Perspectives of laser-chemical isotope separation of a long-lived fission product: cesium-135

Abstract

The cesium isotope 135Cs has an extremely long half-life (τ1/2 = 2.3 · 106 y) and its high water solubility leads to the anxiety of exudation to ground water during geological disposal. Such a LLFP 135Cs would be converted into 136Cs (Its half-life is 13.16 d and it becomes stable 136Ba) by neutron capture reaction. However intermingling 133Cs of which the natural abundance is 100% disturbs this nuclear converting reaction because 133Cs also absorbs neutrons and produces 135Cs again. For separating 135Cs from other cesium isotopes, laser-chemical isotope separation (LCIS) is believed to be suitable mainly due to the light absorption and emission stability. Isotope separation of alkali metal 85Rb/87Rb was successfully achieved, showed 23.9 of head separation factor by LCIS. The measured isotope shift of Cs D2 line is within the reach of available semiconductor lasers having emission line width of less than 1 MHz, which shows that the selective excitation of 135Cs may turn to be possible. It is known that cesium excited to the 62P3/2 state may forms cesium hydride while ground-state cesium does not. Therefore if the lifetime of 62P3/2 state is sufficiently longer than the inverse rate of the chemical reaction, 135Cs can be extracted as cesium hydride. Applicability of the Doppler-free two-photon absorption method for selective excitation and further evaluation on Rydberg states and ionization should be investigated.