Excited states up to 3 MeV in the odd-odd 54 Mn have been populated in the 54 Cr(p,n) 54 Mn reaction at E p = 4.5 MeV. Conversion coefficients ( α K and α π ) were deduced using the NPG method for the first time in 54 Mn. Multipolarities were unambiguously assigned in 16 transitions for the first time using the obtained conversion coefficient values. Spin and parity has been restricted for three transitions.

The 50Cr(p,γ)51Mn proton capture reaction has been used to study the photon strength functions by utilizing primary gamma ray transitions from the entry states to discrete states of known spins and parities. The reaction was conducted with the 3 MV Tandetron accelerator at iThemba LABS which delivered proton beams of 2.5 to 2.740, 2.760 to 3.0 MeV and 3.675 to 4.498 MeV in intervals of 20-25 keV with beam currents of up to 5 μA. In this work the proton capture reaction was employed together with the Average Resonance Capture method to extract the shape of the PSF of 51Mn.

Collective structures in 186Os have been investigated through the 186W(4He, 4n) 186Os reaction, at a beam energy of 48 MeV. The low-lying bands built on the excited 0+2 , 2+2 , and 4+3 states have been extended up to spins of 12+, 15+, and 8+, respectively, and a number of new linking transitions were identified. The features of the collective bands in 186Os, such as level energies, are presented in the context of a systematic study of the neighboring even-even 182-192Os isotopes. In addition, the validity of the K-selection rule, stemming from a description based on axial symmetry of the nuclear shape, is examined. The observed decays between the rotational bands support a description where K is conserved. However, some K-forbidden decays were also identified, suggesting that a model allowing for small K admixtures is probably required. The experimental data are further compared with calculations using a five-dimensional collective Hamiltonian based on covariant density functional theory. The calculations predict that the collective bands are associated with different nuclear shapes, varying in quadrupole deformation, triaxiality, and softness.

The nuclear level density (NLD) and gamma-ray strength function (gamma SF) of 63Ni have been investigated using the Oslo method. The extracted NLD is compared with previous measurements using particle evaporation and those found from neutron resonance spacing. The gamma SF was found to feature a strong low-energy enhancement that could be explained as M1 strength based on large-scale shell model calculations. Comparison of gamma SFs measured with the Oslo method for various Ni isotopes reveals systematic changes to the strength below 5 MeV with increasing mass.

The spectroscopy of 62Cu is studied via the 54Cr(12C, 1 p3n) 62Cu fusion-evaporation reaction. On the basis of the gamma -gamma coincidence analysis, angular distributions from oriented states, and linear polarization measurement, three positive-parity and three negative-parity level sequences in 62Cu are observed, including two new gamma -ray transitions and one new level. The collective structures are discussed in terms of the tilted axis cranking covariant density functional theory. Although not firmly confirmed in experiment, the properties of a magnetic rotational structure with the n(f7/2)-1(p3/2 f5/2)2 circle times v(g9/2)1(p3/2 f5/2)4 configuration have been discussed. Its angular momentum generation is probably due to the shears mechanism.

Excited states in Ba-131 and Ce-133 were studied using in-beam gamma-ray spectroscopy through the Sn-122(C-13, 4n) Ba-133 and Te-125(C-12, 4n) Ce-133 reactions, respectively. A strongly coupled band, associated with the nu g(7/2) [404]7/2(+) configuration, was identified in Ba-131 and Ce-133. It is the first time to observe the nu g(7/2) [404]7/2(+) bands in the N = 75 isotones. The signature partners exhibit considerable energy splitting in comparison with those in the pi g(7/2) [404]7/2(+) bands in the odd-A Ta and Re isotopes. Extensive cranked shell model and quasiparticle-plus-triaxial-rotor model calculations reveal the origin of the signature splitting, which depends not only on the triaxiality, but also on the configuration mixing with nearby low- j orbitals.

With its unique combination of excellent timing properties and good energy resolution, LaBr3:Ce detectors have proven to be effective tool in gamma spectroscopy and in particular fast-timing studies. Eight 2” x 2” LaBr3:Ce detectors used in conjunction with the 16 channel all-digital waveform 500 MHz acquisition module, PIXIE-16 were commissioned at iThemba Laboratory for Accelerator Based Sciences, South Africa. The results presented here give insight of the performance of the 2” x 2” LaBr3 detectors when used in conjunction with a digital pulse processing (DPP) module and electronic read-out module. Initial commissioning experiments were done using radioactive sources, including 60Co, 152Eu and 67Ga. The detectors were then exposed to an in-beam environment using the AFRODITE array with targets 45Sc and 64Ni, with a proton beam energy of 27 MeV.

Background: The shape of most nuclei is described by its quadrupole deformation (showing the deviation from spherical shape) and its triaxiality (showing the deviation from axial symmetry). Nuclei affected by triaxiality show additional collective rotational bands, called gamma bands, that appear at low excitation energy. The gamma bands can be caused by the precession of a nucleus with triaxial shape, but can also arise from small gamma vibrations around an axially symmetric shape. Purpose: The aim of this work is to search for new collective excitations in 187Os in particular related with the gamma degree of freedom. Methods: The rotational structures of 187Os were populated by the 186W(4He, 3n)187Os reaction at a beam energy of 37 MeV. The measurements of gamma - gamma coincidences, angular distribution ratios and gamma -ray intensities were performed using eleven Compton-suppressed Ge clover detectors. Results: The previously known positive-parity band built on the 11/2+ isomer has been extended up and a new excited positive-parity band built above a 15/2+ state has been observed. The 11/2+ band was assigned a nu i13/2 configuration while the new 15/2+ band was associated with a coupling of the valence i13/2 neutron with the gamma band of the even-even core. The quasiparticle-plus-triaxial-rotor model calculations provide a good agreement with the experimental data for both bands. They describe the 15/2+ band as a collective excitation with respect to the 11/2+ band that corresponds to a precession of the intermediate nuclear axis similarly to the precession of a rotating top. Conclusions: As shown by the calculations, the new rotational band can be understood as resulting from the three-dimensional rotation of a triaxially-deformed nucleus. However, a description based on the vibrations of a gamma -soft nuclear shape should also be investigated in order to firmly establish the nature of the excited positiveparity band. Further studies able to distinguish between these alternative descriptions will be beneficial.

Low- and medium-spin negative-parity bands of Os-187 have been studied using the AFRican Omnipurpose Detector for Innovative Techniques and Experiments (AFRODITE) array, following the W-186(He-4, 3n) Os-187 reaction at a beam energy of 37 MeV. In the current work, all the previously known bands have been significantly extended and three new bands have been added to the level scheme. The angular distribution ratio (R-AD) and polarization measurements have been used to assign spin and parity to the observed new levels. The configurations of some of the bands have been modified. The observed bands are interpreted within the cranked shell model (CSM) and cranked Nilsson-Strutinsky-Bogoliubov (CNSB) formalism. Comparison with experimental data shows good agreements. Systematic comparison with the neighboring Os-185 isotope is also discussed.

Excited states in 131Ba and 133Ce were studied using in-beam γ-ray spectroscopy through the 122Sn(13C,4n)131Ba and 125Te(12C,4n)133Ce reactions, respectively. A strongly coupled band, associated with the νg7/2[404]7/2+ configuration, was identified in 131Ba and 133Ce. It is the first time to observe the νg7/2[404]7/2+ bands in the N=75 isotones. The signature partners exhibit considerable energy splitting in comparison with those in the πg7/2[404]7/2+ bands in the odd-A Ta and Re isotopes. Extensive cranked shell model and quasiparticle-plus-triaxial-rotor model calculations reveal the origin of the signature splitting, which depends not only on the triaxiality, but also on the configuration mixing with nearby low-j orbitals.3 MoreReceived 30 July 2021Revised 13 October 2021Accepted 19 November 2021DOI:https://doi.org/10.1103/PhysRevC.104.064304©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCollective levelsElectromagnetic transitionsNuclear structure & decaysTransfer reactionsProperties90 ≤ A ≤ 149Nuclear Physics

Electric monopole (E0) transitions are a highly sensitive probe of the charge distribution of an atomic nucleus. A large E0 transition strength (ρ2(E0)) is a clear indicator of nuclear shape coexistence. In the region between doubly magic 40Ca and 56Ni, E0 transitions have never been observed in the Ti or Cr isotopes, nor in the heavier iron isotopes (56,58Fe). We have performed the first measurements of the E0 transitions in 52Cr via conversion-electron and pair-conversion spectroscopy using the Super-e spectrometer at the Australian National University Heavy Ion Accelerator Facility. We present the first spectra obtained for 52Cr, including the first observation of the E0 transition from the first-excited 0+ state in 52Cr, in both electron-positron pairs and conversion-electron spectroscopy. The preliminary values for the E0 strength in the 1531keV 2+ → 2+ transition in 52Cr is ρ2(E0) × 103 = 470(190), and for the 1728-keV 23+ → 21+ transition, it is ρ2(E0) 103 = 1800(1200). The large E0 strengths observed are consistent with shape coexistence in this region. However, despite the relatively precise observation of the conversion-electron and electron-positron pair intensities, the E0 strengths have large uncertainties. More precise determinations of relevant spectroscopic quantities, such as the state lifetimes and transition mixing ratios for mixed M1 + E2 transitions, are needed to determine the E0 strength more precisely.

The E0 transition depopulating the first-excited 0+ state in 24Mg has been observed for the first time, and the E0 transition strength determined by electron-positron pair and γ-ray spectroscopy measurements performed using the Super-e pair spectrometer. The E0 transition strength is ρ2×103=380(70). A two-state mixing model implies a deformation of the first-excited 0+ state of β2≈1 and a change in the mean-square charge radius of Δ〈r2〉≈1.9fm2, which suggests a significant shape change between the ground state and first-excited 0+ state in 24Mg. The observed E0 strength gives direct evidence of shape coexistence and superdeformation in 24Mg, bringing this nucleus into line with similar behaviour in nearby N=Z nuclei. This result agrees with recent theoretical work on the cluster nature of 24Mg and has potential ramifications for nuclear reactions of astrophysical importance.

The study of electric monopole (E0) transitions between two 0+ states is important because the monopole strength carries vital information about the nuclear structure due to its direct link with the mean squared charge radius r2 and quadrupole deformation parameter β. Therefore, the measurement of internal conversion electrons (ICE) or internal pair formation (IPF) is crucial for E0 transition studies. Transitions between 0+ states do not change angular momentum. Hence, single-photon emission is forbidden, but can decay by conversion electrons or pair formation and two-photon emission which is mostly negligible. In order to implement E0 studies at iThemba LABS, an electron spectrometer that uses a solenoidal magnetic field acting as a lens and a Si(Li) detector has been refurbished and characterized using calibration sources of ICE. Figures of merit have been extracted and compared with simulations. The spectrometer coupled with an array of LaBr3:Ce detectors and Low Energy Photon Spectrometers (LEPS) was successfully implemented for in-beam experiments. Measurements of internal conversion coefficients (ICC) and monopole strengths extracted from in-beam measurements of 72As, 72Ge, and 72Se are presented.