# Professor Phil Walker

## Professor of Physics

Qualifications: BA (1973, Cambridge), PhD (1978, Australian National University)

Email:
Phone: Work: 01483 68 6807
Room no: 18 BA 02

## 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 goes to various European countries, Australia, USA, Canada and Japan to carry out nuclear physics experiments. 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).

Nuclear isomers are discussed in: "Energy Traps in Atomic Nuclei", by Philip Walker and George Dracoulis, Nature 399, 35 (1999). See also "The Atomic Nucleus", by Phil Walker, New Scientist No. 2382 (1 October 2011).

## 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.

"Workshop on Next Generation Isomers": this was run on 2nd April 2007 at the University of Surrey. For photo and talks, see: http://personal.ph.surrey.ac.uk/~phs3ps/isomers07/

"50th Anniversary Symposium on Nuclear Sizes and Shapes": this was run 23rd-25th June 2008 at the University of Surrey. For photo and talks, see: http://www.ph.surrey.ac.uk/cnrp/page_3956

"Workshop on Nuclear Isomers: Structure and Applications": this was run 19th-21st May 2010 at the University of Surrey. For photo and talks, see: http://www.nucleartheory.net/Workshop_2010/

## Publications

### Highlights

• . (2016) 'Review of Metastable States in Heavy Nuclei'. Reports on Progress in Physics, 79 (7)

#### Abstract

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.

• . (2016) 'High-K isomerism in rotational nuclei'. PHYSICA SCRIPTA, 91 (1) Article number ARTN 013010
• . (2014) 'Isomer decay spectroscopy of Sm 164 and Gd 166: Midshell collectivity around N=100'. Physical Review Letters, 113 (26)

#### Abstract

© 2014 American Physical Society.Excited states in the N=102 isotones Gd166 and Sm164 have been observed following isomeric decay for the first time at RIBF, RIKEN. The half-lives of the isomeric states have been measured to be 950(60) and 600(140) ns for Gd166 and Sm164, respectively. Based on the decay patterns and potential energy surface calculations, including β6 deformation, a spin and parity of 6- has been assigned to the isomeric states in both nuclei. Collective observables are discussed in light of the systematics of the region, giving insight into nuclear shape evolution. The decrease in the ground-band energies of Gd166 and Sm164 (N=102) compared to Gd164 and Sm162 (N=100), respectively, presents evidence for the predicted deformed shell closure at N=100.

• . (2013) 'Direct observation of long-lived isomers in Bi212'. Physical Review Letters, 110 (12)

#### Abstract

Long-lived isomers in Bi212 have been studied following U238 projectile fragmentation at 670 MeV per nucleon. The fragmentation products were injected as highly charged ions into a storage ring, giving access to masses and half-lives. While the excitation energy of the first isomer of Bi212 was confirmed, the second isomer was observed at 1478(30) keV, in contrast to the previously accepted value of >1910 keV. It was also found to have an extended Lorentz-corrected in-ring half-life >30 min, compared to 7.0(3) min for the neutral atom. Both the energy and half-life differences can be understood as being due a substantial, though previously unrecognized, internal decay branch for neutral atoms. Earlier shell-model calculations are now found to give good agreement with the isomer excitation energy. Furthermore, these and new calculations predict the existence of states at slightly higher energy that could facilitate isomer deexcitation studies. © 2013 American Physical Society.

• . (2013) 'The ILIMA project at FAIR'. International Journal of Mass Spectrometry, 349-350, pp. 247-254.

#### Abstract

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.

• . (2013) 'Quasi-particle and collective magnetism: Rotation, pairing and blocking in high-K isomers'. Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 726 (4-5), pp. 675-679.

#### Abstract

For the first time, a wide range of collective magnetic g-factors gR, 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, Np-Nn. 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 (Np, Nn), 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 gR 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 gR 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.

• . (2012) 'Long-lived isomers in neutron-rich Z=72-76 nuclides'. PHYSICAL REVIEW C, 86 (5) Article number ARTN 054321

#### Abstract

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.

### Journal articles

• . (2017) 'Long-lived K isomer and enhanced gamma vibration in the neutron-rich nucleus 172Dy: Collectivity beyond double midshell'. Physics Letters B,
[ Status: Accepted ]

#### Abstract

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.

• . (2016) 'Review of Metastable States in Heavy Nuclei'. Reports on Progress in Physics, 79 (7)

#### Abstract

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.

• . (2016) 'New results from isochronous mass measurements of neutron-rich uranium fission fragments with the FRS-ESR-facility at GSI'. European Physical Journal A: Hadrons and Nuclei, 52 (138)

#### Abstract

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.

• . (2016) 'Intensity-sensitive and position-resolving cavity for heavy-ion storage rings'. Nuclear Inst. and Methods in Physics Research, A, 826, pp. 39-47.

#### Abstract

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.

• . (2016) 'First direct mass measurements of stored neutron-rich Cd-129,Cd-130,Cd-131 isotopes with FRS-ESR'. PHYSICS LETTERS B, 754, pp. 288-293.
• . (2016) 'Decay spectroscopy of Sm-160: The lightest four-quasiparticle K isomer'. PHYSICS LETTERS B, 753, pp. 182-186.
• . (2016) 'High-K isomerism in rotational nuclei'. PHYSICA SCRIPTA, 91 (1) Article number ARTN 013010
• . (2016) 'Low-lying states near the I-pi=6(+) isomer in Ag-108'. JOURNAL OF PHYSICS G-NUCLEAR AND PARTICLE PHYSICS, 43 (1) Article number ARTN 015103
• . (2015) 'Study of projectile fragmentation reaction with isochronous mass spectrometry'. PHYSICA SCRIPTA, T166 Article number ARTN 014009
• . (2015) 'First measurement of the beta-decay half-life of Au-206'. EPL, 111 (5) Article number ARTN 52001
• . (2015) 'Core excitations across the neutron shell gap in <sup>207</sup>Tl'. Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 747, pp. 88-92.

#### Abstract

© 2015 The Authors.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.

• . (2015) 'Between atomic and nuclear physics: radioactive decays of highly-charged ions'. JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 48 (14) Article number ARTN 144024
• . (2015) 'Increased isomeric lifetime of hydrogen-like Os-192m'. PHYSICAL REVIEW C, 91 (3) Article number ARTN 031301
• . (2015) 'Shape-coexisting rotation in neutron-deficient Hg and Pb nuclei'. Physical Review C - Nuclear Physics, 91 (3)

#### Abstract

© 2015 American Physical Society.For a shape-soft nucleus, the deformation change with increasing angular momentum of rotation can be significant. Total-Routhian-surface (TRS) calculations include the shape changes, but angular momentum is not conserved (neither is it a good quantum number, nor is it kept unchanged in the whole TRS mesh). In the projected shell model (PSM), the angular momentum appears as a good quantum number, but calculations have usually been performed with fixed deformation. In the present work, by performing angular-momentum projection on the mean-field potential-energy surface (PES), we can obtain an angular-momentum-conserved PES which gives deformation for a rotational state at a given spin. In order to investigate the shape-changing effect, we have chosen neutron-deficient Hg and Pb isotopes in which shape coexistence occurs. We interpret the irregular rotational behavior of the oblate bands at low spin as arising from deformation changes which are induced by collective rotation. At higher spin, the oblate rotational spectrum can also be influenced by the crossing between the K=0 ground-state band and a low-K two-quasineutron band. Calculated g factors for the states of oblate bands are given for future experimental testing, and the intrinsic structures of high-K oblate states are investigated.

• . (2015) 'Structure of <sup>207</sup>Pb populated in <sup>208</sup>Pb + <sup>208</sup>Pb deep-inelastic collisions'. Acta Physica Polonica B, 46 (3), pp. 619-622.

#### Abstract

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.

• . (2015) 'Spectroscopy of the low-lying states near the high spin isomer in <sup>108</sup>Ag'. Acta Physica Polonica B, 46 (3), pp. 703-707.

#### Abstract

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.

• . (2015) 'Isomeric ratios in <sup>206</sup>Hg'. Acta Physica Polonica B, 46 (3), pp. 601-605.

#### Abstract

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-ray tracking array worked properly and that the tracking efficiency was independent of the time relative to the prompt flash.

• . (2015) 'Octupole transitions in the 208Pb region'. Journal of Physics: Conference Series, 580 (1)

#### Abstract

© Published under licence by IOP Publishing Ltd.The 208Pb region is characterised by the existence of collective octupole states. Here we populated such states in 208Pb + 208Pb deep-inelastic reactions. γ-ray angular distribution measurements were used to infer the octupole character of several E3 transitions. The octupole character of the 2318 keV 17- 14+ in 208Pb, 2485 keV 19/2- 13/2+ in 207Pb, 2419 keV 15/2- 9/2+ in 209Pb and 2465 keV 17/2+ 11/2- in 207Tl transitions was demonstrated for the first time. In addition, shell model calculations were performed using two different sets of two-body matrix elements. Their predictions were compared with emphasis on collective octupole states.

• . (2014) 'Isomer decay spectroscopy of Sm 164 and Gd 166: Midshell collectivity around N=100'. Physical Review Letters, 113 (26)

#### Abstract

© 2014 American Physical Society.Excited states in the N=102 isotones Gd166 and Sm164 have been observed following isomeric decay for the first time at RIBF, RIKEN. The half-lives of the isomeric states have been measured to be 950(60) and 600(140) ns for Gd166 and Sm164, respectively. Based on the decay patterns and potential energy surface calculations, including β6 deformation, a spin and parity of 6- has been assigned to the isomeric states in both nuclei. Collective observables are discussed in light of the systematics of the region, giving insight into nuclear shape evolution. The decrease in the ground-band energies of Gd166 and Sm164 (N=102) compared to Gd164 and Sm162 (N=100), respectively, presents evidence for the predicted deformed shell closure at N=100.

• . (2014) 'George Dionisios Dracoulis obituary'. PHYSICS TODAY, 67 (11), pp. 66-66.
• . (2014) 'Isomeric decay spectroscopy of the Bi 217 isotope'. Physical Review C - Nuclear Physics, 90 (3)

#### Abstract

© 2014 American Physical Society.The structure of the neutron-rich bismuth isotope Bi217 has been studied for the first time. The fragmentation of a primary U238 beam at the FRS-RISING setup at GSI was exploited to perform γ-decay spectroscopy, since μs isomeric states were expected in this nucleus. Gamma rays following the decay of a t1/2=3 μs isomer were observed, allowing one to establish the low-lying structure of Bi217. The level energies and the reduced electric quadrupole transition probability B(E2) from the isomeric state are compared to large-scale shell-model calculations.

• . (2014) 'Half-life systematics across the N=126 shell closure: role of first-forbidden transitions in the β decay of heavy neutron-rich nuclei.'. Phys Rev Lett, United States: 113 (2)

#### Abstract

This Letter reports on a systematic study of β-decay half-lives of neutron-rich nuclei around doubly magic (208)Pb. The lifetimes of the 126-neutron shell isotone (204)Pt and the neighboring (200-202)Ir, (203)Pt, (204)Au are presented together with other 19 half-lives measured during the "stopped beam" campaign of the rare isotope investigations at GSI collaboration. The results constrain the main nuclear theories used in calculations of r-process nucleosynthesis. Predictions based on a statistical macroscopic description of the first-forbidden β strength reveal significant deviations for most of the nuclei with N<126. In contrast, theories including a fully microscopic treatment of allowed and first-forbidden transitions reproduce more satisfactorily the trend in the measured half-lives for the nuclei in this region, where the r-process pathway passes through during β decay back to stability.

• . (2014) 'Nuclear physics: Track it to the limit'. Nature Physics, 10 (5), pp. 338-339.
• . (2014) 'Influence of the octupole mode on nuclear high-K isomeric properties'. Physica Scripta, 89 (5)

#### Abstract

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.

• . (2014) 'Magnetic properties of Hf 177 and Hf 180 in the strong-coupling deformed model'. Physical Review C - Nuclear Physics, 89 (4)

#### Abstract

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.

• . (2014) 'Favored configurations for four-quasiparticle K isomerism in the heaviest nuclei'. Physical Review C - Nuclear Physics, 89 (4)

#### Abstract

Configuration-constrained potential-energy-surface calculations are performed including β6 deformation to investigate high-K isomeric states in nuclei around 254No and 270Ds, the heaviest nuclei where there have been some observations of two-quasiparticle isomers, while data for four-quasiparticle isomers are scarce. We predict the prevalent occurrence of four-quasiparticle isomeric states in these nuclei, together with their favored configurations. The most notable examples, among others, are Kπ=20+ states in 266,268Ds and 268,270Cn having very high K value, relatively low excitation energy, and well-deformed axially symmetric shape. The predicted isomeric states, with hindered spontaneous fission and α decay, could play a significant role in the future study of superheavy nuclei. © 2014 American Physical Society.

• . (2014) 'β -decay studies of neutron-rich Tl, Pb, and Bi isotopes'. Physical Review C - Nuclear Physics, 89 (1)

#### Abstract

The fragmentation of relativistic uranium projectiles has been exploited at the Gesellschaft für Schwerionenforschung laboratory to investigate the β decay of neutron-rich nuclei just beyond 208Pb. This paper reports on β-delayed γ decays of 211-213Tl, 215Pb, and 215-219Bi de-exciting states in the daughters 211-213Pb, 215Bi, and 215-219Po. The resulting partial level schemes, proposed with the help of systematics and shell-model calculations, are presented. The role of allowed Gamow-Teller and first-forbidden β transitions in this mass region is discussed. © 2014 American Physical Society.

• . (2014) 'Study of the level structure of 108Ag'. EPJ Web of Conferences, 66

#### Abstract

The high spin structure of 108Ag nucleus has been studied using the reaction 11B + 100Mo 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.

• . (2014) 'New isomers in the neutron-rich region beyond 208Pb'. EPJ Web of Conferences, 66

#### Abstract

The region of neutron-rich nuclei beyond 208Pb has been very difficult to explore due to its high mass and exoticity. However, recent experimental improvements allowed one to perform a quite extended isomer decay spectroscopy of these nuclei. © Owned by the authors, published by EDP Sciences, 2014.

• . (2014) 'Study of 207Tl126 produced in deep-inelastic reactions'. EPJ Web of Conferences, 66

#### Abstract

Deep-inelastic collisions of a 208Pb beam on a 208Pb target were performed using the ATLAS accelerator at Argonne National Laboratory. The Gammasphere detector array was used for the detection of prompt and delayed gamma-rays of the reaction products.207Tl is one proton away from the 208Pb doubly-magic nucleus. Its low-energy level structure is dominated by the single proton-hole states πs1/2-1, πd3/2-1 and πh11/2-1. The 11/2- state is isomeric with T1/2 = 1.33(11) s. The reaction partner of 207Tl is 209Bi, which has arelatively well established level scheme compared to 207Tl. Cross-coincidences between these two nuclei were used to confirm or establish levels above the 11/2- isomeric state in 207Tl. These states are obtained via breaking of the neutron core. Angular correlation analysis was performed on known transitions in 208Pb, proving the applicability of this method for multipolarity assignment. © Owned by the authors, published by EDP Sciences, 2014.

• . (2013) 'Nuclear physics experiments with ion storage rings'. Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms, 317 (PART B), pp. 603-616.

#### Abstract

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.

• . (2013) 'The Collinear Resonance Ionization Spectroscopy (CRIS) experimental setup at CERN-ISOLDE'. Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms, 317 (PART B), pp. 565-569.

#### Abstract

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.

• . (2013) 'New μs isomers in the neutron-rich 210Hg nucleus'. Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 725 (4-5), pp. 292-296.

#### Abstract

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 210Hg: the 8+ isomer expected from the seniority scheme in the νg9/2 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..

• . (2013) 'Schottky mass measurements of heavy neutron-rich nuclides in the element range 70 <= Z <= 79 at the GSI Experimental Storage Ring'. PHYSICAL REVIEW C, 88 (2) Article number ARTN 024310
• . (2013) 'Submicrosecond isomer in Rh-117(45)72 and the role of triaxiality in its electromagnetic decay rate'. PHYSICAL REVIEW C, 88 (2) Article number ARTN 024302
• . (2013) 'β-delayed γ-ray spectroscopy of 203,204Au and 200-202Pt'. Physical Review C - Nuclear Physics, 88 (1)

#### Abstract

The β decay of five heavy, neutron-rich nuclei, 203,204Pt and 200-202Ir, has been investigated following relativistic cold fragmentation reactions of lead projectiles using the FRS+RISING setup at GSI. This paper reports on the study of the low-lying states in the decay daughter nuclei 203,204Au and 200-202Pt. The characteristic γ rays for each nucleus have been determined using β-delayed γ-ray spectroscopy. Tentative level schemes, relative intensities, and apparent β feedings are provided. These data are compared with shell-model calculations, which indicate a substantial contribution to the total β strength from high-energy first-forbidden β-decay transitions in this mass region. © 2013 American Physical Society.

• . (2013) 'The population of metastable states as a probe of relativistic-energy fragmentation reactions'. Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 723 (4-5), pp. 302-306.

#### Abstract

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.

• . (2013) 'A dedicated decay-spectroscopy station for the collinear resonance ionization experiment at ISOLDE'. Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 707, pp. 35-39.

#### Abstract

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.

• . (2013) 'Direct observation of long-lived isomers in Bi212'. Physical Review Letters, 110 (12)

#### Abstract

Long-lived isomers in Bi212 have been studied following U238 projectile fragmentation at 670 MeV per nucleon. The fragmentation products were injected as highly charged ions into a storage ring, giving access to masses and half-lives. While the excitation energy of the first isomer of Bi212 was confirmed, the second isomer was observed at 1478(30) keV, in contrast to the previously accepted value of >1910 keV. It was also found to have an extended Lorentz-corrected in-ring half-life >30 min, compared to 7.0(3) min for the neutral atom. Both the energy and half-life differences can be understood as being due a substantial, though previously unrecognized, internal decay branch for neutral atoms. Earlier shell-model calculations are now found to give good agreement with the isomer excitation energy. Furthermore, these and new calculations predict the existence of states at slightly higher energy that could facilitate isomer deexcitation studies. © 2013 American Physical Society.

• . (2013) 'Core-coupled states and split proton-neutron quasiparticle multiplets in Ag122-126'. PHYSICAL REVIEW C, 87 (3) Article number ARTN 034308
• . (2013) 'New studies on the aspects of nuclear shapes'. Acta Physica Polonica B, 44 (3), pp. 271-282.

#### Abstract

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.

• . (2013) 'Core excitations across the neutron shell gap in 207Tl'. Acta Physica Polonica B, 44 (3), pp. 381-385.

#### Abstract

Deep-inelastic collisions of a 208Pb beam on a 208Pb target were performed using the ATLAS accelerator at Argonne National Laboratory. Prompt and delayed λ-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 λ-rays from its bettercharacterised reaction partner 209Bi. A number of new transitions were found in 207Tl.

• . (2013) 'Quasi-particle and collective magnetism: Rotation, pairing and blocking in high-K isomers'. Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 726 (4-5), pp. 675-679.

#### Abstract

For the first time, a wide range of collective magnetic g-factors gR, 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, Np-Nn. 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 (Np, Nn), 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 gR 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 gR 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.

• . (2013) 'The ILIMA project at FAIR'. International Journal of Mass Spectrometry, 349-350, pp. 247-254.

#### Abstract

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.

• . (2013) 'CRIS: A new method in isomeric beam production'. EPJ Web of Conferences, 63

#### Abstract

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.

• . (2013) 'Mixing effects on K-forbidden transition rates from the 6 isomers in the N = 104 isotones'. Journal of Physics G: Nuclear and Particle Physics, 40 (1)

#### Abstract

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.

• . (2013) 'Structure of nearly degenerate dipole bands in 108Ag'. Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 725 (1-3), pp. 85-91.

#### Abstract

The high spin negative parity states of 108Ag have been investigated with the 11B+100Mo 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 108Ag 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.

• . (2012) 'Long-lived isomers in neutron-rich Z=72-76 nuclides'. PHYSICAL REVIEW C, 86 (5) Article number ARTN 054321

#### Abstract

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.

• . (2012) 'New isomers in the full seniority scheme of neutron-rich lead isotopes: The role of effective three-body forces'. Physical Review Letters, 109 (16)

#### Abstract

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.

• . (2012) 'Hindered decays from a non-yrast four-quasiparticle isomer in 164Er'. Physical Review C - Nuclear Physics, 86 (4)

#### Abstract

The half-life of a Kπ=12 + isomer in 164Er has been measured to be 68(2) ns, and new decay pathways have been identified. These include highly K-forbidden γ-ray transitions directly to the ground-state rotational band, with reduced hindrance values that can be compared with those found for heavier nuclides. The new data support the interpretation that the level density is a key variable in determining K-forbidden transition rates. © 2012 American Physical Society.

• . (2012) 'On the possibility of enhanced fission stability for broken-pair excitations'. Journal of Physics G: Nuclear and Particle Physics, 39 (10)

#### Abstract

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.

• . (2012) 'Competition between collective oblate rotation and non-collective prolate K isomerism in neutron-rich tungsten isotopes'. Science China: Physics, Mechanics and Astronomy, 55 (9), pp. 1613-1617.

#### Abstract

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.

• . (2012) 'Understanding the different rotational behaviors of No-252 and No-254'. PHYSICAL REVIEW C, 86 (1) Article number ARTN 011301
• . (2012) 'Superdeformed multi-quasiparticle high-K states and possible isomers in Pb and Po isotopes'. PHYSICAL REVIEW C, 85 (6) Article number ARTN 064304
• . (2012) 'High-K isomers in neutron-rich zirconium isotopes'. PHYSICAL REVIEW C, 85 Article number 027307
• . (2012) 'Discovery of isomers in dysprosium, holmium, and erbium isotopes with N=94 to 97'. PHYSICAL REVIEW C, 85 Article number 024313
• . (2012) 'Study of isomer production rates for A = 142-152 and Z = 62-67 in fragmentation of a relativistic 208Pb beam'. Acta Physica Polonica B, 43 (2), pp. 253-259.

#### Abstract

We have investigated nuclear fragmentation reactions of a relativistic 208Pb beam.Ten isomeric states for nuclei with A = 142-152 and Z = 62-67 were observed. Measured isomeric ratios were compared, together with values from other experiments, with prediction of theoretical models. The discrepancies between the experimental and theoretical values were discussed in terms of transitions by-passing the isomer that are not included in the models.

• . (2012) 'Storage ring at HIE-ISOLDE: Technical design report: Technical design report'. European Physical Journal: Special Topics, 207 (1), pp. 1-117.

#### Abstract

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.

• . (2012) 'β decay of 102Y produced in projectile fission of 238U'. Journal of Physics: Conference Series, 381 (1)

#### Abstract

The population of 102Zr following the β decay of 102Y produced in the projectile fission of 238U at the GSI facility in Darmstadt, Germany has been studied. 102Y is known to decay into 102Zr via two states, one of high spin and the other low spin. These states preferentially populate different levels in the 102Zr daughter. In this paper the intensities of transitions in 102Zr 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.

• . (2012) 'Magnetic moments of K isomers as indicators of octupole collectivity'. European Physical Journal A, 48 (6), pp. 1-12.

#### Abstract

The relation between the quadrupole-octupole deformation and the structure of high-K isomers in heavy even-even nuclei is studied through a reflection asymmetric deformed shell model including a BCS procedure with constant pairing interaction. Two-quasiparticle states with Kπ = 4-, 5-, 6-, 6+ and 7- are considered in the region of actinide nuclei (U, Pu and Cm) and rare-earth nuclei (Nd, Sm and Gd). The behaviour of two-quasiparticle energies and magnetic dipole moments of these configurations is examined over a wide range in the plane of quadrupole and octupole deformations (β2 and β3. In all considered actinide nuclei, the calculations show that there is pronounced sensitivity of the magnetic moments to the octupole deformation. In the rare-earth nuclei, the calculations for 154, 156Gd show stronger sensitivity of the magnetic moment to the octupole deformation than in the other considered cases. © 2012 SIF, Springer-Verlag Berlin Heidelberg.

• . (2012) 'Technique for resolving low-lying isomers in the experimental storage ring (ESR) and the occurrence of an isomeric state in Re'. Journal of Physics: Conference Series, 381 (1)

#### Abstract

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.

• . (2012) 'Multiple β - Decaying states in 194Re: Shape evolution in neutron-rich osmium isotopes'. Physical Review C - Nuclear Physics, 85 (3) Article number 034301

#### Abstract

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.

• . (2012) 'First measurement of beta decay half-lives in neutron-rich Tl and Bi isotopes'. Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 715 (4-5), pp. 293-297.

#### Abstract

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.

• . (2012) 'New results on mass measurements of stored neutron-rich nuclides in the element range from Pt to U with the FRS-ESR facility at 360-400MeV/u'. Nuclear Physics A, 882, pp. 71-89.

#### Abstract

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.

• . (2012) 'Search for spin-orbit-force reduction at 106,108Zr around r-process path'. AIP Conference Proceedings, 1484, pp. 156-160.

#### Abstract

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.

• . (2012) 'Single-particle isomeric states in 121Pd and 117Ru'. Journal of Physics: Conference Series, 366 (1)

#### Abstract

Neutron-rich nuclei were populated in a relativistic fission of 238U. Gamma-rays with energies of 135 keV and 184 keV were associated with two isomeric states in 121Pd and 117Ru. 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.

• . (2012) 'Residual interactions and the K-mixing-induced fast decay of the three-quasiparticle isomer in Tm-171'. Physical Review C (Nuclear Physics), 85 (2) Article number 024324

#### Abstract

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.

• . (2012) 'AGATA - Advanced GAmma Tracking Array'. Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 668, pp. 26-58.
• . (2011) 'THE ATOMIC NUCLEUS'. NEW SCIENTIST, 211 (2832), pp. I-VIII.
• . (2011) 'Deformation and its influence on K isomerism in neutron-rich Hf nuclei'. PHYSICAL REVIEW C, 83 (6) Article number ARTN 067303
• . (2011) 'Structural Evolution in the Neutron-Rich Nuclei Zr-106 and Zr-108'. PHYSICAL REVIEW LETTERS, 106 (20) Article number ARTN 202501
• . (2011) 'NUCLEAR ASTROPHYSICS Star bursts pinned down'. NAT PHYS, 7 (4), pp. 281-282.
• . (2011) 'Discovery of a nonyrast K-pi=8(+) isomer in Dy-162, and the influence of competing K-mixing mechanisms on its highly forbidden decay'. PHYSICAL REVIEW C, 83 (3) Article number ARTN 034322
• . (2011) 'β-Decay half-lives of very neutron-rich Kr to Tc isotopes on the boundary of the r-process path: An indication of fast r-matter flow'. Physical Review Letters, 106 (5) Article number 052502 , pp. ---.

#### Abstract

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.

• . (2011) 'NEW STRUCTURES IN (HF)-H-178 AND COULOMB EXCITATION OF ISOMERS'. INTERNATIONAL JOURNAL OF MODERN PHYSICS E-NUCLEAR PHYSICS, 20 (2), pp. 474-481.
• . (2011) 'Low-lying level structure of the neutron-rich nucleus Nb-109: A possible oblate-shape isomer'. PHYSICS LETTERS B, 696 (3), pp. 186-190.
• . (2011) 'Effects of high-order deformation on high-K isomers in superheavy nuclei'. Physical Review C - Nuclear Physics, 83 (1) Article number 011303(R) , pp. ---.

#### Abstract

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.

• . (2011) 'Isomeric states observed in heavy neutron-rich nuclei populated in the fragmentation of a (208)Pb beam'. PHYSICAL REVIEW C, 84 (4) Article number ARTN 044313
• . (2011) 'On the stability of high-K isomers in the second well of actinide nuclei'. European Physical Journal A, 47 (11), pp. 1-5.

#### Abstract

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.

• . (2011) 'Development of axial asymmetry in the neutron-rich nucleus Mo-110'. PHYSICS LETTERS B, 704 (4), pp. 270-275.
• . (2010) 'Multi-quasiparticle excitation: Extending shape coexistence in A similar to 190 neutron-deficient nuclei'. PHYSICAL REVIEW C, 82 (4) Article number ARTN 044314
• . (2010) 'Discovery of Highly Excited Long-Lived Isomers in Neutron-Rich Hafnium and Tantalum Isotopes through Direct Mass Measurements'. PHYSICAL REVIEW LETTERS, 105 (17) Article number ARTN 172501
• . (2010) 'Shape coexistence and isomeric states in neutron-rich Tc-112 and Tc-113'. PHYSICAL REVIEW C, 82 (4) Article number ARTN 044312
• . (2010) 'A digital method for separation and reconstruction of pile-up events in germanium detectors'. REVIEW OF SCIENTIFIC INSTRUMENTS, 81 (10) Article number ARTN 103507
• . (2010) 'Direct measurement of the 4.6 MeV isomer in stored bare Sb-133 ions'. PHYSICS LETTERS B, 688 (4-5), pp. 294-297.
• . (2010) 'Configuration dependence of K-forbidden transition rates from three-quasiparticle isomers'. PHYSICAL REVIEW C, 81 (4) Article number ARTN 041304
• . (2010) 'Determination of gas amplification factor by digital waveform analysis of avalanche counter signals'. NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT, 615 (1), pp. 53-56.
• . (2010) 'Structures of Po-201 and Rn-205 from EC/beta(+)-decay studies'. PHYSICAL REVIEW C, 81 (2) Article number ARTN 024322
• . (2010) 'High-K isomers as probes of octupole collectivity in heavy nuclei'. Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 694 (2), pp. 119-122.

#### Abstract

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.

• . (2010) 'K-hindered decay of a six-quasiparticle isomer in Hf176'. Physical Review C - Nuclear Physics, 82 (5) Article number 054316
• . (2010) 'Discovery and investigation of heavy neutron-rich isotopes with time-resolved Schottky spectrometry in the element range from thallium to actinium'. Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 691 (6), pp. 234-237.

#### Abstract

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.

• . (2010) 'Application of digital zero-crossing technique for neutron-gamma discrimination in liquid organic scintillation detectors'. Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 621 (1-3), pp. 498-501.

#### Abstract

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.

• . (2010) 'Time walk correction of CdTe detectors using depth sensing technique'. Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 621 (1-3), pp. 506-512.

#### Abstract

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.

• . (2009) 'Isomeric states in neutron-deficient A similar to 80-90 nuclei populated in the fragmentation of Ag-107'. PHYSICAL REVIEW C, 80 (6) Article number ARTN 064303
• . (2009) 'Nuclear structure "southeast" of Pb-208: Isomeric states in Hg-208 and Tl-209'. PHYSICAL REVIEW C, 80 (6) Article number ARTN 061302
• . (2009) 'beta(-)-delayed spectroscopy of neutron-rich tantalum nuclei: Shape evolution in neutron-rich tungsten isotopes'. PHYSICAL REVIEW C, 80 (6) Article number ARTN 064308
• . (2009) 'Narrowing of the neutron sd-pf shell gap in Na-29'. PHYSICS LETTERS B, 674 (3), pp. 168-171.
• . (2009) 'Fast decay of a three-quasiparticle isomer in Tm-171'. PHYSICAL REVIEW C, 79 (4) Article number ARTN 044321
• . (2009) 'Schottky Mass Measurement of the Hg-208 Isotope: Implication for the Proton-Neutron Interaction Strength around Doubly Magic Pb-208'. PHYSICAL REVIEW LETTERS, 102 (12) Article number ARTN 122503
• . (2009) 'Weakly deformed oblate structures in Os-198(76)122'. PHYSICAL REVIEW C, 79 (3) Article number ARTN 031305
• . (2009) 'Proton-hole excitation in the closed shell nucleus Au-205'. PHYSICS LETTERS B, 672 (2), pp. 116-119.
• . (2009) 'Isomeric states in 208Hg and 209Tl populated in fragmentation of 238U'. Acta Physica Polonica B, 40 (3), pp. 871-874.

#### Abstract

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.

• . (2009) 'Isomeric decay studies in neutron-rich N ≈ 126 nuclei'. International Journal of Modern Physics E, 18 (4), pp. 1002-1007.

#### Abstract

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.

• . (2008) 'Single-particle behavior at N=126: Isomeric decays in neutron-rich Pt-204'. PHYSICAL REVIEW C, 78 (6) Article number ARTN 061302
• . (2008) 'Nuclear structure studies of short-lived neutron-rich nuclei with the novel large-scale isochronous mass spectrometry at the FRS-ESR facility'. NUCLEAR PHYSICS A, 812, pp. 1-12.
• . (2008) 'Nuclear physics - A neutrino's wobble?'. NATURE, 453 (7197), pp. 864-865.
• . (2008) 'High-spin, multiparticle isomers in (121,123)Sb'. PHYSICAL REVIEW C, 77 (3) Article number ARTN 034311
• . (2008) 'Neutron-proton pairing competition in N = Z nuclei: Metastable state decays in the proton dripline nuclei Nb-82(41) and Tc-86(43)'. PHYSICS LETTERS B, 660 (4), pp. 326-330.
• . (2008) 'Rotation-driven prolate-to-oblate shape phase transition in W-190: A projected shell model study'. PHYSICS LETTERS B, 659 (1-2), pp. 165-169.
• . (2008) 'First results with the rising active stopper'. International Journal of Modern Physics E, 17 (SUPPL. 1), pp. 8-20.
• . (2008) 'Erratum to: "Neutron-proton pairing competition in N = Z nuclei: Metastable state decays in the proton dripline nuclei 82 41Nb and 86 43Tc"'. Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 668 (5), pp. 460-460.
• . (2007) 'Two-proton radioactivity - Caught in the act'. NATURE PHYSICS, 3 (12), pp. 836-837.
• . (2007) 'Identification of a high-spin isomer in (99)Mo'. PHYSICAL REVIEW C, 76 (4) Article number ARTN 047303
• . (2007) 'High-spin isomeric structures in exotic odd-odd nuclei: Exploration of the proton drip line and beyond'. PHYSICAL REVIEW C, 76 (3) Article number ARTN 034313
• . (2007) 'Collective rotation and vibration in neutron-rich Hf-180,Hf-182 nuclei'. PHYSICAL REVIEW C, 75 (3) Article number ARTN 034305
• . (2007) 'Discovery of a new long-lived isomeric state in Ce-125'. EUROPEAN PHYSICAL JOURNAL A, 31 (3), pp. 393-394.
• . (2007) 'Isomeric ratio for the I-pi=8(+) yrast state in Pd-96 produced in the relativistic fragmentation of Ag-107'. ACTA PHYSICA POLONICA B, Zakopane, POLAND: 38 (4), pp. 1277-1282.
• . (2007) 'Status of the experimental program on mass measurements of stored exotic nuclei at the FRS-ESR facility'. NUCLEAR PHYSICS A, Rio de Janeiro, BRAZIL: 787, pp. 315C-320C.
• . (2007) 'Isomer spectroscopy using relativistic projectile fragmentation at the N=Z line for A similar to 80 -> 90.'. NUCLEAR PHYSICS A, Rio de Janeiro, BRAZIL: 787, pp. 491C-498C.
• . (2007) 'beta decay of Na-32'. PHYSICAL REVIEW C, 75 (1) Article number ARTN 017302
• . (2007) 'Angular momentum orientation at bandcrossings in rotating nuclei'. JOURNAL OF PHYSICS G-NUCLEAR AND PARTICLE PHYSICS, 34 (1), pp. 123-127.
• . (2007) 'Isomeric decay studies around Pt-204 and Tb-148'. EUROPEAN PHYSICAL JOURNAL-SPECIAL TOPICS, Cortina, ITALY: 150, pp. 165-168.
• . (2007) 'New developments for isochronous mass measurements of short-lived nuclei'. AIP Conference Proceedings, 891, pp. 199-204.

#### Abstract

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.

• . (2006) 'Shapes of neutron-rich A approximate to 190 odd-odd nuclei'. PHYSICAL REVIEW C, 74 (6) Article number ARTN 067303
• . (2006) 'Excited states in neutron-rich W-188 produced by an O-18-induced 2-neutron transfer reaction'. EUROPEAN PHYSICAL JOURNAL A, 30 (2), pp. 391-396.
• . (2006) 'Shape-driving effects in the triaxial nucleus, Xe-128'. PHYSICAL REVIEW C, 74 (3) Article number ARTN 034318
• . (2006) 'Observation of states beyond band termination in Er-156,Er-157,Er-158 and strongly deformed structures in Hf-173,Hf-174,Hf-175'. PHYSICA SCRIPTA, Lund, SWEDEN: T125, pp. 123-126.
• . (2006) 'Prediction and possible observation of an oblate shape isomer in W-190'. PHYSICS LETTERS B, 635 (5-6), pp. 286-289.
• . (2006) 'K-pi=0(+) 2.29 s isomer in neutron-rich Tm-174'. PHYSICAL REVIEW C, 73 (2) Article number ARTN 024306
• . (2006) 'High angular momentum states populated in fragmentation reactions'. PHYSICS LETTERS B, 632 (2-3), pp. 203-206.
• . (2006) 'Shapes of neutron-rich A≈190 odd-odd nuclei'. Physical Review C - Nuclear Physics, 74 (6)

#### Abstract

The occurrence of oblate and prolate shapes is investigated theoretically for odd-odd neutron-rich nuclei with A≈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. © 2006 The American Physical Society.

• . (2006) 'Yrast states in 188,190 Os nuclei'. International Journal of Modern Physics E, 15 (8), pp. 1797-1802.

#### Abstract

Yrast states in neutron rich nuclei 188Os and 190Os have been populated using the deep-inelastic reaction 82Se + 192Os at 460 MeV beam bombarding energy. A thick 192Os target (>50 mg/cm2) with 0.2 mm Ta backing was used to stop all of the recoils in the target, minimising the broadening of the lines due to Doppler shift. High fold γ-γ coincidences were aquired with the 4π spectrometer GASP detector array. Three and two dimensional gamma-ray matrices were used to construct level schemes of the nuclei of interest up to an yrast state of 22. © World Scientific Publishing Company.

• . (2006) 'Neutron-deficient N approximate to 126 nuclei produced in U-238 fragmentation: Population of high-spin states'. Frontiers in Nuclear Structure Astrophysics, and Reactions: FINUSTAR, Kos, GREECE: 831, pp. 114-118.

### Conference papers

• . (2015) 'Heavy Rotation - Evolution of quadrupole collectivity centred at the neutron-rich doubly mid-shell nucleus Dy-170'. AMER INST PHYSICS NUCLEAR STRUCTURE AND DYNAMICS '15, Portoroz, SLOVENIA: 3rd International Conference on Nuclear Structure and Dynamics 1681
• . (2014) 'ANGULAR DISTRIBUTIONS OF gamma RAYS FROM Bi-210 PRODUCED IN Pb-208+Pb-208 DEEP-INELASTIC REACTIONS'. WYDAWNICTWO UNIWERSYTETU JAGIELLONSKIEGO ACTA PHYSICA POLONICA B, Piaski, POLAND: 33rd Mazurian Lakes Conference on Physics 45 (2), pp. 205-210.
• . (2012) 'Technique for Resolving Low-lying Isomers in the Experimental Storage Ring (ESR) and the Occurrence of an Isomeric State in Re-192'. IOP PUBLISHING LTD RUTHERFORD CENTENNIAL CONFERENCE ON NUCLEAR PHYSICS, 2011, Univ Manchester, Manchester, ENGLAND: Rutherford Centennial Conference on Nuclear Physics 381
• . (2011) 'Isomers in neutron-rich lead isotopes populated via the fragmentation of 238U at 1 GeV A'. Institute of Physics Journal of Physics: Conference Series: International Nuclear Physics Conference, Vancouver, Canada: INPC 2010 312 (Section 9)

#### Abstract

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.

• . (2010) 'Precise measurement of nuclear isomers in the storage ring at GSI'. ELSEVIER SCIENCE BV NUCLEAR PHYSICS A, Beijing, PEOPLES R CHINA: 10th International Conference on Nucleus-Nucleus Collisions (NN2009) 834 (1-4), pp. 476C-478C.
• . (2010) 'High-spin isomers: structure and applications'. ELSEVIER SCIENCE BV NUCLEAR PHYSICS A, Beijing, PEOPLES R CHINA: 10th International Conference on Nucleus-Nucleus Collisions (NN2009) 834 (1-4), pp. 22C-27C.
• . (2010) 'Mass and lifetime measurements of stored exotic nuclei'. Proceedings of Science, Heidelberg, Germany: 11th Symposium on Nuclei in the Cosmos (NIC XI)

#### Abstract

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).

• . (2009) 'Structure of neutron-rich nuclei around the N=126 closed shell; the yrast structure of Au-205(126) up to spin-parity I-pi = (19/2(+))'. SPRINGER EUROPEAN PHYSICAL JOURNAL A, Ryn, POLAND: 5th International Conference on Exotic Nuclei and Atomic Masses (ENAM'08) 42 (3), pp. 489-493.
• . (2009) 'beta(-)-DELAYED AND ISOMER SPECTROSCOPY OF NEUTRON-RICH Ta AND W ISOTOPES'. WYDAWNICTWO UNIWERSYTETU JAGIELLONSKIEGO ACTA PHYSICA POLONICA B, Zakopane, POLAND: 43rd Zakopane Conference on Nuclear Physics 40 (3), pp. 875-878.
• . (2009) 'REVISION OF THE K-ISOMER IN W-190(116)'. WYDAWNICTWO UNIWERSYTETU JAGIELLONSKIEGO ACTA PHYSICA POLONICA B, Zakopane, POLAND: 43rd Zakopane Conference on Nuclear Physics 40 (3), pp. 885-888.
• . (2009) 'ISOMERIC RATIOS FOR NUCLEI WITH Z=62-67 AND A=142-152 PRODUCED IN THE RELATIVISTIC FRAGMENTATION OF Pb-208'. WYDAWNICTWO UNIWERSYTETU JAGIELLONSKIEGO ACTA PHYSICA POLONICA B, Zakopane, POLAND: 43rd Zakopane Conference on Nuclear Physics 40 (3), pp. 879-883.
• . (2009) 'ANGULAR MOMENTUM POPULATION IN FRAGMENTATION REACTIONS'. POLISH ACAD SCIENCES INST PHYSICS ACTA PHYSICA POLONICA B, Zakopane, POLAND: 43rd Zakopane Conference on Nuclear Physics 40 (3), pp. 889-892.
• . (2009) 'beta-DELAYED gamma-RAY SPECTROSCOPY OF HEAVY NEUTRON RICH NUCLEI "SOUTH" OF LEAD'. POLISH ACAD SCIENCES INST PHYSICS ACTA PHYSICA POLONICA B, Zakopane, POLAND: 43rd Zakopane Conference on Nuclear Physics 40 (3), pp. 867-870.
• . (2009) 'LARGE-SCALE MASS MEASUREMENTS OF SHORT-LIVED NUCLIDES WITH THE ISOCHRONOUS MASS SPECTROMETRY AT GSI'. WORLD SCIENTIFIC PUBL CO PTE LTD INTERNATIONAL JOURNAL OF MODERN PHYSICS E-NUCLEAR PHYSICS, Lanzhou, PEOPLES R CHINA: 7th International Conference on Nuclear Physics at Storage Rings 18 (2), pp. 346-351.
• . (2009) 'First results with TIGRESS and accelerated radioactive ion beams from ISAC: Coulomb excitation of20,21,29Na'. AIP Conference Proceedings: 20th International Conference on Application of Accelerators in Research and Industry, Fort Worth, Texas: APPLICATION OF ACCELERATORS IN RESEARCH AND INDUSTRY 1099, pp. 754-759.

#### Abstract

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.

• . (2009) 'Structure of N >= 126 nuclei produced in fragmentation of (238)U'. AMER INST PHYSICS CAPTURE GAMMA-RAY SPECTROSCOPY AND RELATED TOPICS, Univ Cologne, Inst Nucl Phys, Cologne, GERMANY: 13th International Symposium on Capture Gamma-Ray Spectroscopy and Related Topics 1090, pp. 145-148.
• . (2009) 'Gamma-ray spectroscopy at TRIUMF-ISAC: The new frontier of radioactive ion beam research'. AIP Conference Proceedings: First Ulaanbaatar Conference on Nuclear Physics and Applications, Ulaanbaatar, Mongolia: NUCLEAR PHYSICS AND APPLICATIONS: First Ulaanbaatar Conference on Nuclear Physics and Applications 1109, pp. 19-26.

#### Abstract

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.

• . (2009) 'New Insights into the Structure of Exotic Nuclei Using the RISING Active Stopper'. AMER INST PHYSICS CAPTURE GAMMA-RAY SPECTROSCOPY AND RELATED TOPICS, Univ Cologne, Inst Nucl Phys, Cologne, GERMANY: 13th International Symposium on Capture Gamma-Ray Spectroscopy and Related Topics 1090, pp. 122-129.
• . (2007) 'Exciting isomers from the first stopped-beam RISING campaign'. EDP SCIENCES S A EUROPEAN PHYSICAL JOURNAL-SPECIAL TOPICS, Cortina, ITALY: 7th International Conference on Radioactive Nuclear Beams 150, pp. 173-176.
• . (2007) 'Recent results in fragmentation isomer spectroscopy with rising'. ELSEVIER SCIENCE BV NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, Ft Worth, TX: 19th International Conference on Application of Accelerators in Research and Industry 261 (1-2), pp. 1079-1083.
• . (2007) 'Superallowed beta decay studies at TRIUMF - Nuclear structure and fundamental symmetries'. ACTA PHYSICA POLONICA B, JAGELLONIAN UNIV, INST PHYSICS ACTA PHYSICA POLONICA B, Zakopane, POLAND: 41st Zakopane Conference on Nuclear Physics 38 (4), pp. 1179-1194.
• . (2007) 'Identification of excited states in the N = Z nucleus Nb-82'. ACTA PHYSICA POLONICA B, JAGELLONIAN UNIV, INST PHYSICS ACTA PHYSICA POLONICA B, Zakopane, POLAND: 41st Zakopane Conference on Nuclear Physics 38 (4), pp. 1271-1275.
• . (2007) 'Identification of isomeric states 'south' of Pb-208 via projectile fragmentation'. ACTA PHYSICA POLONICA B, JAGELLONIAN UNIV, INST PHYSICS ACTA PHYSICA POLONICA B, Zakopane, POLAND: 41st Zakopane Conference on Nuclear Physics 38 (4), pp. 1283-1286.
• . (2007) 'First results from the stopped beam isomer rising campaign at GSI'. ACTA PHYSICA POLONICA B, JAGELLONIAN UNIV, INST PHYSICS ACTA PHYSICA POLONICA B, Zakopane, POLAND: 41st Zakopane Conference on Nuclear Physics 38 (4), pp. 1255-1264.
• . (2007) 'Isomeric states in the light Tc isotopes'. ACTA PHYSICA POLONICA B, JAGELLONIAN UNIV, INST PHYSICS ACTA PHYSICA POLONICA B, Zakopane, POLAND: 41st Zakopane Conference on Nuclear Physics 38 (4), pp. 1265-1269.
• . (2007) 'First results from the stopped RISING campaign at GSI: The mapping of isomeric decays in highly exotic nuclei'. AMER INST PHYSICS Six International Conference of the Balkan Physical Union, Istanbul, TURKEY: 6th International Conference of the Balkan-Physical-Union 899, pp. 19-22.
• . (2007) 'Rare modes of decay from high-spin isomers'. AMER INST PHYSICS PROTON EMITTING NUCLEI AND RELATED TOPICS, Lisbon, PORTUGAL: International Conference on Proton Emitting Nuclei and Related Topics 961, pp. 273-277.
• . (2006) 'Yrast states in Os-188,Os-190 nuclei'. WORLD SCIENTIFIC PUBL CO PTE LTD INTERNATIONAL JOURNAL OF MODERN PHYSICS E-NUCLEAR PHYSICS, Shanghai, PEOPLES R CHINA: International Conference on Frontiers of Nuclear Structure 15 (8), pp. 1797-1802.
• . (2006) 'K=6(+) isomers in Hf, Yb and W nuclei'. WORLD SCIENTIFIC PUBL CO PTE LTD INTERNATIONAL JOURNAL OF MODERN PHYSICS E-NUCLEAR PHYSICS, Shanghai, PEOPLES R CHINA: International Conference on Nuclear Structure Physics 15 (7), pp. 1653-1663.
• . (2006) 'Isomer beams'. WORLD SCIENTIFIC PUBL CO PTE LTD INTERNATIONAL JOURNAL OF MODERN PHYSICS E-NUCLEAR PHYSICS, Shanghai, PEOPLES R CHINA: International Conference on Nuclear Structure Physics 15 (7), pp. 1637-1644.
• . (2006) 'Isomers in the region of neutron-rich Hf-188'. AMER CHEMICAL SOC ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, Atlanta, GA: 231st National Meeting of the American-Chemical-Society 231
• . (2006) 'Gamma-ray spectroscopy at TRIUMF-ISAC'. AIP Conference Proceedings, 819, pp. 249-253.

#### Abstract

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 BaF2 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.

• . (2006) 'Nuclear isomers: stepping stones to the unknown'. AMER INST PHYSICS Capture Gamma-Ray Spectroscopy and Related Topics, Univ Notre Dame, Notre Dame, IN: 12th International Symposium on Capture Gamma-Ray Spectroscopy and Related Topics 819, pp. 16-23.
• . (2006) 'Microsecond and nanosecond isomers populated in fission reactions'. AMER INST PHYSICS FUSION06: Reaction Mechanisms and Nuclear Structure at the Coulomb Barrier, Coulomb Barrier, Venice, ITALY: International Conference on Reaction Mechanisms and Nuclear Structure 853, pp. 342-349.

## 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 Soddy, who referred to them as being “different in their stability and mode of breaking up”, 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, dI, from the ground state. For gamma-ray decay, the transition multipole order, L, must be greater than or equal to dI. 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 isomer references above, and articles in my biography.

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