γ -ray measurements in fast-neutron-induced reactions on Tl203

Abstract

Background: Fast-neutron-induced reactions can be used to characterize reaction mechanisms, investigate nuclear structure, and impose constraints on nuclear models. Because of the difficulty in predicting such effects, experimental data are important to constrain models. Thallium isotopes located close to the doubly magic $^{208}\mathrm{Pb}$ nucleus are important for comparison with shell-model calculations.Purpose: Study the population of excited states in $^{203}\mathrm{Tl}$ and lighter isotopes in such reactions.Methods: $\ensuremath{\gamma}$-ray cross sections were measured. The data were taken by using the Germanium Array for Neutron-Induced Excitations spectrometer. The pulsed neutron source of the Los Alamos Neutron Science Center's Weapons Neutron Research facility provided neutrons in the energy range from 1 to 300 MeV. The time-of-flight technique was used to determine the incident neutron energies.Results: Cross sections for emission of several $\ensuremath{\gamma}$ rays in 20 reaction channels were determined. Candidates for the intruder $9/{2}^{\ensuremath{-}}$ state in $^{203}\mathrm{Tl}$, from the odd proton in the ${h}_{9/2}$ orbital, and the first ${5}^{+}$ states in $^{202,204}\mathrm{Tl}$, from admixture of configurations, were identified.Conclusions:The excitation energy of the candidate $9/{2}^{\ensuremath{-}}, \ensuremath{\pi}{h}_{9/2}$ state is in good agreement with the theoretical prediction from a semi-empirical weak-coupling model and its half-life is in the range of tens of nanoseconds. The feeding of a previously known isomer in $^{202}\mathrm{Tl}$ exhibits similarities with the feeding of other isomers in this mass region.