Fission yield measurements of Rb, Sr, Cs and Ba isotopes far from the center of the isotopic yield distributions in 235U( nth, f)

Abstract

Independent fission yields of 92–99Rb, 94–100Sr, 142–148Cs and 143–149Ba in the thermal neutron fission of 235U were determined at the SOLIS on-line mass separator. The independent fission yields of 99Rb, 147, 148Cs and 145–149Ba were measured for the first time. For all the elements a “wing effect” is apparent, i.e. the experimental independent fission yields in the wings of the isotopic (isobaric) yield distributions are considerably higher and their decline with mass (charge) less steep than predicted by fission yield systematics. To account for this effect a corrected semi-empirical distribution function for fission yields is proposed. Comparison of the experimental fission yields with the predictions of theoretical fission models also shows the existence of the “wing effect”. Since the theory predicts primary fission yields a correction due to prompt neutron emission was calculated. It is shown that the effect of prompt neutron emission is to broaden the isotopic distributions in the wing of heavy isotopes only slightly, and in the wing of light isotopes more significantly, but it does not explain the “wing effect” which seems to originate in the fission process.