The relative yields of Rb and Cs isotopes from thermal-neutron fission of ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ have been redetermined using the mass separator OSTIS, on-line at a neutron guide of the High-Flux Beam Reactor at the Institut Laue-Langevin, Grenoble, France. The separator ion source was a hot oven containing ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ in a graphite matrix. The neutron beam was pulsed. Alkali fission products diffused out of the graphite and were ionized, thus producing a stepwise increase in the analyzed ion beam proportional to the independent fission yield. The ion beam and the fissions in the source were monitored simultaneously. The diffusion of Rb and Cs from the source was exponential in time with half-lives ranging from 2.8 to 18 sec, depending upon the element and source temperature. The independent fission yields of Rb and Cs are normalized by equating their element yields to each other and to a value computed from the charge distributions observed with the recoil separator LOHENGRIN and well established mass yields. Fractional independent yields are deduced from the independent fission yields, and these compare very well with the EOZ model described by Wahl.

NUCLEAR REACTIONS ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$($n$,$f$), $E=\mathrm{thermal}$; measured relative abundances; deduced independent fission yields, fractional chain yields, charge displacement and dispersion, odd-even $Z$ effect. Mass separator, fast neutron detector.

• Received 15 May 1979

DOI:https://doi.org/10.1103/PhysRevC.20.2244