Feeding of Rh and Ag isomers in fast-neutron-induced reactions

Abstract

Background: In ($n,{n}^{\ensuremath{'}}$) reactions on stable Ir and Au isotopes in the mass $A=190$ region, the experimentally established feeding of the isomers relative to the feeding of the corresponding ground states increases with increasing neutron energy, up to the neutron energy where the ($n,2n$) reaction channel opens up, and then decreases.Purpose: In order to check for similar behavior in the mass $A=100$ region, the feeding of isomers and ground states in fast-neutron-induced reactions on stable isotopes in this mass region was studied. This is of especial interest for Rh which can be used as a radiochemical detector.Methods: Excited states were studied using the ($n,{n}^{\ensuremath{'}}\ensuremath{\gamma}$), ($n,2n\ensuremath{\gamma}$), and ($n,3n\ensuremath{\gamma})$ reactions on $^{103}\mathrm{Rh}$ and $^{109}\mathrm{Ag}$. A germanium detector array for $\ensuremath{\gamma}$-ray detection and the broad-spectrum pulsed neutron source of the Los Alamos Neutron Science Center's Weapons Neutron Research facility were used for the measurement. The energy of the incident neutrons was determined using the time-of-flight technique.Results: Absolute partial $\ensuremath{\gamma}$-ray cross sections were measured for 57 transitions feeding isomers and ground states in $^{101,102,103}\mathrm{Rh}$ and $^{107,108,109}\mathrm{Ag}$. The feeding of the isomers was found to be very similar in the corresponding reaction channels and it is compared to the feeding determined for the ground states.Conclusions: The opening of reaction channels at higher neutron energies removes angular momentum from the residual nucleus and reduces the population of the higher-spin isomers relative to the feeding of the lower-spin ground states. Similar behavior was observed in the mass $A=190$ region in the feeding of higher-spin isomers, but the reverse behavior was observed in $^{176}\mathrm{Lu}$ with a lower-spin isomer and a higher-spin ground state.