Isotopic and isotonic effects in fission-fragment mass yields of actinide nuclei

Fission-fragment mass distributions for the target nuclei Th-232, U-233,U-235,U-236,U-238, Pu-238,Pu-239,Pu-240,Pu-242, and Cm-244 have been measured at an incident proton energy E-p = 10.3 MeV, and proton and neutron-number distributions of the fragments have been deduced on the basis of the unchanged-charge-density assumption. It was revealed that the shape of the mass distribution in asymmetric fission strongly depends on the proton number of the compound nucleus and shows only a weak dependence on neutron numbers. It was shown that for nuclei with equal Z(CN) the asymmetric fission-fragment charge distributions practically coincide, i.e., they are almost independent on N-CN. Moreover, for nuclei with equal N-CN from isotonic pairs with N-CN = 144, 146, and 148 the fragment neutron-number distributions vary with a change Of Z(CN). This behaviour of the mass, charge and neutron-number distributions is an evidence for the determinative influence of proton shells on the formation of fragments in asymmetric fission of actinide nuclei. (C) 2002 Published by Elsevier Science B.V.