Independent fission yields of Rb and Cs fromU238induced by fission-spectrum neutrons

The relative independent yields of Rb and Cs from ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ fission induced by fission-spectrum neutrons have been determined by means of a mass spectrograph on line with a nuclear burst reactor. The spectrograph ion source is a tantalum oven containing a target of high purity ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ in a graphite matrix. After irradiation, the radioactive fission products are deposited on the focal plane of the spectrograph and each mass position is analyzed quantitatively from its $\beta$ activities. Independent yields were calculated by normalizing the measurements using chain yield values and fractional independent yields that were calculated from recently measured cumulative yields for Kr and Xe. The results indicate a charge deviation from an unchanged charge distribution of 0.41 ± 0.07 nuclear charge units, a scaling factor of 1.27 for the number of neutrons associated with ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ fission over the prompt number from thermal-neutron fission of ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ for these fission products, and values of $\sigma$ for the isobaric charge distribution of 0.49 ± 0.04 and 0.60 ± 0.02 for Rb and Cs, respectively. An odd-even $Z$ effect is noticeable in that the fractional independent yields are generally lower by (20 ± 10)% for both Rb and Cs than the mean values estimated from a smooth theoretical model. The sum of the estimated independent yields for the Rb isotopes agrees within error with that for the isotopes of the complementary element Cs.