Characteristics of light charged particle emission in spontaneous and neutron induced fission of Cm and Cf isotopes

Talking about fission, people think almost automatically about a nucleus that splits into two heavy pieces. However, in 1946 the phenomenon of ternary fission has been discovered, in which the two fission fragments are accompanied by a light charged particle. Ternary fission data are of interest for nuclear physics, however also the nuclear industry requests accurate data for ternary fission yields, especially 3H and 4He, since they are at the origin of the production of He gas and the radioactive tritium gas in the reactor. This work contains a study of the emission probabilities and the characteristics of the energy distributions of the most common ternary particles (3H, 4He and 6He) at different excitation energies. A series of 'isotope couples' has been investigated: the neutron induced fission, performed at the ILL in Grenoble, of 243Cm and the spontaneous fission, performed at the IRMM in Geel, of 244Cm, 245Cm(n,f) – 246Cm(SF), 247Cm(n,f) – 248Cm(SF), 249Cf(n,f) – 250Cf(SF) and 251Cf(n,f) – 252Cf(SF). With all these new results, measured with suited DE-E telescope detectors, a significant enlargement of the existing data base was realised. The data set permits to make some important observations. First of all we can conclude that the average energy for a certain ternary particle remains constant within the uncertainties. Another striking observation is that for the same compound nucleus, the FWHM for the ternary alpha energy distribution is systematically 0.3 MeV smaller for spontaneous fission than for neutron induced fission. Furthermore the FWHM of a certain ternary particle linearly increases with increasing Z2/A. The correlation between the emission probabilities of the ternary particles and a number of experimental parameters has been investigated and the importance of the cluster preformation probability for 4He and 6He particles was demonstrated.