arXiv subjects

T. J. Gray

Publications and source records attributed to T. J. Gray.

22 records · Page 2Linked to original sources

arXiv2019

Early signal of emerging nuclear collectivity in neutron-rich $^{129}$Sb

Radioactive $^{129}$Sb, which can be treated as a proton plus semi-magic $^{128}$Sn core within the particle-core coupling scheme, was studied by Coulomb excitation. Reduced electric quadrupole transition probabilities, $B(E2)$, for the $2^+$ $\times$ $πg_{7/2}$ multiplet members and candidate $πd_{5/2}$ state were measured. The results indicate that the total electric quadrupole strength of $^{129}$Sb is a factor of 1.39(11) larger than the $^{128}$Sn core, which is in stark contrast to the expectations of the empirically successful particle-core coupling scheme. Shell-model calculations performed with two different sets of nucleon-nucleon interactions suggest that this enhanced collectivity is due to constructive quadrupole coherence in the wavefunctions stemming from the proton-neutron residual interactions, where adding one nucleon to a core near a double-shell closure can have a pronounced effect. The enhanced electric quadrupole strength is an early signal of the emerging nuclear collectivity that becomes dominant away from the shell closure.

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arXiv2019

First-excited state $g$ factors in the stable, even Ge and Se isotopes

Transient-field $g$-factor measurements in inverse kinematics were performed for the first-excited states of the stable, even isotopes of Ge and Se. The $g$ factors of $^{74}$Ge and $^{74}$Se were measured simultaneously using a cocktail beam, which eliminates most possible sources of systematic error in a relative $g$-factor measurement. The results are $g(^{74}{\rm Se})/g(^{74}{\rm Ge})=1.34(7)$, $g(^{70}{\rm Ge})/g(^{74}{\rm Ge}) = 1.16(15)$, $g(^{72}{\rm Ge})/g(^{74}{\rm Ge})=0.92(13)$, $g(^{76}{\rm Ge})/g(^{74}{\rm Ge})=0.88(5)$, $g(^{76}{\rm Se})/g(^{74}{\rm Se})=0.96(7)$, $g(^{78}{\rm Se})/g(^{74}{\rm Se})=0.82(5)$, $g(^{80}{\rm Se})/g(^{74}{\rm Se})=0.99(7)$ and $g(^{82}{\rm Se})/g(^{74}{\rm Se})=1.19(6)$. The measured $g$-factor ratios are in agreement with ratios from previous measurements, despite considerable variation in previous reported absolute values. The absolute values of the $g$ factors remain uncertain, however the Rutgers parametrization was used to set the transient-field strength and then compare the experimental $g$ factors with shell-model calculations based on the JUN45 and jj44b interactions. Modest agreement was found between experiment and theory for both interactions. The shell model calculations indicate that the $g(2^+_1)$ values and trends are determined largely by the balance of the spin carried by orbital motion of the protons.

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arXiv2019

Spectroscopy and excited-state $g$~factors in weakly collective ${^{111}}$Cd: confronting collective and microscopic models

The even cadmium isotopes near the neutron midshell have long been considered good examples of vibrational nuclei. However, the vibrational nature of these nuclei has been questioned based on E2 transition rates that are not consistent with vibrational excitations. In the neighbouring odd-mass nuclei, the g factors of the low-excitation collective states have been shown to be more consistent with a deformed rotational core than a vibrational core. Beyond the comparison of vibrational versus rotational models, recent advances in computational power have made shell-model calculations feasible for Cd isotopes, which may give insights into the emergence and nature of collectivity in the Cd isotopes. Collective excitations in the A ~ 100 region were studied through magnetic moments and electromagnetic transitions in 111Cd. The spectroscopy of 111Cd has been studied following Coulomb excitation. Angular correlation measurements, transient-field g-factor measurements and lifetime measurements by the Doppler-broadened line shape method were performed. The structure of the nucleus was explored in relation to particle-vibration versus particle-rotor interpretations. Large-scale shell-model calculations were performed with the SR88MHJM Hamiltonian. Excited-state g factors have been measured, spin assignments examined and lifetimes determined. Attention was given to the reported $5/2^{+}$ 753-keV and $3/2^{+}$ 755-keV states. The $3/2^{+}$ 755-keV level was not observed; evidence is presented that the reported $3/2^+$ state was a misidentification of the $5/2^{+}$ 753-keV state. It is shown that the g factors and level structure of 111Cd are not readily explained by the particle-vibration model. A particle-rotor approach has both successes and limitations. The shell-model approach successfully reproduces much of the known low-excitation structure in 111Cd.

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arXiv2017

Perturbed angular distributions with LaBr$_3$ detectors: the $g$ factor of the first ${10^+}$ state in $^{110}$Cd revisited

The Time Differential Perturbed Angular Distribution technique with LaBr$_3$ detectors has been applied to the $I^π= \frac{11}{2}^-$ isomeric state ($E_x = 846$ keV, $τ=107$~ns) in $^{107}$Cd, which was populated and recoil-implanted into a gadolinium host following the $^{98}$Mo($^{12}$C, $3n$)$^{107}$Cd reaction. The static hyperfine field strength of Cd recoil implanted into gadolinium was thus measured, together with the fraction of nuclei implanted into field-free sites, under similar conditions as pertained for a previous implantation perturbed angular distribution $g$-factor measurement on the $I^π= 10^+$ state in $^{110}$Cd. The $^{110}$Cd $g(10^+)$ value was thereby re-evaluated, bringing it into agreement with the value expected for a seniority-two $νh_{\frac{11}{2}}$ configuration.

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