FINE-STRUCTURE OF MASS AND CHARGE-DISTRIBUTION IN LOW-ENERGY FISSION

Studies of finer details in mass and charge distribution in fission leads to a better understanding of the fission process. Experimental determination of independent and cumulative yields using radiochemical techniques as well as mass spectrometers and fission product recoil separators form the basis of such studies. It has been established that closed shells as well as an even number of nucleons influence both mass and charge distributions. The magnitudes of these effects may be estimated from existing experimental yield data and various fission models. Using our measurements of several fission yields and those existing in the literature we have calculated even-odd proton and neutron effects for various low energy fissioning systems. Where enough data existed, direct calculations were made, whereas for other cases the Z(p)-model of WAHL has been used. It is found that the even-odd proton effect is well established and pronounced in thermal neutron fission of U-235 and U-233. Lesser effects were found for reactor neutron induced fission of Th-232, thermal neutron fission of Pu-239 and spontaneous fission of Cm-245 and Cf-249. No effect seems to exist in the thermal neutron fission of Pu-241 and the spontaneous fission of Cf-252. The even-odd neutron effect is found to be much lower than the corresponding proton effect in U-235 and U-233 fissions and is nonexistent in the rest of the fissioning systems.