Bent Herskind was a Gamma-Ray Grand Master, who helped trigger the modern revolution in γ -ray spectroscopy, opening the door to new vistas in nuclear structure physics. His story is a remarkable one and the contributions he made to the field in terms of its scientific richness and to the development of ever more powerful detector systems, were unique. The enthusiasm and excitement he put into everything were infectious and the brilliance of his insights was compelling. His legacy will live on, not only in the significant discoveries that he made, but also in the way he instilled his deep commitment to unravelling nature’s secrets in this quest and the pleasure of physics discovery with his close collaborators. This review article summarises his career and highlights some of his scientific achievements in which the authors had the privilege to collaborate with him.

Observed rotational bands that terminate or appear to terminate at very high spin are analyzed within the configuration constrained cranked Nilsson-Strutinsky (unpaired CNS or CNSB with pairing) formalism. Spin values for the nuclei discussed reach or come close to the maximum spin that can be built within the Z,N=50–82 shells. Configurations are distinguished not only by the number of particles in high-j and low-j shells within each N shell but, in some cases, also by the number of particles in pseudospin partners like d5/2g7/2 and s1/2d3/2. Configurations in Dy156 and Hf164, which terminate at I≈60, are well understood in terms of their occupation of open j shells or groups of j shells. The bands in Dy156 are tentatively observed up to termination while the bands in Hf164 are still a few spin units away. These terminating states are built with up to 18 aligned particles or 18 particles+holes outside a core. The core is built from nucleons in filled j shells, which gives no contribution to the spin. Analysis of the high-spin bands in Xe125,126 and Ce131,132 suggests that bands in Xe126 and Ce132 are observed to terminate at similar spin values, where terminating bands in Xe126 are observed high above yrast. It is remarkable that the deformed mean field, plus single-particle configurations, is able to provide such a comprehensive description of known experimental levels in nuclei up to spin 60ℏ and beyond. It is also impressive that the model can relate alignments of single-particle spin vectors to changes in shape with the nuclear spin. Published by the American Physical Society 2024

The level structure of 158Er has been studied using the Gammasphere spectrometer via the 114Cd(48Ca, 4n) reaction at 215 MeV with both thin (self-supporting) and thick (backed) targets. The level scheme has been con-siderably extended with more than 200 new transitions and six new rotational structures, including two strongly coupled high -K bands. Configuration assignments for the new structures are based on their observed alignments, B(M1)/B(E2) ratios of reduced transition probabilities, excitation energies, and comparisons with neighboring nuclei and theoretical calculations. With increasing angular momentum, this nucleus exhibits Coriolis-induced alignments of both neutrons and protons before it then undergoes a rotation-induced transition from near-prolate collective rotation to a noncollective oblate configuration. This transition occurs via the mechanism of band termination around spin 45 h over bar in three rotational structures. Two distinct lifetime branches, consistent with the crossing of a collective "fast" rotational structure by an energetically favored "slow" terminating sequence, are confirmed for the positive-parity states, and similar behavior is established in the negative-parity states. Weak-intensity, high-energy transitions are observed to feed into the terminating states. At the highest spins,

Abstract While being on a picnic, if you are not sure whether you brought the boiled or the raw eggs in your basket, there is a simple way of telling without smashing them open: Simply spin them. The boiled egg will spin very fast when you rotate it, whereas the raw egg will resist the spinning motion. Nuclear physicists make nuclei spin to study their internal features. Some 50 years ago a most astonishing effect was observed in rapidly rotating nuclei. This surprise discovery, known as backbending, which is a unique phenomenon in the finite, many-body quantum system, triggered a revolution of our studies into the structure of the atomic nucleus that continues to the present day.

Maximal ground-state deformation should occur when both proton and neutron Fermi surfaces are located at midshell. However, subshell gaps that stabilize large deformation can exist at proton or neutron values other than midshell. One such gap may occur at Z = 60 in the rare-earth region, as the energy of the first 2+ states in even-even nuclei are often lowest in an isotonic chain for neodymium (Z = 60) rather than the midshell isotopes of dysprosium (Z = 66). Further evidence of this deformed gap has now been observed by investigating the signature splitting systematics of the nu i13/2 bands found in the odd-N, rare-earth nuclei. These were aided by the present observation of the nu i13/2 band in 159Gd and the confirmation of the same structure in 155Sm via the transfer of a neutron from a 160Gd beam to a 154Sm target.

A Gd-160 beam was accelerated to an energy of 1000 MeV and, separately, bombarded thick targets of Sm-154 and Dy-164 in order to observe neutron-rich, rare-earth nuclei via deep-inelastic collision processes. Gammasphere was utilized to observe gamma-ray emissions. Many new states and transitions were observed in Gd-160 as a result of so-called unsafe Coulomb excitation. The ground-state band in Gd-160 has been extended to I-pi= 20(+) and a rotational band based on the K-pi = 4(+) state, previously associated with a hexadecapole vibration, was observed up to 18(+). The quasiparticle configuration of the K-pi = 4(+) band has been determined, and its unusual alignment behavior may result from a possible quenching of static neutron pairing. In addition, the band based on the [523]5/2 quasineutron orbital in Gd-161 was extended from 11/2(-) to 33/2(-) and also displays the same unusual alignment behavior.

Rotational structures have been measured using the Jurogam II and GAMMASPHERE arrays at low spin following the Gd-155(alpha, 2n) Dy-157 and Nd-148(C-12, 5n) Dy-155 reactions at 25 and 65 MeV, respectively. We report high-K bands, which are conjectured to be the first candidates of a K-pi = 2(+)gamma vibrational band, built on the [505]11/2(-) neutron orbital, in both odd-A Dy-155,Dy-157 isotopes. The coupling of the first excited K = 0(+) states or the so-called beta vibrational bands at 661 and 676 keV in Dy-154 and Dy-156 to the [505]11/2(-) orbital, to produce a K-pi = 11/2(-) band, was not observed in both Dy-155 and Dy-157, respectively. The implication of these findings on the interpretation of the first excited 0(+) states in the core nuclei Dy-154 and Dy-156 are also discussed.

High spin states in the isotope 194Hg were populated using the 150Nd (48Ca,4n) reaction at a beam energy of 213 MeV. A sequence of dip ole transitions has been observed above 8 MeV excitation energy. Cross-over transitions have also been identified. An interpretation connecting this sequence to a nuclear shape change is attempted. A comparison with similar structures in the neighbouring Hg isotopes is also attempted.

A comprehensive systematic study is made for the collective beta and gamma bands in even-even isotopes with neutron numbers N = 88 to 92 and proton numbers Z = 62 (Sm) to 70 (Yb). Data, including excitation energies, B(E0) and B(E2) values, and branching ratios from previously published experiments are collated with new data presented for the first time in this study. The experimental data are compared to calculations using a five-dimensional collective Hamiltonian (5DCH) based on the covariant density functional theory (CDFT). A realistic potential in the quadrupole shape parameters V (beta, gamma) is determined from potential energy surfaces (PES) calculated using the CDFT. The parameters of the 5DCH are fixed and contained within the CDFT. Overall, a satisfactory agreement is found between the data and the calculations. In line with the energy staggering S(I) of the levels in the 2(gamma)+ bands, the potential energy surfaces of the CDFT calculations indicate gamma-soft shapes in the N = 88 nuclides, which become gamma rigid for N = 90 and N = 92. The nature of the 0(2)(+) bands changes with atomic number. In the isotopes of Sm to Dy, they can be understood as beta vibrations, but in the Er and Yb isotopes the 0(2)(+) bands have wave functions with large components in a triaxial superdeformed minimum. In the vicinity of Sm-152, the present calculations predict a soft potential in the beta direction but do not find two coexisting minima This is reminiscent of Sm-152 exhibiting an X(5) behavior. The model also predicts that the 0(3)(+) bands are of two-phonon nature, having an energy twice that of the 0(2)(+) band. This is in contradiction with the data and implies that other excitation modes must be invoked to explain their origin.

Garrett et al. systematically investigated band-crossing frequencies resulting from the rotational alignment of the first pair of i(13/2 )neutrons (AB) in rare-earth nuclei. In that study, evidence was found for an odd-even neutron number dependence attributed to changes in the strength of neutron pairing correlations. The present paper carries out a similar investigation at higher rotational frequencies for the second pair of aligning i(13/2 ) neutrons (BC). Again, a systematic difference in band-crossing frequencies is observed between odd-N and even-N Er, Yb, Hf, and W nuclei, but in the BC case, it is opposite to the AB neutron-number dependence. These results are discussed in terms of a reduction of neutron pairing correlations at high rotational frequencies and of the effects of Pauli blocking on the pairing field by higher-seniority configurations. Also playing a significant role are the changes in deformation with proton and neutron numbers, the changes in location of single-particle orbitals as a function of quadrupole deformation, and the position of the Fermi surface with regard to the various Omega components of the neutron i(13/2 ) shell.

Previous experiments observed a 4^{+} state in the N=28 nucleus ^{44}S and suggested that this state may exhibit a hindered E2-decay rate, inconsistent with being a member of the collective ground state band. We populate this state via two-proton knockout from a beam of exotic ^{46}Ar projectiles and measure its lifetime using the recoil distance method with the GRETINA γ-ray spectrometer. The result, 76(14)_{stat}(20)_{syst}  ps, implies a hindered transition of B(E2;4^{+}→2_{1}^{+})=0.61(19) single-particle or Weisskopf units strength and supports the interpretation of the 4^{+} state as a K=4 isomer, the first example of a high-K isomer in a nucleus of such low mass.

An experiment populating low/medium-spin states in Dy-156 was performed to investigate the possibility of tetrahedral symmetry in this nucleus. In particular, focus was placed on the low-spin, negative-parity states since recent theoretical studies suggest that these may be good candidates for this high-rank symmetry. The states were produced in the Nd-148(C-12, 4n) reaction and the Gammasphere array was utilized to detect the emitted. rays. B(E2)/B(E1) ratios of transition probabilities from the low-spin, negative-parity bands were determined and used to interpret whether these structures are best associated with tetrahedral symmetry or, as previously assigned, to octupole vibrations. In addition, several other negative-parity structures were observed to higher spin and two new sequences were established.

Radioactive ^{136}Te has two valence protons and two valence neutrons outside of the ^{132}Sn double shell closure, providing a simple laboratory for exploring the emergence of collectivity and nucleon-nucleon interactions. Coulomb excitation of ^{136}Te on a titanium target was utilized to determine an extensive set of electromagnetic moments for the three lowest-lying states, including B(E2;0_{1}^{+}→2_{1}^{+}), Q(2_{1}^{+}), and g(2_{1}^{+}). The results indicate that the first-excited state, 2_{1}^{+}, composed of the simple 2p⊕2n system, is prolate deformed, and its wave function is dominated by excited valence neutron configurations, but not to the extent previously suggested. It is demonstrated that extreme sensitivity of g(2_{1}^{+}) to the proton and neutron contributions to the wave function provides unique insight into the nature of emerging collectivity, and g(2_{1}^{+}) was used to differentiate among several state-of-the-art theoretical calculations. Our results are best described by the most recent shell model calculations.

We report the results of a study of rotational bands in Ra-219 via the Pb-208(C-14, 3n) reaction to look for evidence that this nucleus is statically octupole deformed. We add 19 gamma rays not previously observed to the level scheme and extend the two most strongly populated alternating parity bands to J = 51/2 and 45/2. The magnitude of the energy splitting between the spin- parity doublets in the two bands appears to exclude the possibility that Ra-219 has a static octupole deformation.

The neutron-rich nucleus ^{144}Ba (t_{1/2}=11.5  s) is expected to exhibit some of the strongest octupole correlations among nuclei with mass numbers A less than 200. Until now, indirect evidence for such strong correlations has been inferred from observations such as enhanced E1 transitions and interleaving positive- and negative-parity levels in the ground-state band. In this experiment, the octupole strength was measured directly by sub-barrier, multistep Coulomb excitation of a post-accelerated 650-MeV ^{144}Ba beam on a 1.0-mg/cm^{2} ^{208}Pb target. The measured value of the matrix element, ⟨3_{1}^{-}∥M(E3)∥0_{1}^{+}⟩=0.65(+17/-23) eb^{3/2}, corresponds to a reduced B(E3) transition probability of 48(+25/-34)  W.u. This result represents an unambiguous determination of the octupole collectivity, is larger than any available theoretical prediction, and is consistent with octupole deformation.

In 1974 Aage Bohr and Ben Mottelson predicted the different 'phases' that may be expected in deformed nuclei as a function of increasing angular momentum and excitation energy all the way up to the fission limit. While admitting their picture was highly conjectural they confidently stated '...with the ingenious experimental approaches that are being developed, we may look forward with excitement to the detailed spectroscopic studies that will illuminate the behaviour of the spinning quantised nucleus'. High resolution gamma-ray spectroscopy has indeed been a major tool in studying the structure of atomic nuclei and has witnessed numerous significant advances over the last four decades. This article will select highlights from investigations at the Niels Bohr Institute, Denmark, and Daresbury Laboratory, UK, in the late 1970s and early 1980s, some of which have continued at other national laboratories in Europe and the USA to the present day. These studies illustrate the remarkable diversity of phenomena and symmetries exhibited by nuclei in the angular momentum-excitation energy plane that continue to surprise and fascinate scientists.

The first rotational sequences have been assigned to the odd-odd nucleus Re-168. Coincidence relationships of these structures with rhenium x rays confirm the isotopic assignment, while arguments based on the gamma-ray multiplicity (K-fold) distributions observed with the new bands lead to the mass assignment. Configurations for the two bands were determined through analysis of the rotational alignments of the structures and a comparison of the experimental B(M1)/B(E2) ratios with theory. Tentative spin assignments are proposed for the pi h(11/2)nu i(13/2) band, based on energy level systematics for other known sequences in neighboring odd-odd rhenium nuclei, as well as on systematics seen for the signature inversion feature that is well known in this region. The spin assignment for the pi h(11/2)nu(h(9/2)/f(7/2)) structure provides additional validation of the proposed spins and configurations for isomers in the Au-176 -> Ir-172 -> Re-168 alpha-decay chain.