Ternary Fission of 240Pu and 242Pu

Using the facilities of the Brookhaven Graphite Research Reactor (BGRR) continuation of ternary fission investigations has been made by extending previous experiments to cover the fissioning systems 240Pu and 242Pu. The experiment involves fission fragment detection and energy analysis by three solid-state detectors positioned 120° apart in a plane about a fission source. The output pulse from each detector is digitally analysed and recorded event by event whenever a parallel triple coincidence circuit is satisfied. Earlier data, indicating clear distinctions in the ternary fission of these two nuclei, have been confirmed with much greater statistical confidence. The frequency of occurrence is approximately one per 106 binary-fission eventst the total-kinetic-energy release is slightly less than that for binary fission, about 150 MeV and 165 MeV for the systems 240Pu and 242Pu, respectively. In general, we find that the characteristics of this fissioning pair (240Pu and 242Pu)are in many respects remarkably similar to the features observed for the comparable fissioning pair, 234U* and 236U*. The differences between members in each pair closely parallel each other; this is especially true when the individual fragment-kinetic-energy spectra and the fragment mass distributions are compared. The distinction between the individual fragment-kinetic-energy spectra clearly rules out the possibility of Coulomb scattering, accidental, or spurious events as probable causes of the triple-coincidence data. Evidence of lightmass fragment production in or near the range 30-40 amu and 50-60 amu is also observed. The fission type ratio (I/II) defined in previous work has been evaluated as 0.6 ± 0.1 for 240Pu and 2.7 ± 0.8 for 242Pu further emphasizing the distinction between the two fissioning systems and reconfirming, in our interpretation, the extensive role which underlying shell-structure effects exhibit with respect to the tripartite mode of fission. (author)