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We report on a comparison between the theoretically predicted and experimentally measured spectra of the first-forbidden nonunique β-decay transition ^{137}Xe(7/2^{-})→^{137}Cs(7/2^{+}). The experimental data were acquired by the EXO-200 experiment during a deployment of an AmBe neutron source. The ultralow background environment of EXO-200, together with dedicated source deployment and analysis procedures, allowed for collection of a pure sample of the decays, with an estimated signal to background ratio of more than 99 to 1 in the energy range from 1075 to 4175 keV. In addition to providing a rare and accurate measurement of the first-forbidden nonunique β-decay shape, this work constitutes a novel test of the calculated electron spectral shapes in the context of the reactor antineutrino anomaly and spectral bump.

CITATION: Niu, C. Y., et al. 2018. Spectroscopic study of the possibly triaxial transitional nucleus 75Ge. Physical Review C, 97(3):034322, doi:10.1103/PhysRevC.97.034322.

Lifetime of the $3/{2}^{\ensuremath{-}}$ first excited state in $^{37}\mathrm{S}$ populated by the ${\ensuremath{\beta}}^{\ensuremath{-}}$ decay of $^{37}\mathrm{P}$ has been measured using $\ensuremath{\beta}\ensuremath{-}\ensuremath{\gamma}$ delayed coincidence technique. The $B(E2;7/{2}^{\ensuremath{-}}\ensuremath{\rightarrow}3/{2}^{\ensuremath{-}})$ value in $^{37}\mathrm{S}$ deduced from the lifetime comes close to the $B(E2;{0}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})$ value in weakly deformed $^{38}\mathrm{S}$ but deviates significantly from that in spherical $^{36}\mathrm{S}$. This manifests that $^{37}\mathrm{S}$ is a weakly deformed rather than spherical nucleus.

Lifetime measurements of yrast levels in $^{167}\mathrm{W}$ were measured using the recoil-distance Doppler-shift method. The differential decay-curve method was applied for a lifetime determination. Excited states of the nucleus $^{167}\mathrm{W}$ were populated by the reaction $^{142}\mathrm{Nd}$ ($^{28}\mathrm{Si}, 3n$) at a beam energy of 144 MeV. The energy spectra and measured transition quadrupole moments inferred from the lifetimes of $^{167}\mathrm{W}$ are compared with the predictions of the cranked Nilsson-Strutinsky-Bogoliubov calculations. The changes of deformations and shapes with increasing spin due to the $\ensuremath{\gamma}$-polarization effect of aligned particles are discussed. The signature inversion visible in the negative parity yrast band is explained to be related to the triaxial shapes.

High-spin states of $^{123}\mathrm{Ba}$ have been studied via the $^{108}\mathrm{Cd}(^{19}\mathrm{F},3np)^{123}\mathrm{Ba}$ fusion-evaporation reaction at a beam energy of 90 MeV. Several $E1$ transitions linking the positive-parity $\ensuremath{\nu}({d}_{5/2}+{g}_{7/2})$ band and negative-parity $\ensuremath{\nu}{h}_{11/2}$ band are observed in $^{123}\mathrm{Ba}$ for the first time. Evidence for the existence of octupole correlations in $^{123}\mathrm{Ba}$ is presented based on the systematic comparisons of the $B(E1)/B(E2)$ branching ratios and the energy displacements in odd-$A$ Ba isotopes. The characteristics of octupole correlation in the odd-$A\phantom{\rule{0.28em}{0ex}}^{123,125}\mathrm{Ba}$ are explained by the state-of-the-art multidimensionally-constrained relativistic mean-field model and cluster model based on the dinuclear system concept.

The spectroscopy of $^{73}\text{Ge}$ has been investigated via the $^{70}\text{Zn}(^{7}\text{Li},3np)^{73}\text{Ge}$ fusion-evaporation reaction. A previously known band built on the $\ensuremath{\nu}({f}_{5/2})$ orbital is extended up to higher-spin states and a new rotational band built on the $\ensuremath{\nu}({g}_{9/2})$ orbital is established. The observed band crossing and shape evolution induced by the alignment in $^{73}\text{Ge}$ are studied in comparison with the similar structures in the neighboring $N=41$ isotones and the cranked Woods--Saxon--Strutinsky calculations by means of total-Routhian-surface (TRS) methods. The inversion of the order of the ${g}_{9/2}$ neutron and the ${g}_{9/2}$ proton crossing frequency observed in the negative-parity $\ensuremath{\nu}({f}_{5/2})$ bands between $^{73}\text{Ge}$ and $^{75}\text{Se}$, $^{77}\text{Kr}$ can be ascribed to their different triaxial deformations. At higher spins, both the $\ensuremath{\nu}({f}_{5/2})$ and $\ensuremath{\nu}({g}_{9/2})$ bands in $^{73}\text{Ge}$ undergo a shape evolution with sizable alignments occurring. The characters of large signature splitting in $\ensuremath{\nu}({g}_{9/2})$ band of $^{73}\text{Ge}$ are discussed.

High-spin states of the semimagic nucleus $^{141}\mathrm{Pr}$ have been investigated by in-beam $\ensuremath{\gamma}$-spectroscopic techniques using the fusion-evaporation $^{138}\mathrm{Ba}(^{7}\mathrm{Li},\phantom{\rule{0.16em}{0ex}}4n)$ reaction at a beam energy of 38 MeV. The level scheme of $^{141}\mathrm{Pr}$ has been extended up to $\ensuremath{\sim}6.2$ MeV in excitation energy and $35/2 \ensuremath{\hbar}$ in spin. A total of 14 new levels and 30 new transitions have been assigned to $^{141}\mathrm{Pr}$ on the basis of $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidence information. The observed level structures are discussed in light of the available experimental data and shell model calculations with the oxbash code. According to the calculations, most of the observed levels of $^{141}\mathrm{Pr}$ are from the proton excitations above the $Z=50$ shell. Possible evidence for the neutron excitations across the $N=82$ gap in $^{141}\mathrm{Pr}$ is also discussed and compared with other $N=82$ isotones.

High-spin states of $^{115}\mathrm{In}$ have been studied using the $^{114}\mathrm{Cd}$ ($^{7}\mathrm{Li},\ensuremath{\alpha}2\mathrm{n}$) reaction at a beam energy of 48 MeV. A total of 13 new transitions have been observed and added to the level scheme of $^{115}\mathrm{In}$. Most of the states in $^{115}\mathrm{In}$ can be interpreted in terms of the weak coupling of a ${g}_{9/2}$ proton hole to the core states of $^{116}\mathrm{Sn}$ or a ${g}_{7/2}$ proton to the core states of $^{114}\mathrm{Cd}$. A $\ensuremath{\Delta}I=1$ band with the $\ensuremath{\pi}{({g}_{9/2})}^{\ensuremath{-}1}\ensuremath{\bigotimes}\ensuremath{\nu}{({h}_{11/2})}^{2}$ configuration was suggested as an oblate band built on the ``stapler'' mechanism with the aid of the tilted axis cranking model based on covariant density functional theory.

The high-spin level structure of the semi-magic nucleus $^{91}\mathrm{Nb}$ has been investigated via the ${}^{82}$Se(${}^{14}$N,$5n$)$^{91}\mathrm{Nb}$ reaction at a beam energy of 60 MeV. Based on these experimental results, a new level scheme is established that modifies and extends earlier schemes. Shell-model calculations have been carried out to interpret the level structure of $^{91}\mathrm{Nb}$. The calculated results show that the inclusion of neutron particle-hole excitation across the $N=50$ shell gap is essential to adequately describe states above $I=(21/2\phantom{\rule{0.16em}{0ex}}--\phantom{\rule{0.16em}{0ex}}23/2)\ensuremath{\hbar}$, with excitation energies above $\ensuremath{\sim}$6.0 MeV.

Excited states of ${}^{116}$Sb were investigated using in-beam $\ensuremath{\gamma}$-ray spectroscopy techniques with the ${}^{114}$Cd(${}^{7}$Li, $5n$) reaction. About 25 new states and 35 new transitions were added into the level scheme of ${}^{116}$Sb. The previously known two rotational bands based on the $\ensuremath{\pi}{g}_{9/2}^{}{}^{\ensuremath{-}1}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{11/2}$ and $\ensuremath{\pi}{g}_{9/2}^{}{}^{\ensuremath{-}1}\ensuremath{\bigotimes}\ensuremath{\nu}{d}_{5/2}$ configurations have been extended up to (16)${}^{\ensuremath{-}}$ and (13)${}^{+}$ states, respectively. A new $\ensuremath{\Delta}I=1$ band was identified, and interpreted in terms of the triaxial particle rotor model calculations.

High spin states of ${}^{112}$In were investigated using the reaction ${}^{110}$Pd(${}^{7}$Li,5$n$)${}^{112}$In at a beam energy of 50 MeV. A new level sequence has been found in this nucleus and the configuration $\ensuremath{\pi}{g}_{9/2}^{\ensuremath{-}2}{g}_{7/2}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{11/2}$ before the alignment and the configuation $\ensuremath{\pi}{g}_{9/2}^{\ensuremath{-}2}{g}_{7/2}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{11/2}^{3}$ after the alignment are assigned. The self-consistent titled axis cranking relativistic mean-field model calculations are performed to interpret the rotational structure and the characteristic features of antimagnetic rotation have been presented for the band after backbend.

High-spin states in the odd-odd ${}^{128}$I have been studied experimentally using the ${}^{124}$Sn(${}^{7}$Li, 3n$\ensuremath{\gamma}$)${}^{128}$I reaction at beam energies of 28 and 32 MeV. A level scheme built on the known ${T}_{1/2}=175$ ns, (${6}^{\ensuremath{-}})$ isomer has been established. The low-lying states in the level scheme have been proposed to be associated with the $\ensuremath{\pi}{d}_{5/2}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{11/2}$ and $\ensuremath{\pi}{g}_{7/2}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{11/2}$ two-quasiparticle excitations. The strongly populated negative-parity states, forming a bandlike structure, have been assigned to be based on the $\ensuremath{\pi}{g}_{7/2}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{11/2}$ configuration. The $\ensuremath{\pi}{h}_{11/2}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{11/2}$ multiplet ${9}^{+}$, ${10}^{+}$, and ${11}^{+}$ members have been identified according to the systematics of the corresponding level structures observed in the lighter odd-odd iodine isotopes. The two-quasiparticle configuration assignments are supported by the nucleon pair approximation and empirical shell-model calculations.

Excited states in ${}^{165}$Er have been populated using the ${}^{160}$Gd(${}^{9}$Be, 4$n$) reaction at beam energies of 42 and 45 MeV. A band consisting of two levels at excitation energies of 1317.6 and 1505.9 keV, both of which decay strongly to the $11/{2}^{\ensuremath{-}}[505]$ band, is observed and assigned as the $\ensuremath{\gamma}$-vibrational band built on the $11/{2}^{\ensuremath{-}}[505]$ configuration. In addition, a newly identified level is proposed as a three-quasiparticle state.

High-spin states in $^{161}\mathrm{Er}$ have been studied experimentally using the $^{150}\mathrm{Nd}$($^{16}\mathrm{O}$,5$\mathit{n}$) reaction at a beam energy of 86 MeV. The $5/{2}^{+}[642]$, $3/{2}^{\ensuremath{-}}[521]$, and $11/{2}^{\ensuremath{-}}[505]$ bands are extended up to high-spin states, and particularly the $\ensuremath{\alpha}=\ensuremath{-}1/2$ branch of the ground state $3/{2}^{\ensuremath{-}}[521]$ band is revised significantly. The relatively enhanced $E1$ transitions from the $3/{2}^{\ensuremath{-}}[521]$ band to the $5/{2}^{+}[642]$ band are observed. The band properties are analyzed within the framework of a triaxial particle-rotor model, and near-prolate shape and triaxial deformation are proposed to the $3/{2}^{\ensuremath{-}}[521]$ and $5/{2}^{+}[642]$ bands, respectively. Signature inversion occurs in the $3/{2}^{\ensuremath{-}}[521]$ band after the band crossing in $^{161}\mathrm{Er}$, and the systematics of the signature inversion associated with the $3/{2}^{\ensuremath{-}}[521]$ configuration are discussed. By analyzing the properties of the relatively enhanced $E1$ transitions, it is found that the $R(E1/E2)$ values show angular momentum dependence before the band crossing, and these enhanced $E1$ transitions could be attributed to octupole softness.

Excited states of $^{118}\mathrm{Sn}$ have been populated in the $^{116}\mathrm{Cd}$($^{7}\mathrm{Li}$,$1p4n$) reaction with a beam energy of $50$ MeV. The previously known level scheme is extended in the low-spin region as well as to higher spins. The systematic behavior of the intruder deformed bands in the Sn isotopes is discussed. Total Routhian surface calculations and configuration-fixed constrained triaxial relativistic mean-field approaches are employed for analysis of the nuclear structure of $^{118}\mathrm{Sn}$.

High-spin states of (156)Yb have been studied via the (144)Sm((16)O,4n)(156)Yb fusion-evaporation reaction at beam energy 102 MeV. The positive-parity yrast band and negative-parity cascade have been extended up to higher-spin states, respectively. In the present work, the negative-parity sequence above the 25(-) state was found to be irregular and fragment into many parallel branches. This pattern may related to the excitation from the nucleon in the Z=64,N=82 core. The characteristics of alignment plot and E-GOS curve for the positive-parity yrast sequence in (156)Yb indicate that this nucleus may undergo an evolution from quasivibrational to quasirotational structure with increasing angular momentum. Based on a systematic summary of the available experimental alignments for the even-A (156,158,160,162,164)Yb isotopes, the structural evolutions induced by the increase in angular momentum, as well as by the change in neutron numbers, in these even-A Yb isotopes have been discussed in comparison with the cranked Woods-Saxon-Strutinsky calculations by means of total-Routhian-surface (TRS) methods.

The high-spin states of the odd-odd nucleus $^{90}\mathrm{Nb}$ have been investigated with in-beam \ensuremath{\gamma}-spectroscopic techniques via the $^{76}\mathrm{Ge}$($^{19}\mathrm{F},5n)^{90}\mathrm{Nb}$ reaction at a beam energy of 80 MeV. $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidences were measured using a $\ensuremath{\gamma}$-ray detector array. Twenty new $\ensuremath{\gamma}$ rays have been assigned to $^{90}\mathrm{Nb}$ and the level scheme has been extended up to an excitation energy of 8.095 MeV at spin $18\ensuremath{\hbar}$. The level structure of $^{90}\mathrm{Nb}$ at high spin states has been well reproduced using semiempirical shell-model calculations in the model space $\ensuremath{\pi}(1{p}_{1/2},0{f}_{5/2},0{g}_{9/2})\ensuremath{\nu}(0{g}_{9/2})$. The results show that the excitation of protons plays an important role in generating the high-spin states of $^{90}\mathrm{Nb}$.