups of nuclei: with pronounced, shallow, and missing minima in the 2qp energy surfaces with respect to the octupole deformation. The result indicates regions of nuclei with octupole softness as well as with possible octupole deformation in the high-K isomeric states. These findings show the need for further theoretical analysis as well as of detailed experimental measurements of magnetic moments in heavy deformed nuclei. © 2014 The Royal Swedish Academy of Sciences.
The reaction of a pulsed 18O beam on a 164Dy target was studied in the first experiment with the NuBall array at the IPN Orsay, France. Excited state half-lives were measured using the fast timing method with 20 LaBr3(Ce) detectors. The timing characteristics of the fully digital acquisition system is briefly discussed. A value for the previously unknown half-life of the first excited 4+ state in 178W is presented.
Shell gap at the magic number N = 82 is important to reproduce the 2nd peak of r-process abundance. If a spin-orbit force is reduced in a very neutron-rich region, a shell quenching at N = 82 and a new shell closure at N = 70 are predicted. A shell evolution by the spin-orbit-force reduction can be searched for through the shape evolution of Zr isotopes around an expected double magic nuclei, 110Zr(Z=40,N=70). We performed β-γ and isomer spectroscopy at RIBF to observe low-lying states in 106,108Zr. The present results indicate a well deformed shape for 106,108Zr. The drastic reduction of the spin-orbit force most likely does not occur around 110Zr on an r-process path. © 2012 American Institute of Physics.
The neutron-rich lead isotopes, up to Pb216, have been studied for the first time, exploiting the fragmentation of a primary uranium beam at the FRS-RISING setup at GSI. The observed isomeric states exhibit electromagnetic transition strengths which deviate from state-of-the-art shell-model calculations. It is shown that their complete description demands the introduction of effective three-body interactions and two-body transition operators in the conventional neutron valence space beyond Pb208. © 2012 American Physical Society.
The neutron-rich isotopes 211;213Tl, beyond the N = 126 shell-closure, have been studied for the first time in isomer γ-ray decay, exploiting the fragmentation of a primary uranium beam at the FRS-RISING (FRagment Separator-Rare ISotopes INvestigation at GSI) setup at GSI. The observed isomeric states in 211;213Tl show a deviation from the seniority-like scheme of 209Tl. The possible interpretation of the data is discussed on the basis of energy-level systematics and shell-model calculations.
The recent experimental observation of collective rotational bands up to I > 60h in Hf-175 presents theoretical challenges. It is shown here that total Routhian surface calculations are able to explain the yrast high-spin behavior, with collective oblate states favored at I similar to 35h and strongly deformed prolate states at the highest spins. The collective oblate rotation terminates in noncollective prolate states. Comparisons are made with ultimate cranker calculations, and theoretical quadrupole moments are evaluated.
An extended decay scheme for Xe-128 has been constructed by using data from the Sn-124(Be-9, 5n)Xe-128 reaction at a beam energy of 58 MeV. Bands have been identified as being built on several intrinsic states, including a proposed 9/2(-)[514] circle times 1/2(+)[400] two-quasineutron configuration that forms the K-pi=5(-) intrinsic state at 2228 keV, and on a previously assigned K-pi=8(-) intrinsic state at 2786 keV. A half-life of 73(3) ns has been measured for the latter. Theoretical calculations have been performed by using the configuration-constrained blocking method based on a nonaxial Woods-Saxon potential. Large gamma deformation and gamma softness are predicted for the ground state and the K-pi=5(-) intrinsic state, whereas a nearly axially symmetric shape is predicted for the K-pi=8(-) two-quasiparticle configuration. The low value of the hindrance factor for the E1 transition depopulating the K-pi=8(-) intrinsic state is discussed in the context of analogous transitions in neighbouring N=74 isotones.
The wave-function composition for the low-lying states in Na-29 was explored by measuring their electromagnetic properties using the Coulomb-excitation technique. A beam of Na-29 ions, postaccelerated to 70 MeV, bombarded a Pd-110 target with a rate of up to 600 particles per second at the recently commissioned ISAC-II facility at TRIUMF. Six segmented HPGe clover detectors of the TIGRESS gamma-ray spectrometer were used to detect deexcitation gamma rays in coincidence with scattered or recoiling charged particles in the segmented silicon detector, BAMBINO. The reduced transition matrix element vertical bar < 5/2(1)(+)vertical bar vertical bar E2 vertical bar vertical bar 3/2(gs)(+)>vertical bar in Na-29 was derived to be 0.237(21) e b from the measured gamma-ray yields for both projectile and target. This first-time measured value is consistent with the most recent Monte Carlo shell-model calculation, indicating a significant admixture of both sd and pf components in the wave function, and also providing evidence for the narrowing of the neutron sd-pf shell gap from similar to 6 MeV for stable nuclei to similar to 3 MeV for Na-29. (C) 2009 Elsevier B.V. All rights reserved.
Configuration-constrained calculations of potential-energy surfaces in even-even superheavy nuclei reveal systematically the existence at low excitation energies of multiquasiparticle states with deformed axially symmetric shapes and large angular momenta. These results indicate the prevalence of long-lived, multiquasiparticle isomers. In a quantal system, the ground state is usually more stable than the excited states. In contrast, in superheavy nuclei the multiquasiparticle excitations decrease the probability for both fission and alpha decay, implying enhanced stability. Hence, the systematic occurrence of multiquasiparticle isomers may become crucial for future production and study of even heavier nuclei. The energies of multiquasiparticle states and their alpha decays are calculated and compared to available data.
Using the 168Er(10B,5n) reaction at a beam energy of 68 MeV, new data have been obtained for the population and decay of a T1/2 = 148 ns, Kπ = 21/2− three-quasiparticle isomer at 1717 keV in 173Ta. Revised decay energies and intensities have been determined, together with newly observed members of a rotational band associated with the isomer. By comparison with other isomers in the A ≈ 180 deformed region, the 173Ta isomer properties help to specify the key degrees of freedom that determine K-forbidden transition rates. In particular, when all three quasiparticles are of the same nucleon type, there is a strong dependence of the E2 reduced hindrance factor on the isomer excitation energy.
We investigate the role of two-quasiparticle isomeric states along the proton drip line, using configuration-constrained potential-energy-surface calculations. In contrast to even-even nuclei, odd-odd nuclei can have coexisting low-lying two-quasiparticle states. The low excitation energy and high angular momentum can lead to long-lived isomers. Also, because of the hindrance by spin selection, the probabilities of beta and proton decays from high-spin isomers can be reduced significantly. The present calculations reproduce reasonably well the available data for observed isomers in such nuclei. Unobserved high-spin isomers are predicted, which could be useful for future experimental studies of exotic nuclei at and beyond the proton drip line.
The odd-even differences of nuclear masses are strongly influenced by mean-field and odd-nucleon blocking effects. When such effects are taken into account, the determination of the pairing interaction strength needs to be modified, resulting in larger pairing gaps. This method leads to an improved description for both moments of inertia and backbending frequencies of rotational bands, with no additional parameters.
Isomers in near-spherical Z = 51, antimony isotopes are reported here for the first time using fusion-fission reactions between Al-27 and a pulsed Hf-178 beam of energy, 1150 MeV. gamma rays were observed from the decay of isomeric states with half-lives, T-1/2 = 200(30) and 52(3) mu s, and angular momenta I = (25/2) and I-pi = 23(+)/2, in Sb-121,Sb-123, respectively. These states are proposed to correspond to nu(h(11/2))(2) configurations, coupled to an odd d(5/2) or g(7/2) proton. Nanosecond isomers were also identified at I-pi = 19(-)/2 [T-1/2 = 8.5(5) ns] in Sb-121 and I-pi = (15(-)/2) [T-1/2 = 37(4) ns] in Sb-123. Information on spins and parities of states in these nuclei was obtained using a combination of angular correlation and intensity-balance measurements. The configurations of states in these nuclei are discussed using a combination of spin/energy systematics and shell-model calculations for neighboring tin isotones and antimony isotopes.
The structure of the deformed, doubly odd nuclide Re-180 has been studied by gamma-ray and conversion-electron spectroscopy using the Yb-174(B-11,5n) reaction with a pulsed 71 MeV beam of B-11 ions. Several of the previously known intrinsic states have been given revised spin and parity assignments. Rotational bands are observed with K-pi=(4(+)),(5(-)),(7(+)),8(+),9(-),13(+),14(-),15(-),16(+),21(-), and (22(+)). Among these, a four-quasiparticle t band is identified, which is already energetically favored at its bandhead compared to the corresponding two-quasiparticle band; and two six-quasiparticle bands are identified and associated with a tau=13 mu s isomer. The observed structures, including g factors and alignments, are interpreted with the aid of Nilsson-plus-BCS calculations and configuration-constrained potential energy surface calculations. Reduced-hindrance values are obtained for K-forbidden transitions, illustrating the important role of the K quantum number for near-yrast isomers.
The projected shell model is used to study the multi-quasiparticle and collective excitations of 178Hf. With an axially symmetric basis, the spin-16 isomer at 2.4 MeV appears to be well separated in energy/spin space from other configurations. However, projected energy surface calculations suggest that 178Hf has significant softness to axially asymmetric shapes, which can strongly modify the level distribution. The implications for photodeexcitation of the isomer are discussed.
In this letter we report on new information on the shape evolution of the very neutron-rich 92;94Se nuclei from an isomer-decay spectroscopy experiment at the Radioactive Isotope Beam Factory at RIKEN. High-resolution germanium detectors were used to identify delayed -rays emitted following the decay of these nuclei. New transitions are reported extending the previously known level schemes. The isomeric levels are interpreted as originating from high-K quasi-neutron states with an oblate deformation of B ~ - 0:25, with the high-K state in 94Se being metastable and K hindered. Following this, 94Se is the lowest-mass nucleus known to date with K-forbidden decay and a substantial K hindrance. Furthermore, it is the first observation of an oblate K isomer in a deformed nucleus. From the decay patterns, an oblate deformation is suggested for the 94Se60 ground-state band, in line with the predictions of recent beyond-mean-field calculations.
Using the pairing-deformation-frequency self-consistent total-Routhiansurface and configuration-constrained potential-energy-surface calculations, we have studied nuclear deformation and its effect on the structure of nuclei. It was found that the high-order multipolarity-six (β6) deformation plays a significant role in superheavy nuclei. Possible non-collective highspin isomeric states which locate in the second well of actinide nuclei have been investigated with the predictions of excitation energies and configurations. High-spin isomers can extend shape coexistence in A ∼ 190 neutrondeficient nuclei. Triaxiality with γ 30 is found in the ground and excited rotational states of the A ∼ 70 germanium isotopes. Octupole correlations have also been discussed in different mass regions. In recent experiments, the textbook nucleus 158Er has been reached at ultrahigh spins around 65∼. We have studied 158Er ultrahigh-spin states by means of the self-consistent tilted-axis-cranking method based on the Nilsson shell correction and the Skyrme-Hartree-Fock model. The calculation with a ≈ 12 ° triaxialstrongly-deformed (TSD) excited configuration can well reproduce the observed large transitional quadrupole moment. It is demonstrated that the TSD minimum at negative γdeformation which appears in the principalaxis-cranking approach is a saddle point if allowing the rotational axis to change direction.
A previously unreported isomer has been identified in Mo-99 at an excitation energy of E-x = 3010 keV, decaying with a half-life of T-1/2 = 8(2) ns. The nucleus of interest was produced following fusion-fission reactions between a thick Al-27 target frame and a Hf-178 beam at a laboratory energy of 1150 MeV. This isomeric state is interpreted as an energetically favored, maximally aligned configuration of nu h (11/2) circle times pi(g (9/2))(2).
The single closed-neutron-shell, one proton-hole nucleus 207Tl was populated in deep-inelastic collisions of a 208Pb beam with a 208Pb target. The yrast and near-yrast level scheme has been established up to high excitation energy, comprising an octupole phonon state and a large number of core excited states. Based on shell-model calculations, all observed single core excitations were established to arise from the breaking of the N=126 neutron core. While the shell-model calculations correctly predict the ordering of these states, their energies are compressed at high spins. It is concluded that this compression is an intrinsic feature of shell-model calculations using two-body matrix elements developed for the description of two-body states, and that multiple core excitations need to be considered in order to accurately calculate the energy spacings of the predominantly three-quasiparticle states.
A Fermi-aligned (i 13 2)2 rotational band, with K≈8, has been observed in an even-even nucleus, 180W. Its structure corresponds to a t-band in the tilted-cranking representation. It crosses the ground-state band at I = 16 h {combining short stroke overlay}, giving rise to backbending in the yrast sequence. Contrary to the usual interpretation, backbending in this case is not caused by the s-band. Comparison is made with other possible t-band crossings in the A ≈ 180 region. © 1993.
The suggestion that some atomic nuclei would be able to exist in more than one stable or metastable configuration was proposed by Soddy in 1917. Subsequently, the first experimental example of such an isomeric pair was reported by Hahn in 1921, in the form of two metastable states of 234Pa, then known as UZ and UX2. Nowadays, of the 3437 nuclides listed in the most recent NUBASE evaluation, 1318 have at least one metastable excited state with a half-life of 100 ns or longer. The present work reviews historical aspects of nuclear isomers, and the dfferent physical mechanisms that lead to their formation. Selected frontiers of contemporary isomer research are discussed, with an emphasis on remote regions of the nuclear landscape. Some possibilities for the electromagnetic manipulation of isomers are included.
High-spin terminating bands in heavy nuclei were first identified in nuclei around Er-158(90). While examples of terminating states have been identified in a number of erbium isotopes, almost nothing is known about the states lying beyond band termination. In the present work, the high-spin structure of Er-156,Er-157,Er-158 has been studied using the Gammasphere spectrometer. The subject of triaxial superdeformation and 'wobbling' modes in Lu nuclei has rightly attracted a great deal of attention. Very recently four strongly or superdeformed (SD) sequences have been observed in Hf-174, and cranking calculations using the Ultimate Cranker code predict that such structures may have significant triaxial deformation. We have performed two experiments in an attempt to verify the possible triaxial nature of these bands. A lifetime measurement was performed to confirm the large (and similar) deformation of the bands. In addition, a high-statistics, thin-target experiment took place to search for linking transitions between the SD bands, possible wobbling modes, and new SD band structures.
Isomeric ratios have been measured for high-spin states in Po84198,200,206,208, At85208,209,210,211, Rn86210,211,212,213,214, Fr87208,211,212,213,214, Ra88210,211,212,214,215, and Ac89215 following the projectile fragmentation of a 1 AGeV U beam by a Be target at GSI Helmholtzzentrum für Schwerionenforschung. The fragments were separated in the fragment separator (FRS) and identified by means of energy loss and time-of-flight techniques. They were brought to rest at the centre of the RISING gamma-ray detector array and intensities of gamma rays emitted in the decay of isomeric states with half-lives between 100 ns and 40 μs and spin values up to 55/2ℏ were used to obtain the corresponding isomeric ratios. The data are compared to theoretical isomeric ratios calculated in the framework of the abrasion-ablation model. Large experimental enhancements are obtained for high-spin isomers in comparison to expected values. © 2013 Elsevier B.V.
The structure of nuclear isomeric states is reviewed in the context of their role in contemporary nuclear physics research. Emphasis is given to high-spin isomers in heavy nuclei, with A & 150. The possibility to exploit isomers to study some of the most exotic nuclei is a recurring theme. In spherical nuclei, the role of octupole collectivity is discussed in detail, while in deformed nuclei the limitations of the K quantum number are addressed. Isomer targets and isomer beams are considered, along with applications related to energy storage, astrophysics, medicine, and experimental advances.
High-K isomers are long-lived excitations of deformed atomic nuclei. Their structure is built from broken nucleon pairs that generate high angular momentum, K, along the nuclear symmetry axis. The partial conservation of this quantity leads to strong inhibition of electromagnetic decay, and hence to isomerism. The present work examines the hindrance factors for a range of multipole orders, with a focus on highly K-forbidden E1 transitions from multi-quasiparticle isomers in the A ~ 170 region of nuclei. Allowing for a general 104 inhibition of E1 transitions, there is good accord with other multipole orders. A key feature is that the inhibition declines for isomers that are more highly excited, relative to a rigid rotor of the same total angular momentum. Comparison is also made with K-forbidden E1 transitions in the quasicontinuum, and similar inhibition properties are found.
Deep-inelastic collisions of a 208Pb beam on a 208Pb target were performed using the ATLAS accelerator at Argonne National Laboratory. Prompt and delayed γ-rays from the reaction products were detected using the GAMMASPHERE detector array. The cross-coincidence method was used to identify transitions in 207Tl, by gating on γ-rays from its better-characterised reaction partner 209Bi. A number of new transitions were found in 207Tl.
The high spin negative parity states of Ag have been investigated with the B+Mo reaction at 39 MeV beam energy using the INGA facility at TIFR, Mumbai. From the γ-γ coincidence analysis, an excited negative parity band has been established and found to be nearly degenerate with the ground state band. The spin and parity of the levels are assigned using angular correlation and polarization measurements. This pair of degenerate bands in Ag is studied using the recently developed microscopic triaxial projected shell model approach. The observed energy levels and the ratio of the electromagnetic transition probabilities of these bands in this isotope are well reproduced by the present model. Further, it is shown that the partner band has a different quasiparticle structure as compared to the yrast band. © 2013 Elsevier B.V.
A digital timing method aiming to minimize the time walk caused by the depth-dependent pulse shape variations in CdTe detectors has been developed. Detector pulses are digitized at the preamplifier stage and a full digital process is carried out to deduce and correct the time walk according to the interaction depth. A time resolution of 6.52 ns FWHM at an energy threshold of 150 keV with a CdTe detector (10×10×1 mm3) is achieved, which is close to the intrinsic resolution of the detector. The method improves the time resolution with no loss of detection efficiency and it is easy to implement. It is confirmed that the slow mobility and the short lifetime of the holes are major obstacles for further improvement in the timing performance of the CdTe detectors. The method is applicable to any semiconductor detector.
A systematic study of the population probabilities of nanosecond and microsecond isomers produced following the projectile fragmentation of U-238 at 750 MeV/nucleon has been undertaken at the SIS/FRS facility at GSI. Approximately 15 isomeric states in neutron-deficient nuclei around A similar to 190 were identified and the corresponding. isomeric ratios determined. The results are compared with a model based on the statistical abrasion-ablation description of relativistic fragmentation and simple assumptions concerning gamma cascades in the final nucleus (sharp cutoff). This model represents an upper limit for the population of isomeric states in relativistic projectile fragmentation. When the decay properties of the states above the isomer are taken into account, as opposed to the sharp cutoff approximation, a good agreement between the experimental and calculated angular momentum population is obtained.
The population of Zr following the β decay of Y produced in the projectile fission of U at the GSI facility in Darmstadt, Germany has been studied. Y is known to decay into Zr via two states, one of high spin and the other low spin. These states preferentially populate different levels in the Zr daughter. In this paper the intensities of transitions in Zr observed are compared with those from the decay of the low-spin level studied at the TRISTAN facility at Brookhaven National Laboratory and of the high-spin level studied at the JOSEF separator at the Kernforschungsanlage Jülich. © Published under licence by IOP Publishing Ltd.
The fission of high-K, two-quasiparticle isomers is considered, with specific reference to No, No and Fm. The published experimental evidence is discussed in relation to configuration-constrained potential-energy-surface calculations, which suggest that the high-K isomers should be less susceptible to fission than their corresponding ground states. © 2012 IOP Publishing Ltd.
A recent experiment using projectile fragmentation of a Au beam on a Be target, combined with the fragment recoil separator and experimental storage ring at ring at GSI, has uncovered an isomeric state in Re at 267(10) keV with a half-life of ∼60 s. The data analysis technique used to resolve the isomeric state from the ground state is discussed. © Published under licence by IOP Publishing Ltd.
The quenching of the experimental spectroscopic factor for the proton decay of 151mLu from the short-lived d3=2 isomeric state has been a long standing problem. In the present work, the proton energy value and half-life of this isomer were remeasured to be 1295(5) keV and 15.4 0.8 s, respectively, in an experiment at the Accelerator Laboratory of the University of Jyvaskyla. The re ned experimental data can resolve the discrepancy in the spectroscopic factor with the WKB approximation. It is also found that the proton formation probability extracted from the present measurements is much larger than that from the adopted data before, indicating no signi cant hindrance for the proton decay of 151mLu.
An algorithm for digital implementation of the zero-crossing method for n/γ discrimination in liquid organic scintillators is described. The method exhibits good performance at low energies and requires little computational effort, which makes it suitable for compact real-time neutron detectors.
Recent experiments open up the possibility to investigate oblate rotation-aligned states and prolate high-K isomers in neutron-rich tungsten isotopes. In the present work, we perform the projected-shell-model calculations for A ̃ 190 tungsten nuclei. The W results are compared with experimental data. The observed 8+ isomer is assigned as a two-quasiproton Kφ = 8 configuration. Low-lying high-K four-quasiparticle states are predicted. Of particular interest is the prediction of the Kφ = 20+ state in 190,192W, which may form a long-lived spin trap. In competition with the prolate high-K states, rotational alignment leads to near-yrast collective oblate rotation. © Science China Press and Springer-Verlag Berlin Heidelberg 2012.
Neutron-rich nuclei were populated in a relativistic fission of U. Gamma-rays with energies of 135 keV and 184 keV were associated with two isomeric states in Pd and Ru. Half-lives of 0.63(5) μs and 2.0(3) μs were deduced and the isomeric states were interpreted in terms of prolate deformed single-particle states.
The influence of the octupole deformation on the structure of high-K isomeric states in the region of heavy even-even actinide nuclei is studied through a reflection asymmetric deformed shell model (DSM). Two-quasiparticle states with high-K val- ues are constructed by taking into account the pairing effect through a DSM+BCS procedure with constant pairing interaction. The behaviour of two-quasiparticle ener- gies and magnetic dipole moments of K = 6+, 6− and 8− configurations, applicable to mass numbers in the range A = 234 − 252, was examined over a wide range of quadrupole and octupole deformations. A pronounced sensitivity of the magnetic moments to the octupole deformation is found. The result suggests a possibly im- portant role for high-K isomers in determining the degree of octupole deformation in heavy actinide nuclei.
238U projectile fragments have been created at the entrance of the fragment separator FRS, spatially separated in flight within 0.45 μs and injected into the storage-cooler ring ESR at 7.9 Tm corresponding to about 70% light velocity. Accurate new mass values and lifetime information of the stored exotic nuclei in the element range from platinum to uranium have been obtained with single-particle Schottky spectrometry. In this experiment the new isotopes of 236Ac, 224At, 221Po, 222Po, and 213Tl were discovered. The isotopes were unambiguously identified and their masses measured. In addition, the time-correlated data have provided information on the lifetime of the new nuclides. The discovery of isotopes along with accurate mass measurement has been achieved for the first time at the FRS-ESR facility. The results will contribute to the knowledge of the decay products from the r-process nuclei and enable a crucial test of the predictive power of modern nuclear mass and half-life models.
The β decay of 207Hg into the single-proton-hole nucleus 207Tl has been studied through γ-ray spectroscopy at the ISOLDE Decay Station (IDS) with the aim of identifying states resulting from coupling of the πs−11/2, πd−13/2, and πh−111/2 shell model orbitals to the collective octupole vibration. Twenty-two states were observed lying between 2.6 and 4.0 MeV, eleven of which were observed for the first time, and 78 new transitions were placed. Two octupole states (s1/2-coupled) are identified and three more states (d3/2-coupled) are tentatively assigned using spin-parity inferences, while further h11/2-coupled states may also have been observed for the first time. Comparisons are made with state-of-the-art large-scale shell model calculations and previous observations made in this region, and systematic underestimation of the energy of the octupole vibrational states is noted. We suggest that in order to resolve the difference in predicted energies for collective and noncollective t=1 states (t is the number of nucleons breaking the 208Pb core), the effect of t=2 mixing may be reduced for octupole-coupled states. The inclusion of mixing with t=0,2,3 excitations is necessary to replicate all t=1 state energies accurately.
Excited states in neutron-rich nuclei located south-east of 132Sn are investigated by shell-model calculations. A new shell-model Hamiltonian is constructed for the present study. The proton–proton and neutron–neutron interactions of the Hamiltonian are obtained through the existing CD-Bonn G matrix results, while the proton–neutron interaction across two major shells is derived from the monopole based universal interaction plus the M3Y spin–orbit force. The present Hamiltonian can reproduce well the experimental data available in this region, including one-neutron separation energies, level energies and the experimental B(E2)B(E2) values of isomers in 134,136,138Sn, 130Cd, and 128Pd. New isomers are predicted in this region, e.g. in 135Sn, 131Cd, 129Pd, 132,134In and 130Ag, in which almost no excited states are known experimentally yet. In the odd–odd 132,134In and 130Ag, the predicted very long E 2 life-times of the low-lying 5−5− states are discussed, demanding more information on the related proton–neutron interaction. The low-lying states of 132In are discussed in connection with the recently observed γ rays. The predicted 19/2−19/2− isomer in 129Pd could decay by both electromagnetic transitions and neutron emission with comparable partial life-times, making it a good candidate for neutron radioactivity, a decay mode which is yet to be discovered.
The preliminary results from the RISING Stopped Beam Isomer Campaign are presented, with specific focus on results of the initial experiment to investigate isomeric decays along the N=Z line around A similar to 80-90 following the projectile fragmentation of a Ag-107 primary beam at an energy of 750 MeV per nucleon. A description of the technical aspects behind the design of the RISING array is presented, together with evidence for previously unreported isomeric decays in Tc-87,Tc-88 and the N=Z nuclei Nb-82(41) and Tc-86(43).
A new decay-spectroscopy station (DSS) has been developed to be coupled to the collinear resonance ionization spectroscopy (CRIS) beam line at CERN-ISOLDE. The system uses a rotatable wheel with ten 20μg/ carbon foils as beam implantation sites for the efficient measurement of charged decay products. Silicon detectors are placed on either side of the carbon foil in an optimal geometry to cover a large solid angle for detecting these charged particles. In addition to the silicon detectors at the on-beam axis position, a second pair of off-beam axis detectors are placed at the wheel position 108° away, allowing longer-lived species to be studied. Up to three high purity germanium detectors can be placed around the chamber for particle-gamma correlated measurement. The radioactive beam is transported through the CRIS beam line before implantation into a carbon foil at the DSS. All materials used in the DSS are UHV-compatible to maintain high vacuum conditions required by the CRIS beam line. This paper describes the DSS and presents the first data collected at the setup during the commissioning run with Fr. © 2012 Elsevier B.V.
Excited states have been studied in ____iso{159}{Sm}, ____iso{161}{Sm}, ____iso{162}{Sm} (Z~=~62), ____iso{163}{Eu} (Z~=~63), and ____iso{164}{Gd} (Z~=~64), populated by isomeric decay following 238U projectile fission at RIBF, RIKEN. The isomer half-lives range from 50 ns to 2.6 ____mus. In comparison with other published data, revised interpretations are proposed for ____iso{159}{Sm} and ____iso{163}{Eu}. The first data for excited states in ____iso{161}{Sm} are presented, where a 2.6 ____mus isomer is assigned a three-quasiparticle, K^____pi = 17/2^- structure. The interpretation is supported by multi-quasiparticle Nilsson-BCS calculations, including the blocking of pairing correlations. A consistent set of reduced E1 hindrance factors is obtained. Limited evidence is also reported for isomeric decay in 163Sm, 164Eu and 165Eu.
A comprehensive study of the low-lying states of 108Ag, near the isomeric state at Ei = 110 keV with Jπ = 6+ and T1/2 = 438 y, has been presented. Spectroscopy of these states has been carried out using the reaction 100Mo(11B, 3nγ)108Ag at 39 MeV beam energy using INGA. The multipolarities and electromagnetic nature of the transitions have been assigned based on the angular correlation and polarization measurements. The experimentally identified states have been compared to the results of the Projected Hartree-Fock calculations to understand the configurations involved in these states.
Background: The interplay between collective and single-particle degrees of freedom is an important structure aspect to study. The nuclei in theA ≈ 180 mass region are often denoted as good examples to study such problems because these nuclei are known to exhibit many rotational bands based on multi-quasiparticle K isomers. Purpose: A large set of high-quality experimental data on high-K isomeric states in the A ≈ 180 mass region has accumulated. A systematic description of them is a theoretical challenge as it requires a method going beyond the usual mean field with multi-quasiparticle configurations built in the shell-model basis. The K-isomer data provide an ideal testing ground for theoretical models. Method: The recently extended projected shell model (PSM) by the Pfaffian method is employed with a sufficiently large configuration space including up to 10 quasiparticles. The restoration of rotational symmetry which is broken in the deformed mean field is obtained by means of angular-momentum projection. With axial symmetry in the basis deformation, each multi-quasiparticle state, classified by a K quantum number, represents the major component of a rotational K band. Shell-model diagonalization in such a projected basis defines the K mixing, which is the key ingredient of the present method. Results: Quasiparticle structure and rotational properties of high-K isomers in even-even neutron-rich 174−186W isotopes are described. The rotational evolution of the yrast and near-yrast bands is discussed with successive band crossings. Multi-quasiparticle K isomers and associated rotational bands in each W isotope are studied with detailed quasiparticle configurations given. Electromagnetic transition properties are also studied and the calculated B(E2), B(M1), and g-factors are compared with experiment if data exist. Conclusions: Many nuclei of the A ≈ 180 mass region exhibit properties of an axially symmetric shape and K is approximately a good quantum number. For such nuclei, the extended PSM assuming an axially symmetric basis but including K mixing through diagonalization of the two-body Hamiltonian is an appropriate method to study multi-quasiparticle K isomers and K violations in these states. For special examples where one finds highly K-forbidden transitions the present model needs to be further improved.
206Hg was populated in the fragmentation of an E∕A = 1 GeV 208Pb beam at GSI. It was part of a campaign to study nuclei around 208Pb via relativistic Coulomb excitation. The observation of the known isomeric states confirmed the identification of the fragmentation products. The isomeric decays were also used to prove that the correlations between beam identification detectors and the AGATA γ-ray tracking array worked properly and that the tracking efficiency was independent of the time relative to the prompt flash.
A heavy-ion storage ring can be adapted for use as an isochronous mass spectrometer if the ion velocity matches the transition energy of the ring. Due to the variety of stored ion species, the isochronous condition cannot be fulfilled for all the ions. In order to eliminate the measurement uncertainty stemming from the velocity spread, an intensity-sensitive and position-resolving cavity is proposed. In this article we first briefly discuss the correction method for the anisochronism effect in the measurement with the cavity. Then we introduce a novel design, which is operated in the monopole mode and offset from the central beam orbit to one side. The geometrical parameters were optimized by analytic and numerical means in accordance with the beam dynamics of the future Collector Ring at FAIR. Afterwards, the electromagnetic properties of scaled prototypes were measured on a test bench. The results were in good agreement with the predictions.
Neutron-rich nuclei beyond N = 126 in the lead region were populated by fragmenting a 238U beam at 1 GeV A on a Be target and then separated by the Fragment Separator (FRS) at GSI. Their isomeric decays were observed, enabling study of the shell structure of neutron-rich nuclei around the Z=82 shell closure. Some preliminary results are reported in this paper.
We have identified a mechanism of collective nuclear de-excitation in a Bose–Einstein condensate of 135Cs atoms in their isomeric state, 135mCs, suitable for the generation of coherent gamma photons. The process described here relies on coherence transfer from the Bose–Einstein condensate to the photon field, leading to collective decay triggered by spontaneous emission of a gamma photon. The mechanism differs from single-pass amplification, which cannot occur in atomic systems due to the nuclear recoil and the associated large shift between absorption and emission lines, nor does it require the large densities necessary for standard Dicke super-radiance. This overcomes the limitations that have been hindering the production of coherent gamma photons in many systems. Therefore, we propose an approach for generation of coherent gamma rays, which relies on a combination of well established techniques of nuclear and atomic physics, and can be realized with currently available technology.
Actinide nuclei are found to be good candidates for the formation of high angular momentum, broken-pair excitations in the second minimum of the potential-energy surface. Configuration-constrained calculations of the energy surfaces, including reflection asymmetry, give predictions of the properties of high-K states in the second well. In addition to excitation energies, spins and parities, the calculations indicate increased barriers towards fission, consistent with the extended half-lives observed experimentally.
Several new bands have been identified in 130Ba, among which there is one with band-head spin 8+. Its properties are in agreement with the Fermi-aligned νh211/2, 7/2−[523] ⊗9/2−[514]Nilsson configuration. This is the first observation of a two-quasiparticle t-band in the A =130mass region. The t-band is fed by a dipole band involving two additional h11/2protons. The odd-spin partners of the proton and neutron S-bands and the ground-state band at high spins are also newly identified. The observed bands are discussed using several theoretical models, total Routhians surfaces (TRS), tilted axis cranking (TAC), particle rotor model (PRM) and projected shell model (PSM), which strongly suggest the coexistence of prolate and oblate shapes polarized by rotation aligned two-proton and two-neutron configurations, as well as prolate collective rotations around axes with different orientations. With the new results, 130Ba presents one of the best and most complete sets of collective excitations that a γ-soft nucleus can manifest at medium and high spins, revealing a diversity of shapes and rotations for the nuclei in the A =130mass region.
The band built on the Kπ = 8−, T1/2 = 9.4 ms isomer of 130Ba has been identified, filling the gap in the systematics of the dipole bands built on the 8− isomers in the N = 74 isotones from 128Xe to 138Gd. The use of the GALILEO array in conjunction with its ancillaries EUCLIDES and Neutron Wall, helped to firmly place the newly identified transitions on top of the long-lived isomer. The extracted gK and gR gyromagnetic factors are in agreement with the 7/2+[404] ⊗ 9/2−[514] two-neutron Nilsson configuration. Particle-rotor model calculations give an understanding of the limited degree of K mixing. The experimental information on the Kπ = 8− isomer of 130Ba is now the most complete among the K isomers of the N = 74 isotones.
Background: The γ softness of 136Nd makes it possible to study the shape changes induced by two-proton or two-neutron excitation. Purpose: We measure the lifetimes of two-quasiparticle states of the bands based on the 10+ states at 3296 and 3279 keV to investigate the shape change induced by the alignment of two protons or two neutrons in the h11/2 orbital. Methods: The recoil-distance Doppler shift method was used for the study of 136Nd studies, which was formed by the fusion reaction 120Sn(20Ne, 4n)136Nd, at Ebeam = 85 MeV. Calculations were performed within the microscopic-macroscopic approach, based on the deformed Woods-Saxon single-particle potential and the Yukawa-plus-exponential macroscopic energy. Results: The lifetime of the 10+ state at 3279 keV of 136Nd was measured to be T 10+1/2 = 1.63(9) ns. The lifetimes of the 2+ state at 374 keV and of the 12+ state at 3686 keV of the ground band were also measured to be T 2+1/2 = 26.5(14) ps and T 12+1/2 = 22.5(14) ps. Conclusions: The measured lifetime of 10+ the state at 3279 keV together with other observables confirm the structure change in 136Nd. A rather small reduced hindrance of the electromagnetic decay of the 10+ state at 3279 keV would be consistent with its K-mixed character.
An experiment addressing electron capture (EC) decay of hydrogen-like 142Pm60+ions has been conducted at the experimental storage ring (ESR) at GSI. The decay appears to be purely exponential and no modulations were observed. Decay times for about 9000 individual EC decays have been measured by applying the single-ion decay spectroscopy method. Both visually and automatically analysed data can be described by a single exponential decay with decay constants of 0.0126(7)s−1for automatic analysis and 0.0141(7)s−1for manual analysis. If a modulation superimposed on the exponential decay curve is assumed, the best fit gives a modulation amplitude of merely 0.019(15), which is compatible with zero and by 4.9 standard deviations smaller than in the original observation which had an amplitude of 0.23(4).
Isochronous mass spectrometry was applied to measure isomeric yield ratios of fragmentation reaction prod- ucts. This approach is complementary to conventional g -ray spectroscopy in particular for measuring yield ratios for long-lived isomeric states. Isomeric yield ratios for the high-spin I = 19=2¯h states in the mirror nuclei 53Fe and 53Co are measured to study angular momentum population following the projectile fragmentation of 78Kr at energies of 480 A MeV on a beryllium target. The 19/2 state isomeric ratios of 53Fe produced from different projectiles in literature have also been extracted as a function of mass number difference between projectile and fragment (mass loss). The results are compared to ABRABLA07 model calculations. The isomeric ratios of 53Fe produced using different projectiles suggest that the theory underestimates not only the previously reported dependence on the spin but also the dependence on the mass loss.
For the first time, a wide range of collective magnetic g-factors g, obtained from a novel analysis of experimental data for multi-quasi-particle configurations in high-K isomers, is shown to exhibit a striking systematic variation with the relative number of proton and neutron quasi-particles, N-N. Using the principle of additivity, the quasi-particle contribution to magnetism in high-K isomers of Lu-Re, Z=71-75, has been estimated. Based on these estimates, band-structure branching ratio data are used to explore the behavior of the collective contribution as the number and proton/neutron nature (N, N), of the quasi-particle excitations, change. Basic ideas of pairing, its quenching by quasi-particle excitation and the consequent changes to moment of inertia and collective magnetism are discussed. Existing model calculations do not reproduce the observed g variation adequately. The paired superfluid system of nucleons in these nuclei, and their excitations, present properties of general physics interest. The new-found systematic behavior of g in multi-quasi-particle excitations of this unique system, showing variation from close to zero for multi-neutron states to above 0.5 for multi-proton states, opens a fresh window on these effects and raises the important question of just which nucleons contribute to the 'collective' properties of these nuclei. © 2013 Elsevier B.V.
Nuclei in the A similar to 180 region have been populated and investigated in a series of multinucleon transfer and deep-inelastic reactions involving an 11.4 MeV per nucleon Xe-136 beam produced by the GSI UNILAC accelerator, impinging on a selection of tantalum and tungsten targets. The reaction products were released from a thermal ion source and subsequently mass selected using the GSI on-line mass separator. The unexpectedly high yield of gamma rays associated with the decay of the well established K-pi=37/2(-),t(1/2)=51.4 min isomer in Hf-177(72) and anomalous half-life characteristics associated with this decay lead to these data being interpreted as the beta(-) decay of a high-K isomer in the mother nucleus, Lu-177. By comparing the experimental findings with the predictions obtained from multi-quasiparticle blocked-BCS-Nilsson calculations, the proposed decay is suggested to be from a K-pi=39/2(-) five-quasiparticle state in Lu-177(71). A half-life of 7 +/- 2 min is determined for this beta-decay path which is estimated to have an excitation energy of approximate to3.9 MeV above the Lu-177 ground state.
The problem of the very different E2 decay rates from the two-quasineutron K = 6 isomers in the N = 104 isotones Er, Yb, Hf, and W is investigated using the triaxial projected shell model with inclusion of multi-quasiparticle configurations. It is demonstrated that the highly K-forbidden transition from the 6 isomer to the ground-state band is sensitive to mixing with the 6 state of the γ-vibrational band. Thus the inter-band transitions, and lifetimes, depend on the relative position of the γ-band and the isomeric state in each isotone. © 2013 IOP Publishing Ltd.
The isotope shifts and hyperfine splitting have been measured in 144-154Sm I using the crossed-beam laser fluorescence method. Transitions at 598.98 nm and 570.68 nm were investigated for all isotopes except 146Sm and 153Sm, in which measurements were only obtained at 570.68 nm. Laser-induced fluorescence has not previously been reported for 145Sm: the hyperfine structure for the ground state of this isotope leads to mu =1.123 mu N, Q(spectroscopic)=-0.60e2b2 and delta (r2) (144-145)=0.11 fm2. The magnetic dipole and electric quadrupole moments of the odd isotopes and the changes in mean square radii of the even ones are shown to be consistent with the information obtained from nuclear spectroscopy.
Tilted-axis rotation, arising from Fermi-aligned configurations, has been observed for the first time to cause backbending in an odd-proton nucleus. In 181Re, two t-bands are found to be energetically favored relative to the usual rotation-aligned s-bands, presenting an alternative form of cold nuclear rotation. Interactions between the bands are weak, and unambiguous comparisons with tilted-axis-cranking calculations can be made.
The anomalously fast decay of a 19/2+ three-quasiparticle isomer in 171Tm was interpreted recently as an example of K mixing induced by a very small mixing matrix element but a (random) close proximity to a collective state. To understand the source of the residual interaction we have generalized the projected shell model by introducing two-body octupole and hexadecupole forces into the Hamiltonian and expanding the model space with inclusion of specific three-quasiparticle configurations. It is found that the K mixing is built up from small interactions transferred through numerous highly excited configurations that contain high-j orbitals. While the chance near-degeneracy enhances the transition strength, the octupole correlation and Coriolis coupling produce the mixing matrix element.
The beta decay of Li-11 has been investigated at TRIUMF-ISAC using a high-efficiency array of Compton suppressed HPGe detectors. From a line-shape analysis of the Doppler-broadened peaks observed in the Be-10 gamma spectrum, both the half-lives of states in Be-10 and the energies of the beta-delayed neutrons feeding those states were obtained. Furthermore, it was possible to determine the excitation energies of the parent states in Be-11 with uncertainties comparable to those obtained from neutron spectroscopy experiments. These data suggest that the beta decay to the 8.81 MeV state in Be-11 occurs in the Li-9 core and that one neutron comprising the halo of Li-11 survives in a halolike configuration after the beta-delayed neutron emission from this level.
The yrast structure of 207Pb above the 13=2+ isomeric state has been investigated in deep-inelastic collisions of 208Pb and 208Pb at ATLAS, Argonne National Laboratory. New and previously observed transitions were measured using the Gammasphere detector array. The level scheme of 207Pb is presented up to ∼ 6 MeV, built using coincidence and γ-ray intensity analyses. Spin and parity assignments of states were made, based on angular distributions and comparisons to shell model calculations.
Neutron-rich isotopes around lead, beyond N= 126, have been studied exploiting the fragmentation of an uranium primary beam at the FRS-RISING setup at GSI. For the first time β-decay half-lives of Bi and Tl isotopes have been derived. The half-lives have been extracted using a numerical simulation developed for experiments in high-background conditions. Comparison with state of the art models used in r-process calculations is given, showing a systematic underestimation of the experimental values, at variance from close-lying nuclei. © 2012 Elsevier B.V.
We have measured the optical isotope shifts of146Sm and151Sm by laser resonance fluorescence. From these measurements the changes in the mean square nuclear radii are: Δ〈r2〉 (A=144 to 146)=0.266(10) fm2, and Δ〈r2〉 (A=151 to 152)=0.262(10) fm2. These results, together with those of the stable isotopes, show that the average nuclear expansion of samarium can be accounted for by the liquid drop model with deformations.
Two high-spin regularly spaced rotational bands with large dynamical moments of inertia have been identified in Hf-175 with the Gammasphere spectrometer. These new bands are very similar to the previously identified triaxial superdeformed bands in the hafnium nuclei. However, the new bands in Hf-175 have been linked into the known level scheme and thereby provide the first firm spin assignments for these structures in this region. In order to understand the new bands, theoretical calculations have been performed based on the ULTIMATE CRANKER code. The new bands in Hf-175 are deduced to be built upon highly deformed structures. No experimental evidence for triaxiality was established and this work suggests that the structure of the so-called "triaxial" superdeformed bands in the Hf nuclei may be quite different from those identified in the lighter mass Lu nuclei. Since the two highly deformed bands in Hf-175 are associated with different deformations, this work also identifies the role of the intruder orbits in polarizing the nuclear shape.
The Collinear Resonance Ionization Spectroscopy (CRIS) experiment at ISOLDE, CERN, uses laser radiation to stepwise excite and ionize an atomic beam for the purpose of ultra-sensitive detection of rare isotopes, and hyperfine-structure measurements. The technique also offers the ability to purify an ion beam that is heavily contaminated with radioactive isobars, including the ground state of an isotope from its isomer, allowing decay spectroscopy on nuclear isomeric states to be performed. The isomeric ion beam is selected by resonantly exciting one of its hyperfine structure levels, and subsequently ionizing it. This selectively ionized beam is deflected to a decay spectroscopy station (DSS). This consists of a rotating wheel implantation system for alpha- and beta-decay spectroscopy, and up to three germanium detectors around the implantation site for gamma-ray detection. Resonance ionization spectroscopy and the new technique of laser assisted nuclear decay spectroscopy have recently been performed at the CRIS beam line on the neutron-deficient francium isotopes. Here an overview of the two techniques will be presented, alongside a description of the CRIS beam line and DSS. © Owned by the authors, published by EDP Sciences, 2013.
Picosecond lifetimes of medium spin states in Lu-165 were measured for the first time. The reaction used to populate the nucleus of interest was La-139(Si-30,4n)Lu-165 at a beam energy of 135 MeV. The beam was provided by the XTU-tandem accelerator of Laboratori Nazionali di Legnaro, Italy. By using the differential decay curve method, lifetimes of 19 states in four different rotational bands were obtained. Therefrom the B(E2) values and the transitional quadrupole moments were deduced. The obtained Q(t) for the different bands are compared with total Routhian surface (TRS) calculations and particle-rotor-model calculations. The TRS calculations predict different axial symmetric shapes for the bands built on the 9/2(-)[514], 9/2(+)[404], and 1/2(-)[541] configurations, with a gamma softness for the 9/2(-)[514] configuration. This band has also been studied using the particle-rotor model, the results of which, however, are consistent with a triaxial shape with a gamma value of -15(p).
We propose to install a storage ring at an ISOL-type radioactive beam facility for the first time. Specifically, we intend to setup the heavy-ion, low-energy ring TSR at the HIE-ISOLDE facility in CERN, Geneva. Such a facility will provide a capability for experiments with stored secondary beams that is unique in the world. The envisaged physics programme is rich and varied, spanning from investigations of nuclear ground-state properties and reaction studies of astrophysical relevance, to investigations with highly-charged ions and pure isomeric beams. The TSR might also be employed for removal of isobaric contaminants from stored ion beams and for systematic studies within the neutrino beam programme. In addition to experiments performed using beams recirculating within the ring, cooled beams can also be extracted and exploited by external spectrometers for high-precision measurements. The existing TSR, which is presently in operation at the Max-Planck Institute for Nuclear Physics in Heidelberg, is well-suited and can be employed for this purpose. The physics cases as well as technical details of the existing ring facility and of the beam and infrastructure requirements at HIE-ISOLDE are discussed in the present technical design report.
Using, for the first time, configuration-constrained potential-energy-surface calculations with the inclusion of β6 deformation, we find remarkable effects of the high-order deformation on the high-K isomers in 254No, the focus of recent spectroscopy experiments on superheavy nuclei. For shapes with multipolarity six, the isomers are more tightly bound and, microscopically, have enhanced deformed shell gaps at N=152 and Z=100. The inclusion of β6 deformation significantly improves the description of the very heavy high-K isomers.
The occurrence of oblate and prolate shapes is investigated theoretically for odd-odd neutron-rich nuclei with A approximate to 190. Using the cranked Woods-Saxon-Strutinsky method, including configuration constraints, it is found that collective oblate rotation coexists with high-K prolate rotation, for tantalum and rhenium isotopes with N=115 and 117.
A study of neutron-rich isotopes in the A=185 region of the nuclear chart has uncovered long-lived (>1s) isomers in several isotopes of hafnium, tantalum, tungsten, rhenium, and osmium. The region was accessed via the use of projectile fragmentation with the UNILAC-SIS accelerators at GSI. Fragmentation products of 197Au were passed through the fragment separator (FRS) and injected into the experimental storage ring (ESR), where single-ion identifications could be made. Evidence is presented for isomers in 183184 186Hf, 186187Ta,186W, 190192 194Re, and 195Os with excitation energies in the range of 0.13.0 MeV. The lightest of these nuclides have well deformed prolate shapes, while the heaviest are transitional and susceptible to shape changes. Their properties are interpreted with the help of multi-quasiparticle and potential-energy- surface calculations. ©2012 American Physical Society.
In-flight fission of a 345 MeV per nucleon 238U primary beam on a 2 mm thick 9Be target has been used to produce and study the decays of a range of neutron-rich nuclei centred around the doubly mid-shell nucleus 170Dy at the RIBF Facility, RIKEN, Japan. The produced secondary fragments of interest were identified eventby- event using the BigRIPS separator. The fragments were implanted into the WAS3ABI position sensitive silicon active stopper which allowed pixelated correlations between implants and their subsequent β-decay. Discrete γ-ray transitions emitted following decays from either metastable states or excited states populated following beta decay were identified using the 84 coaxial high-purity germanium (HPGe) detectors of the EURICA spectrometer, which was complemented by 18 additional cerium-doped lanthanum bromide (LaBr3)
The -decay half-lives of 94 neutron-rich nuclei 144−151Cs, 146−154Ba, 148−156La, 150−158Ce, 153−160Pr, 156−162Nd, 159−163Pm, 160−166Sm, 161−168Eu, 165−170Gd, 166−172Tb, 169−173Dy, 172−175Ho and two isomeric states 174mEr, 172mDy were measured at Radioactive Isotope Beam Factory (RIBF), providing a new experimental basis to test theoretical models. Striking, large drops of -decay half-lives are observed at neutron-number N = 97 for 58Ce, 59Pr, 60Nd, 62Sm, and N = 105 for 63Eu, 64Gd, 65Tb, 66Dy. Features in the data mirror the interplay between pairing effects and microscopic structure. r-Process network calculations performed for a range of mass models and astrophysical conditions show that the 57 half-lives measured for the first time play an important role in shaping the abundance pattern of rare-earth elements in the solar system.
The high spin structure of Ag nucleus has been studied using the reaction B + Mo at 39 MeV with Indian National Gamma Array (INGA) at TIFR-BARC accelerator facility. From the two- and higher-fold coincidence analysis of the emitted γ-rays, the level structure of the nucleus is built, with addition of around ∼ 60 new transitions. A new positive parity dipole band has been observed and significant additions have been made in the low spin region. A pair of nearly degenerate, negative parity, dipole bands is established, which is studied using the triaxial projected shell model (TPSM). © Owned by the authors, published by EDP Sciences, 2014.
Masses of 238U projectile fragments have been measured with time-resolved Schottky Mass Spectrometry (SMS) at the FRS-ESR facility at GSI. The exotic nuclei were created in the production target at the entrance of the fragment separator FRS, spatially separated in flight and injected into the storage-cooler ring ESR at about 70% light velocity. This means the ions were mainly bare or carried only a few electrons, e.g., the population of Li-like ions was below 1% for Pt fragments. Accurate newmass values of 33 neutron-rich, stored exotic nuclei in the element range from platinum to uranium have been obtained for the first time. In total more than 150 nuclides including references with well-known masses have been covered in this large-area SMS measurement. A novel data analysis has been applied which reduces the systematic errors by taking into account the velocity profile of the cooler electrons and the residual ion-optical dispersion in this part of the storage ring. The experiment, the data analysis, and the mass values are presented. The experimental data are compared with theoretical predictions demonstrating systematic deviations of up to 1500 keV from modern mass models.
The level structure of 172Dy has been investigated for the first time by means of decay spectroscopy following in-flight fission of a 238U beam. A long-lived isomeric state with T1/2 = 0.71(5) s and K π = 8 − has been identified at 1278 keV, which decays to the ground-state and γ-vibrational bands through hindered electromagnetic transitions, as well as to the daughter nucleus 172Ho via allowed β decays. The robust nature of the K π = 8 − isomer and the ground-state rotational band reveals an axially-symmetric structure for this nucleus. Meanwhile, the γ-vibrational levels have been identified at unusually low excitation energy compared to the neighboring well-deformed nuclei, indicating the significance of the microscopic effect on the non-axial collectivity in this doubly mid-shell region. The underlying mechanism of enhanced γ vibration is discussed in comparison with the deformed Quasiparticle Random-Phase Approximation based on a Skyrme energy-density functional.
Excited states in 63,65,67Mn were studied via in-beam γ-ray spectroscopy following knockout reactions from 68Fe. Similar level schemes, consisting of the 11/2−, 9/2−, 7/2− and 5/2 g.s.− level sequence, connected by I→I−1 transitions, were established, the first time for 65,67Mn. Their level structures show features consistent with strongly-coupled rotational bands with K=5/2 . State-of-the-art shell-model calculations with the modified LNPS effective interaction reproduce the observed levels remarkably well and suggest the dominance of 4-particle-4-hole neutron configurations for all the states. The data on the low-lying excited states of odd-mass 53−67Mn provide a textbook example of nuclear structure evolution from weak coupling through decoupling to strong coupling along a single isotopic chain on the n-rich side of the β stability line. These results help to deepen our understanding of the N=40 “island of inversion”.
We investigate the role of two-quasiparticle isomeric states along the proton drip line, using configuration-constrained potential-energy-surface calculations. In contrast to even-even nuclei, odd-odd nuclei can have coexisting low-lying two-quasiparticle states. The low excitation energy and high angular momentum can lead to long-lived isomers. Also, because of the hindrance by spin selection, the probabilities of beta and proton decays from high-spin isomers can be reduced significantly. The present calculations reproduce reasonably well the available data for observed isomers in such nuclei. Unobserved high-spin isomers are predicted, which could be useful for future experimental studies of exotic nuclei at and beyond the proton drip line.
The neutron-rich dysprosium isotopes ¹⁶⁸Dy₁₀₂ and ¹⁶⁹Dy₁₀₃ have been investigated using the EURICA γ-ray spectrometer, following production via in-flight fission of a high-intensity uranium beam in conjunction with isotope separation through the BigRIPS separator at RIBF in RIKEN Nishina Center. For ¹⁶⁸Dy, a previously unreported isomer with a half-life of 0.57(7) μs has been identified at an excitation energy of 1378 keV, and its presence affirmed independently using γ-γ-γ coincidence data taken with Gammasphere via two-proton transfer from an enriched ¹⁷⁰Er target performed at Argonne National Laboratory. This isomer is assigned Jπ = Kπ = (4⁻) based on the measured transition strengths, decay patterns, and the energy systematics for two-quasiparticle states in N = 102 isotones. The underlying mechanism of two-quasiparticle excitations in the doubly midshell region is discussed in comparison with the deformed QRPA and multi-quasiparticle calculations. In ¹⁶⁹Dy, the B(E2) value for the transition de-exciting the previously unreported Kπ = (1/2⁻) isomeric state at 166 keV to the Kπ = (5/2⁻) ground state is approximately two orders of magnitude larger than the E2 strength for the corresponding isomeric-decay transition in the N = 103 isotone ¹⁷³Yb, suggesting the presence of a significant γ-vibrational admixture with a dominant neutron one-quasiparticle component in the isomeric state.
Neutron-rich nuclei in the lead region, beyond N=126, have been studied at the FRS-RISING setup at GSI, exploiting the fragmentation of a primary uranium beam. Two isomeric states have been identified in Hg: the 8 isomer expected from the seniority scheme in the νg shell and a second one at low spin and low excitation energy. The decay strength of the 8 isomer confirms the need of effective three-body forces in the case of neutron-rich lead isotopes. The other unexpected low-lying isomer has been tentatively assigned as a 3 state, although this is in contrast with theoretical expectations. © 2013 Elsevier B.V..
Recent experimental data on the low-lying states in W-190 show a change in the E(4(1)(+))/E(2(1)(+)) behavior compared to less neutron-rich neigbors. Self-consistent axially-deformed Hartree-Fock calculations, using a separable monopole interaction, of nuclei in the vicinity of W-190 are performed to systematically examine the evolution of ground state quadrupole deformations. It is found that the neutron number N=116 causes a coexistence of oblate and prolate shapes, with a weak dependence on proton number, thereby hindering the development of these isotones as well-deformed rotors.
A detailed study of the structure of the doubly mid-shell nucleus View the MathML source has been carried out, following isomeric and β decay. We have measured the yrast band up to the spin-parity Jπ=6+ state, the K=2γ -vibration band up to the 5+ state, a low-lying negative-parity band based on a 2− state that could be a candidate for the lowest energy octupole vibration state within this nucleus, and a candidate for the Kπ=6+ two quasi-particle isomer. This state was determined to have an excitation energy of 1643.91(23) keV and a half life of 0.99(4) μs, with a reduced hindrance for its decay to the ground-state band an order of magnitude lower than predicted by NpNn systematics. This is interpreted as being due to γ -vibrational mixing from a near degeneracy of the isomer and the 6+ state of the γ band. Furthermore, the parent nucleus 170Tb has been determined to have a half-life of View the MathML source s with a possible spin-parity of 2−.
High-spin states in neutron-rich Hf-180 and Hf-182 nuclei were populated through inelastic and transfer reactions with a Xe-136 beam incident on a thin Hf-180 target, and investigated using particle-gamma coincidence techniques. New collective band structures were observed, and previously known rotational and vibrational bands in these nuclei were extended to higher angular momenta. No obvious nucleon alignment was observed in the ground state band of either nucleus up to h omega=0.43 MeV, a significant delay compared to lighter even-even Hf isotopes. Woods-Saxon cranking calculations were performed to predict the nature of the first band crossings and shape evolution in Hf-180,Hf-182.
An alternative description of tilted rotation, observed in deformed nuclei, is presented using the projected shell model approach. It is shown that a strong configuration mixing among the projected states is responsible for the appearance of the tilted bands near to, but slightly above, the yrast line in even-even tungsten isotopes. Various tilted bands in 184Os are also predicted.
decays from heavy, neutron-rich nuclei with A∼190 have been investigated following their production via the relativistic projectile fragmentation of an E/A=1 GeV 208Pb primary beam on a ∼2.5 g/cm2 9Be target. The reaction products were separated and identified using the GSI FRagment Separator (FRS) and stopped in the RISING active stopper. γ decays were observed and correlated with these secondary ions on an event-by-event basis such that γ-ray transitions following from both internal (isomeric) and β decays were recorded. A number of discrete, β-delayed γ-ray transitions associated with β decays from 194Re to excited states in 194Os have been observed, including previously reported decays from the yrast Iπ=(6+) state. Three previously unreported γ-ray transitions with energies 194, 349, and 554 keV are also identified; these transitions are associated with decays from higher spin states in 194Os. The results of these investigations are compared with theoretical predictions from Nilsson multi-quasiparticle (MQP) calculations. Based on lifetime measurements and the observed feeding pattern to states in 194Os, it is concluded that there are three β−-decaying states in 194Re.
The Isomeric beams, LIfetimes and MAsses (ILIMA) collaboration will exploit heavy-ion storage rings at the Facility for Antiproton and Ion Research (FAIR) for the study of exotic nuclei. Single-ion sensitivity and exceptional production rates of bare or few-electron radioactive ions, with atomic numbers up to Z = 92, promise access to a wide range of short-lived nuclides for the first time. Measuring the masses, lifetimes and decay modes of ground and isomeric states with t > 10 μs will reveal key features of nuclear structure and nuclear astrophysics, extending, for example, to r-process waiting-point nuclides in the Pb region. © 2013 Elsevier B.V. All rights reserved.
Masses of uranium fission fragments have been measured with the FRagment Separator (FRS) combined with the Experimental Storage Ring (ESR) at GSI. A 410-415 MeV/u 238U projectile beam was fast extracted from the synchrotron SIS-18 with an average intensity of 109/spill. The projectiles were focused on a 1g/cm2 beryllium target at the entrance of the FRS to create neutron-rich isotopes via abrasion-fission. The fission fragments were spatially separated with the FRS and injected into the isochronous storage ring ESR for fast mass measurements without applying cooling. The Isochronous Mass Spectrometry (IMS) was performed under two different experimental conditions, with and without B ____(____rho____)-tagging at the high-resolution dispersive central focal plane of the FRS. The evaluation has been done for the combined data sets from both experiments with a new method of data analysis. The use of a correlation matrix has provided experimental mass values for 23 different neutron-rich isotopes for the first time and 6 masses with improved values. The new masses were obtained for nuclides in the element range from Se to Ce. The applied analysis has given access even to rare isotopes detected with an intensity of a few atoms per week. The novel data analysis and systematic error determination are described and the results are compared with extrapolations of experimental values and theoretical models.
The β-decay half-lives of 38 neutron-rich isotopes from 36Kr to 43Tc have been measured; the half-lives of 100Kr, 103–105Sr, 106–108Y, 108–110Zr, 111,112Nb, 112–115Mo, and 116,117Tc are reported here. The results when compared with previous standard models indicate an overestimation in the predicted half-lives by a factor of 2 or more in the A≈110 region. A revised model based on the second generation gross theory of β decay better predicts the measured half-lives and suggests a more rapid flow of the rapid neutron-capture process (r-matter flow) through this region than previously predicted.
Measurements of the optical isotope shift and hyperfine structure of 153Sm are described. The quadrupole moments and mean square charge radii of samarium isotopes are discussed qualitatively in relation to nuclear deformation.
We report on the first measurement of the half-lives of and four-quasiparticle states in the even-even nucleus 178W. The sub-nanosecond half-lives were measured by applying the centroid shift method to data taken with LaBr3(Ce) scintillator detectors of the NuBall array at the ALTO facility in Orsay, France. The half-lives of these states only became experimentally accessible by the combination of several experimental techniques - scintillator fast timing, isomer spectroscopy with a pulsed beam, and the event-by-event calorimetry information provided by the NuBall array. The measured half-lives are and for the and states, respectively. The decay transitions include weakly hindered E1 and E2 branches directly to the ground-state band, bypassing the two-quasiparticle states. This is the first such observation for an E1 transition. The interpretation of the small hindrance hinges on mixing between the ground-state band and the t-band.
We report on the design, installation, and test of an experimental facility for the production of ultra-cold atomic isotopes and isomers of cesium. The setup covers a broad span of mass numbers and nuclear isomers, allowing one to directly compare chains of isotopes and isotope/isomer pairs. Cesium nuclei are produced by fission or fusion-evaporation reactions using primary proton beams from a 130 MeV cyclotron impinging upon a suitable target. The species of interest is ejected from the target in ionic form, electrostatically accelerated, mass separated, and routed to a science chamber. Here, ions are neutralized by implantation in a thin foil, and extracted by thermal diffusion. A neutral vapor at room temperature is thus formed and trapped in a magneto-optical trap. Real-time fluorescence imaging and destructive absorption imaging provide information on the number of trapped atoms, their density, and their temperature. Tests with a dedicated beam of 133Cs + ions at 30 KeV energy confirm neutralization, evaporation, and laser cooling to 150 μ K, with an average atomic density of 1010 cm−3. Availability of cold and dense atomic samples of Cs isotopes and isomers opens new avenues for high-precision measurements of isotopic and isomeric shifts thereby gaining deeper insight into the nuclear structure, as well as for sensitive measurements of isotopes’ concentration ratios in trace quantities. The facility also constitutes the core for future experiments of many-body physics with nuclear isomers.
Heavy neutron-rich nuclei were populated via relativistic energy fragmentation of a E/A=1 GeV 208Pb beam. The nuclei of interest were selected and identified by a fragment separator and then implanted in a passive plastic stopper. Delayed rays following internal isomeric decays were detected by the RISING array. Experimental information was obtained on a number of nuclei with Z=73-80 (Ta-Hg), providing new information both on the prolate-oblate transitional region as well as on the N=126 closed shell nuclei.
This contribution will report on the experimental work on the level structure of Dy-168. The experimental data have been taken as part of the EURICA decay spectroscopy campaign at RIBF, RIKEN in November 2014. In the experiment, a U-238 primary beam is accelerated up to 345 MeV/u with an average intensity of 12 pnA. The nuclei of interest are produced by in-flight fission of U-238 impinging on Be target with a thickness of 5 mm. The excited states of Dy-168 have been populated through the decay from a newly identified isomeric state and via the beta decay from Tb-168. In this contribution, scientific motivations, experimental procedure and some preliminary results for this study are presented.
Mass-separated 187Ta114 in a high-spin isomeric state has been produced for the first time by multinucleon transfer reactions, employing an argon gas-stopping cell and laser ionization. Internal γ rays revealed a T1/2 = 7.3 ± 0.9 s isomer at 1778 ± 1 keV , which decays through a rotational band with perturbations associated with the approach to a prolate-oblate shape transition. Model calculations show less influence from triaxiality compared to heavier elements in the same mass region. The isomer-decay reduced E 2 hindrance factor fν = 27 ± 1 supports the interpretation that axial symmetry is approximately conserved.
β-decay spectroscopy of 173,174Ho (Z = 67, N = 106,107) was conducted at Radioactive Isotope Beam Factory at RIKEN by using in-flight fission of a 345-MeV/u 238U primary beam. A previously unreported isomeric state at 405 keV with half-life of 3.7(12) μs and a spin and parity of (3/2+) is identified in 173Ho. Moreover, a new state with a spin and parity of 9- was discovered in 174Er. The experimental log ft values of 5.84(20) and 5.25(18) suggest an allowed-hindered β decay from the ground state of 174Ho to the Kπ = 8- isomeric state in 174Er. Configuration-constrained potential energy surface (PES) calculations were performed and the predictions are in reasonable agreement with the experimental results.
The year 2021 marks exactly 100 years since Otto Hahn discovered the first example of nuclear isomerism. The existence of long-lived nuclear excited states opens a window on nuclear structure and applications. From isomers, the availability of electromagnetic decay pathways enables coupling to the atomic electrons, such that nuclear and atomic transitions become interdependent. The nuclear decay process of internal conversion is the most well known. However, observation of its inverse, nuclear excitation by free electron capture, is controversial and requires further research. In this work, the relationship between nuclear and atomic transitions is outlined, and examples are discussed of the use of external electromagnetic radiation to manipulate nuclear transitions associated with isomers.
The Rare-RI Ring (R3) is a recently commissioned cyclotron-like storage ring mass spectrometer dedicated to mass measurements of exotic nuclei far from stability at Radioactive Isotope Beam Factory (RIBF) in RIKEN. The first application of mass measurement using the R3 mass spectrometer at RIBF is reported. Rare isotopes produced at RIBF, 127Sn, 126In, 125Cd, 124Ag, 123Pd, were injected in R3. Masses of 126In, 125Cd, and 123Pd were measured whereby the mass uncertainty of 123Pd was improved. This is the first reported measurement with a new storage ring mass spectrometery technique realized at a heavy-ion cyclotron and employing individual injection of the pre-identified rare nuclei. The latter is essential for the future mass measurements of the rarest isotopes produced at RIBF. The impact of the new 123Pd result on the solar r-process abundances in a neutron star merger event is investigated by performing reaction network calculations of 20 trajectories with varying electron fraction Ye. It is found that the neutron capture cross section on 123Pd increases by a factor of 2.2 and β-delayed neutron emission probability, P1n, of 123Rh increases by 14\%. The neutron capture cross section on 122Pd decreases by a factor of 2.6 leading to pileup of material at A=122, thus reproducing the trend of the solar r-process abundances. The trend of the two-neutron separation energies (S2n) was investigated for the Pd isotopic chain. The new mass measurement with improved uncertainty excludes large changes of the S2n value at N=77. Such large increase of the S2n values before N=82 was proposed as an alternative to the quenching of the N=82 shell gap to reproduce r-process abundances in the mass region of A=112-124.
The neutron-rich 213Pb isotope was produced in the fragmentation of a primary 1 GeV A 238U beam, separated in FRS in mass and atomic number, and then implanted for isomer decay γ-ray spectroscopy with the RISING setup at GSI. A newly observed isomer and its measured decay properties indicate that states in 213Pb are characterized by the seniority quantum number that counts the nucleons not in pairs coupled to angular momentum J=0. The conservation of seniority is a consequence of a geometric phase associated with particle-hole conjugation, which becomes observable in semi-magic nuclei where nucleons half-fill the valence shell. The γ-ray spectroscopic observables in 213Pb are thus found to be driven by two mechanisms, particle-hole conjugation and seniority conservation, which are intertwined through a Berry phase.
Excited states in 174Re have been populated in fusion-evaporation reactions at the Australian National University, and γ-ray spectroscopy has been used to determine the level structure and to deduce the underlying nucleon configurations. The half-life of the bandhead of the Kπ=8−band has been measured to be 2.7(4) ns. A band built on an isomeric state of spin-parity (14−) and a half-life of 53(5) ns has been observed here for the first time, and has been determined to have a four-quasiparticle structure. Contrasting reduced-hindrance values for its decay are discussed in terms of deformation and configuration changes, as indicated by configuration-constrained potential energy surface calculations. The technical staff at the ANU 14UD accelerator facility are thanked for their excellent support. Funding is acknowledged from the UK Science and Technology Facilities Council under Grant No. ST/P005314/1Support for the ANU Heavy Ion Accelerator Facility operations through the Australian National Collaborative Research Infrastructure Strategy (NCRIS) program is acknowledged.
The β decay of 208Hg into the one-proton hole, one neutron-particle 20881Tl127 nucleus was investigated at CERN-ISOLDE. Shell-model calculations describe well the level scheme deduced, validating the proton-neutron interactions used, with implications for the whole of the N>126, Z
The β decay of 192Re117, which lies near the boundary between the regions of predicted prolate and oblate deformations, has been investigated using the KEK Isotope Separation System (KISS) in RIKEN Nishina Center. This is the first case in which a low-energy beam of rhenium isotope has been successfully extracted from an argon gas-stopping cell using a laser-ionization technique, following production via multi-nucleon transfer between heavy ions. The ground state of 192Re has been assigned Jπ=(0−) based on the observed β feedings and deduced logft values towards the 0+ and 2+ states in 192Os, which is known as a typical γ-soft nucleus. The shape transition from axial symmetry to axial asymmetry in the Re isotopes is discussed from the viewpoint of single-particle structure using the nuclear Skyrme-Hartree-Fock model.
The nucleus 212Po has been produced through the fragmentation of a 238U primary beam at 1 GeV/nucleon at GSI, separated with the FRagment Separator, FRS, and studied via isomer γ-decay spectroscopy with the RISING setup. Two delayed previously unknown γ rays have been observed. One has been attributed to the E3 decay of a 21− isomeric state feeding the α-emitting 45-s (18+) high-spin isomer. The other γ-ray line has been assigned to the decay of a higher-lying 23+ metastable state. These are the first observations of high-spin states above the 212Po (18+) isomer, by virtue of the selectivity obtained via ion-by-ion identification of 238U fragmentation products. Comparison with shell-model calculations points to shortfalls in the nuclear interactions involving high-j proton and neutron orbitals, to which the region around Z∼100 is sensitive.
Hindrance factors for K-forbidden E2 decays from multiquasiparticle isomers are analyzed in relation to mixing matrix elements that are often associated with chance near degeneracies, and a functional relationship is established. Focusing on effective matrix elements for ΔK=6 decays from three-quasiparticle isomers, significant configuration dependence is demonstrated.
De-excitation γ rays associated with an isomeric state of 186 Ta were investigated. The isomers were produced in multinucleon transfer reactions between a 136 Xe beam and a natural W target, and were collected and separated by the KEK Isotope Separation System. Two γ transitions with energies of 161.1(2) and 186.8(1) keV associated with an isomeric decay were observed for the first time. The half-life of the isomeric state of the neutral atom 186m Ta was deduced as 17(2) s. Based on the comparison with the previous measurements of the isomeric state using the ESR storage ring at GSI Darmstadt and the coupling of angular momenta of individual particle orbitals in odd-odd nuclei, a decay scheme of 186m Ta was proposed.
The structures of isomeric and collective states in neutron-rich hafnium isotopes have been investigated within the paired cranked Nilsson-Strutinsky-Bogoliubov (CNSB), and the unpaired cranked Nilsson-Strutinsky (CNS) formalisms. We show that by combining these two models, a good understanding of the formation of multiquasiparticle prolate isomers is achieved. The calculations show at angular momenta I≥35, well-deformed oblate collective rotation strongly competes energetically with the prolate noncollective states. Comparison is made with experimental data, where available, and with other model calculations.
This contribution will report on the experimental work on the level structure of 168Dy. The experimental data have been taken as part of the EURICA decay spectroscopy campaign at RIBF, RIKEN in November 2014. In the experiment, a 238U primary beam is accelerated up to 345 MeV/u with an average intensity of 12 pnA. The nuclei of interest are produced by in-flight fission of 238U impinging on Be target with a thickness of 5 mm. The excited states of 168Dy have been populated through the decay from a newly identified isomeric state and via the β decay from 168Tb. In this contribution, scientific motivations, experimental procedure and some preliminary results for this study are presented.
Neutron-rich 160,161,162Sm isotopes have been populated at the RIBF, RIKEN via β first time. β-coincident γ rays were observed in all three isotopes including γ rays from the isomeric decay of 160Sm and 162Sm. The isomers in 160Sm and 162Sm have previously been observed but have been populated via β decay for the first time. The isomeric state in 162Sm is assigned a configuration based on the decay pattern. The level schemes of 160Sm and 162Sm are presented. The ground states in the parent nuclei 160Pm and 162Pm are both assigned a configuration based on the population of states in the daughter nuclei. Blocked BCS calculations were performed to further investigate the spin-parities of the ground states in 160Pm, 161Pm, and 162Pm, and the isomeric state in 162Sm
High-K isomer decay rates are compared and interpreted, with an emphasis on the spin degree of freedom. It is argued that high-K values do not in themselves lead to K mixing. Rather, evidence is presented that the most important consideration is the isomer energy relative to a rotor whose moment of inertia is approximately 85____% of the rigid-body value. The high-spin limit to the occurrence of high-K isomers is then discussed in connection with predictions of competing oblate rotation-aligned structures. Finally, some observations are made regarding the use of K isomers as a tool to access exotic nuclei, including superheavy elements, and exotic nuclear structures.
The 8π spectrometer at TRIUMF-ISAC consists of 20 Compton-suppressed germanium detectors and various auxiliary devices. The Ge array, once used for studies of nuclei at high angular momentum, has been transformed into the world's most powerful device dedicated to radioactive-decay studies. Many improvements in the spectrometer have been made, including a high-throughput data acquisition system, installation of a moving tape collector, incorporation of an array of 20 plastic scintillators for β-particle tagging, 5 Si(Li) detectors for conversion electrons, and 10 BaF detectors for fast-lifetime measurements. Experiments can be performed where data from all detectors are collected simultaneously, resulting in a very detailed view of the nucleus through radioactive decay. A number of experimental programmes have been launched that take advantage of the versatility of the spectrometer, and the intense beams available at TRIUMF-ISAC. © 2006 American Institute of Physics.
We present information on the excited states in the prolate-deformed, neutron-rich nuclei 165;167Tb100;102. The nuclei of interest were synthesised following in-flight fission of a 345 MeV per nucleon 238U primary beam on a 2 mm 9Be target at the Radioactive Ion-Beam Factory (RIBF), RIKEN, Japan. The exotic nuclei were separated and identified event-by-event using the BigRIPS separator, with discrete energy gamma-ray decays from isomeric states with half-lives in the s regime measured using the EURICA gamma-ray spectrometer. Metastable-state decays are identified in 165Tb and 167Tb and interpreted as arising from hindered E1 decay from the 7 2 [523] single quasi-proton Nilsson configuration to rotational states built on the 3 2 [411] single quasi-proton ground state. These data correspond to the first spectroscopic information in the heaviest, odd-A terbium isotopes reported to date and provide information on proton Nilsson configurations which reside close to the Fermi surface as the 170Dy doubly-midshell nucleus is approached.
The population of metastable states produced in relativistic-energy fragmentation of a U-238 beam has been measured. For states with high angular momentum, I=17h and I=21.5h, a higher population than expected has been observed, with the discrepancy increasing with angular momentum. By considering two sources for the angular momentum, related to single-particle and collective motions, a much improved description of the experimental results can be obtained. In addition, new results on the structure of Fr-208, Ra-211 and Ac-216 are reported.
Knowledge of atomic masses is indispensable in nuclear structure and nuclear astrophysics. In the last two decades, majority of experimental data on nuclear masses stems from measurements of stored ions. The ions can be stored at low energies in a Penning trap or at significantly higher energies in a storage ring. In both cases revolution frequencies of ions trapped in a confining magnetic field are determined which enables high-precision mass measurements. In this contribution we sketch the experimental techniques and recent results. Future perspectives at the new generation radioactive beam facilities are outlined. © Copyright owned by the author(s).
High‐resolution gamma‐ray spectroscopy is essential to fully exploit the unique scientific opportunities at the next generation radioactive ion beam facilities such as the TRIUMF Isotope Separator and Accelerator (ISAC). At ISAC the 8π spectrometer and its associated auxiliary detectors is optimize for β‐decay studies while TIGRESS an array of segmented clover HPGe detectors has been designed for studies with accelerated beams. This paper gives a brief overview of these facilities and also presents recent examples of the diverse experimental program carried out at the 8π spectrometer.
A projectile fragmentation experiment has been performed to populate the neutron-rich A ∼ 190 mass region, approaching the Z = 82, N = 126 closed shell. A previously unreported isomer is found in 200Pt, being the first new example, from fragmentation reactions, of a seniority 4 state established from γ-γ coincidences.
High spin states in the neutron rich 188Os and 190Os nuclei have been populated using the 82Se + 192Os deep-inelastic reaction. The level schemes are extended up to spin I ≈21. The observed new structures are tentatively interpreted as fragments of rotational bands built on multi-quasiparticle configurations.
High-spin states in 177Ta have been studied using the 170Er(11B,4n) reaction. New intrinsic states have been observed corresponding to 3-, 5-, and 7-quasiparticle high-K structures. Rotational bands built on most of the 3-and 5-quasiparticle states and some transitions above the 7-quasiparticle states have been identified. Several isomers have been found, the longest lived being the 49/2-, 7-quasiparticle state with a meanlife of 192 μs. Configurations for the observed intrinsic states have been assigned on the basis of gK values, alignments, and decay properties. While the properties of most of the bands are consistent with their proposed configurations, the behavior of some, including the one built on Kπ = 21/2- 3-quasiparticle isomer, is not well understood. Multiquasiparticle blocking calculations based on the Lipkin-Nogami method, are in good agreement with the excitation energies of the experimentally observed states and all the predicted states close to the yrast line up to spin 49/2 have been observed experimentally. The calculations predict the existence of a 9-quasiparticle 67/2- yrast trap at ∼ 8.5 MeV.
The TRIUMF‐ISAC Gamma‐Ray Escape Suppressed Spectrometer (TIGRESS) is a state‐of‐the‐art γ‐ray spectrometer being constructed at the ISAC‐II radioactive ion beam facility at TRIUMF. TIGRESS will be comprised of twelve 32‐fold segmented high‐purity germanium (HPGe) clover‐type γ‐ray detectors, with BGO∕CsI(Tl) Compton‐suppression shields, and is currently operational at ISAC‐II in an early‐implementation configuration of six detectors. Results have been obtained for the first experiments performed using TIGRESS, which examined the A = 20, 21, and 29 isotopes of Na by Coulomb excitation.
One of the most successful descriptions of the structure of atomic nuclei is the spherical shell model. It, however, becomes impractical when moving away from closed-shell nuclei. Instead, it is the interplay between the macroscopic shape degrees of freedom and the microscopic nature of the underlying single-particle structure in a deformed basis that determines the nuclear structure. Being the heaviest nucleus precisely in the middle of, known, closed proton and neutron shells, 170Dy has become a central calibration point for tests of collective models of nuclear physics. However, besides one candidate transition from a previous experiment in Legnaro, Italy, no experimental information is available for this nucleus. Using the EURICA setup at RIKEN, which couples the worlds highest intensity in-flight fission facility with a high-efficiency HPGe array, an experiment in November 2014 produced 170Dy nuclei by in-flight fission of a 238U beam. The results from this experiment provide a wealth of information on this elusive nucleus, including the evolution of quadrupole collectivity, rigidity and higher order deformations, as well as the long sought for isomeric K = 6+ state, predicted to be exceptionally pure at mid-shell. These results provide us with a rich level scheme for discussing both single-particle and collective structures at mid-shell.
The nuclear structure of neutron-rich N > 126 nuclei has been investigated following their production via relativistic projectile fragmentation of a E/A = 1 GeV (238)U beam on a Be target. The preliminary analysis indicates the presence of previously unreported isomeric states in the N = 128 isotones (208)Hg and (209)Tl.
We report on the preliminary results from a study of the decay of the I-pi = 8(+) T-1/2 = 2 mu s isomer in Pd-96 performed as part of the Stopped-Beam RISING campaign within the Rare Isotope Investigation at GSI (RISING). The Pd-96 ions were produced following the projectile fragmentation of a 750 MeV per nucleon Ag-107 primary beam. The reaction products were separated and identified by the in-flight method using the GSI Fragment Separator. The residues of interest were stopped in a perspex stopper surrounded by an array of 15, seven-element germanium Cluster detectors. One of the goals of the current work is to investigate the population of high-spin states produced projectile fragmentation reactions using isomeric ratio measurements to infer information on the angular momentum population distribution. In this short contribution the method and results of determining the isomeric ratio for the I-pi = 8(+) microsecond isomer in Pd-96 nucleus are presented.
It is well known that high-K isomers have strongly hindered electromagnetic decay rates. However, the situation is more subtle in shape-transition regions, where there are competing high-spin isomers associated with shape changes. Nevertheless, a previous analysis of E2 decay rates from isomers in N ≈ 76 (A ≈ 130) nuclides enabled a degree of separation to be made between high-K prolate states and low-K oblate states. The analysis exploited the dependence of E2 reduced hindrance factors, fν, on the product of the valence nucleon numbers, NpNn. The present work investigates the application of these ideas to data for neutron-rich Z ≈ 76 (A ≈ 180) nuclides. Despite few such isomer data being available, it is found that the reduced hindrance factors can be used to help in distinguishing between three different $E2$-transition types. In particular, the decays from high-K prolate isomers to the ground-state band are more hindered than the corresponding decays from low-K oblate isomers. Other decays from low-K oblate isomers have collective E2 transition rates. These features may be helpful in future studies of nuclides in the neutron-rich Z ≈ 76 region.
The structure of 183W has been studied by employing the 176Yb(14C,α 3n) reaction at 68 MeV. Five previously known rotational structure with one-quasiparticle configurations have been extended to higher spin states, and five new rotational bands with three- and five-quasiparticle configurations and a γ-vibration of a one-quasiparticle structure have been newly identified. In the ν7/2-[503] and ν11/2+[615] rotational structures, a signal of an admixture of an octupole- vibrational structure has been observed in their in-band B(M1)/B(E2) ratios and gΚ factors. In the Κπ = 19-M rotational band, a Coriolis effect on the ν1/2-[510] neutron has been identified. In all, 17 Κ-forbidden transitions have been observed. Energies of intrinsic states below 4 MeV have been calculated based on the Blocked BCS theory, and they are used in support of the configuration assignments. © 2000 Elsevier Science B.V. All rights reserved. PACS: 21.10.Re; 21.10.Tg; 23.20.En; 23.20.Lv; 27.70.+q.