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

Fission-fragment mass distributions for the target nuclei 232Th, 233,235,236,238U, 238,239,240,242Pu, and 244Cm have been measured at an incident proton energy Ep=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 ZCN the asymmetric fission-fragment charge distributions practically coincide, i.e., they are almost independent on NCN. Moreover, for nuclei with equal NCN from isotonic pairs with NCN=144,146, and 148 the fragment neutron-number distributions vary with a change of ZCN. 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.