A. D. Ayangeakaa

Two 0 states at 1294.5 and 1774.0 keV, together with three 2 and one 4 levels, were identified or unambiguously spin-parity assigned for the first time in Zr utilizing -ray spectroscopy and - angular correlation techniques with the Gammasphere spectrometer, following the decay of neutron-rich, mass separated Y isotopes. Comparisons with recent Monte Carlo Shell-Model (MCSM) calculations indicate that these two states are candidates for the bandhead of a sequence in a shape-coexisting spherical minimum predicted to be located around 1500 keV. According to the measured relative B(E2) transition probabilities, the 0 state exhibits decay properties which more closely align with those predicted for a spherical shape, while the 0 level is suggested to be associated with a weakly-deformed shape similar to one related to the 0 state.

The electromagnetic properties of low-lying states in 74 Ge have been investigated using a multi-step Coulomb excitation measurement carried out at the ATLAS facility of the Argonne National Laboratory. The experimental setup consisted of the GRETINA multidetector array coupled to the heavy-ion counter, CHICO2. This coupling allowed for precise Doppler correction and clean kinematic separation of the scattered particles. The quadrupole collectivity and triaxiality of the ground-state and quasi-gamma bands--which were Coulomb excited to spins 8+ and 4+, respectively--have been investigated in the framework of the generalized triaxial rotor model and the spherical shell model. Preliminary results from the analysis will be presented in the context of similar studies performed for the neighboring 72, 76 Ge [1, 2] nuclei.

NUCLEAR REACTIONS 3 He (32 S, α) 31 S, E= 128 MeV; measured reaction products, Eγ, Iγ; deduced γ-ray energies, levels, T 1/2, resonances. Comparison with the shell-model code NuShellX calculations. Modern Markov chain Monte Carlo-based Bayesian statistical techniques. The Doppler Shift Lifetimes (DSL2) facility at the TRIUMF Isotope Separator and Accelerator (ISAC-II) facility.

Chirality is a distinct signature that characterizes triaxial shapes in nuclei. We report the first observation of chirality in the nucleus Pr using a high-statistics Gammasphere experiment with the Sb(O,4n)Pr reaction. Two chiral-partner bands with the configuration have been identified in this nucleus. Angular distribution analyses of the connecting transitions between the two chiral partners have revealed a dominant dipole character. Quasiparticle triaxial rotor model calculations are in good agreement with the experiment. This is the first time that both signatures of triaxiality--chirality and wobbling--have been observed in the same nucleus.

Medium and high-spin level sequences in Cu 64 were investigated using the complex Mg 26 (Ca 48, α p 5 n γ) multinucleon transfer reaction. The experiment was performed at the ATLAS accelerator facility at the Argonne National Laboratory using the Gammasphere array and the fragment mass analyzer (FMA). Two high-spin, quasirotational bands consisting of stretched-E 2 transitions were observed in coincidence with the known low-spin structure for the first time. These bands share remarkable similarities with highly deformed and/or superdeformed bands observed in the A≈ 60–70 mass region. In addition, a regular dipole sequence with weak E 2 crossover transitions was observed. A general discussion of the observed structures, complemented by theoretical calculations carried out within the framework of the adiabatic and configuration-fixed constrained covariant density functional theory and the quantum …

Two 0+ states at 1294.5 and 1774.0 keV, together with three 2+ and one 4+ levels, were identified or unambiguously spin-parity assigned for the first time in Zr 100 utilizing γ-ray spectroscopy and γ− γ angular correlation techniques with the Gammasphere spectrometer, following the β− decay of neutron-rich, mass-separated Y 100, 100 m isotopes. Comparisons with recent Monte Carlo shell-model calculations indicate that these two states are candidates for the bandhead of a sequence in a shape-coexisting spherical minimum predicted to be located around≈ 1500 keV. According to the measured relative B (E 2) relative transition probabilities, the 0 5+ state exhibits decay properties which more closely align with those predicted for a spherical shape, while the 0 4+ level is suggested to be associated with a weakly deformed shape similar to one related to the 0 2+ state.

Excited structures in Ce 146 were populated in β decay of the high-spin state in the neutron-rich nucleus La 146. The beam was produced by the Californium Rare Isotope Breeder Upgrade (CARIBU) facility at Argonne National Laboratory, reaccelerated by the ATLAS accelerator, and implanted on a moving-tape system in the middle of the GAMMASPHERE array. The decay scheme of the high-spin, β-decaying state in La 146 was revised with respect to previous studies and evaluated nuclear data. The structure of La 146 is discussed in the framework of the deformed Nilsson model and systematics of known quasiparticle structures in the region.

We recently performed a nuclear resonance fluorescence experiment in 56 Fe with 100% linearly polarized photons for incident beam energies below the neutron separation energy at the High-Intensity Gamma-ray Source (HIγS) facility at Triangle Universities Nuclear Laboratory (TUNL). From the observed dipole states and their decay pattern and transition strengths, we will deduce the gamma-strength function of 56 Fe. With the upgrade of the high-efficiency clover detector array and the increased γ-ray flux at HIγS, we also aim to study the low-energy primary γ-ray transitions induced by incident photon beams using coincidence mode. In this talk, we will give an overview of the experiment and present the preliminary results on the dipole strength distribution in 56 Fe below the neutron separation energy, as well as the comparison with literature data and theoretical calculations based on the quasi-particle phonon …

A ground-state rotational band in the fissile nucleus Rf 254 was observed for the first time. Levels up to spin 14 ℏ and excitation energy of 1.56 MeV were observed. The Rf 254 nuclei were produced using the Pb 206 (Ti 50, 2 n) fusion-evaporation reaction. It is the weakest reaction channel ever studied using in-beam γ-ray spectroscopic methods. The reaction products were separated from the beam in the Argonne gas-filled analyzer (AGFA). The Rf 254 nuclei were implanted into a double-sided Si strip detector at the AGFA focal plane and tagged with subsequent ground-state spontaneous fission decays using temporal and spatial correlations. Prompt γ rays in coincidence with the Rf 254 recoils were detected in the Gammasphere array of Ge detectors. In order to identify the ground-state rotational band in Rf 254, a method for identifying rotational bands in low statistics γ-ray spectra was developed. The deduced …

The level structure of Er 158 has been studied using the Gammasphere spectrometer via the Cd 114 (Ca 48, 4 n) reaction at 215 MeV with both thin (self-supporting) and thick (backed) targets. The level scheme has been considerably 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 (M 1)/B (E 2) 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 ℏ in …

The deformation properties of the low-lying states in Ge 76 have been investigated following a safe-energy Coulomb excitation measurement with the GRETINA tracking array and CHICO2 heavy-ion counter at the ATLAS accelerator facility at Argonne National Laboratory. A comprehensive set of transition and static E 2 matrix elements were extracted from the measured differential Coulomb cross sections and compared with results of configuration-interaction shell-model calculations and computations carried out within the framework of the generalized triaxial rotor model. The remarkable agreement between the calculated and experimental data supports a near-maximum triaxial deformation for the ground state of Ge 76. In addition, the degree of softness of the asymmetry in Ge 76 and Se 76 was investigated using rotational invariants generated from configuration-interaction shell-model wave functions computed …

The atomic masses of Ag 111, 113, Pd 111–113, Rh 111–113, Ru 111–113, and Mo 111, 112 have been measured during the online commissioning experiments of the ZeroDegree multi-reflection time-of-flight Mass Spectrograph (ZD MRTOF-MS) at the RIKEN RI beam factory. The mass of Mo 112 has been determined. For the previously known masses, a good agreement between our results and the 2020 Atomic Mass Evaluation has been observed in most cases. The determined two-neutron separation energies for Mo isotopes up to N= 70 show a smooth trend. In this work, the performed experiment and analysis procedure are presented. The theoretical interest in the measured region is highlighted, and the results are discussed in terms of the various mass surface formulas including the new mass data. Furthermore, a comparison between our results and global theoretical mass models is given, and we …

Level lifetimes provide key insight into the structure of atomic nuclei and serve as stringent tests of theoretical descriptions. Though several methods for determining level lifetimes exist for both reaction measurements and decay studies, here the focus is on techniques involving the direct measurement of time differences between population and subsequent depopulation of excited states. The techniques presented herein are broadly applicable across multiple timing ranges, but the approach is specifically described for the β γ timing method. A multi-step, amplitude-dependent time walk correction and timing resolution corrections were employed to address the data analysis complications that arise from using thick scintillators for electron detection. Subsequently, a new Monte Carlo method utilizing measured detector responses obtained from the data, coupled with chi-square minimization, is presented for extracting …

A search for the decay of the 2 (2)(+) near-threshold resonance in C-14, located 142 keV above the neutron separation energy, was performed at the Argonne National Laboratory with the GRETINA?-ray spectrometer coupled to the ORRUBA Si detectors. The sensitivity of the experiment was of the order of 2.6 x10 (-5). This is comparable to the gamma branch calculated by the Shell Model Embedded in the Continuum, which predicts a significant enhancement of the 2 (2)(+)? 0 transition probability, as a consequence of the collectivization of the near-threshold state.

Neutron-rich Ni isotopes have been attracting much attention on the account of a phenomenon known as shape coexistence, which has been observed in 66 Ni, doubly-magic 68 Ni (N= 40), and 70 Ni. The low-spin structures of the semi-magic 64 Ni and 62 Ni (Z= 28) isotopes are the latest to be investigated for shape coexistence.. The presence of this phenomenon in near closed-shell nuclei is indicative of the rapid change in structure that can happen with either the addition or removal of a few protons or neutrons, by which states associated with deformed shapes can be present at relatively low excitation energy beside the anticipated spherical ones. Finding proof of these shapes in 64 Ni and 62 Ni was stimulated by Monte Carlo shell-model (MCSM) calculations that account for these shapes in the heavier Ni isotopes and disclose the intricate landscape of coexisting shapes. A combination of four distinct …

The decay from the 2+ 2 near-Threshold resonance in 14C, located 142 keV above the neutron emission threshold, was searched for in a fusionevaporation experiment at Argonne National Laboratory with the GODDESS setup, comprising the GRETINA?-ray spectrometer coupled to the ORRUBA charged particle detector. The Shell Model Embedded in the Continuum predicts a significant enhancement of the 2+ 2 0 transition probability, owing to a collectivization of the near-Threshold state. The corresponding branch is expected to be of the order of 5× 10-5, which is comparable with the sensitivity of this experiment.

An extensive study of the level structure of 62Co has been performed following a complex multinucleon transfer reaction, 26Mg(48Ca, 2α3npγ)62Co, at beam energies of 275, 290, and 320 MeV. The combination of the Gammasphere array, the fragment mass analyzer, and a focal-plane ionization chamber was used to identify and delineate excited levels in 62Co. A considerable extension to the 62Co level scheme is proposed with firm spin-parity values assigned on the basis of angular distribution and correlation analyses. Various level sequences built upon states of single-particle character have been observed, and an interpretation of these structures in the framework of the spherical shell model is presented. At moderate spins, two dipole bands have been observed and, based on their phenomenological study, a possible magnetic rotation character is suggested. However, theoretical calculations performed using the particle rotor model support magnetic rotation for only one of these dipole bands.

The structure of Ni 64, the heaviest stable Ni isotope, has been investigated via high-statistics, multistep safe Coulomb excitation to search for shape coexistence, a phenomenon recently observed in neutron-rich Ni 66 and Ni 70 as well as in doubly magic, N= 40, Ni 68. The study was motivated by recent, state-of-the-art Monte Carlo shell-model calculations (MCSM), where a Hamiltonian with effective interactions incorporating the monopole tensor force predicts the existence of shape coexistence, also in the lower-mass Ni 62, 64 isotopes. A set of transition and static E 2 matrix elements for both yrast and near-yrast structures was extracted from the differential Coulomb excitation cross sections. From comparisons between the new results and MCSM as well as other shell-model calculations, a clearer picture of the structure of Ni 64 emerges. Specifically, the low-spin states are shown to be dominated by proton and …

Compton polarimeters have played an important role in the study of nuclear structure physics, but have often been limited in their applications because of relatively low γ-ray detection efficiency. With the advent of γ-ray tracking detector arrays, which feature nearly 4 π solid angle coverage and the ability to identify the location of Compton-scattering events to within a few millimeters, this limitation can be overcome. Here we present a characterization of the performance of the Gamma Ray Energy Tracking In-beam Nuclear Array (GRETINA) as a Compton polarimeter using the 24 Mg (p, p′) reaction at 2.45 MeV proton energy. We also discuss a new capability added to the simulation package UCGretina to simulate the emission of polarized photons, and compare it to the measured data. Finally, we use these simulations to predict the performance of the Gamma Ray Energy Tracking Array (GRETA).

The nature of the yrast negative-parity band and its chiral-like partner band in Ag 104 is investigated experimentally and theoretically. Lifetimes of states in the negative-parity yrast band and positive-parity band based on the 4424-keV level are measured using Doppler shift attenuation technique. Lifetimes of three more states have been determined along with the upper limit for the lifetime of the highest observed yrast states. Further, lifetimes known from earlier studies are determined with better precision. The level scheme of Ag 104 has also been extended with the addition of new enhanced E 1 transitions linking the positive-parity band based on the 4424-keV levels and the yrast negative-parity and its partner band. B (E 1) and/or B (E 1)/B (M 1) values for the transitions from the positive-parity band to the yrast and its partner band have been determined for the first time; these suggest strong octupole correlation …