Isotopic distributions of thermal-neutron-induced fission fragments of near-symmetric fission of Pu 239,241 determined using calorimetric low-temperature detectors

Isotopic distributions were measured for the light fragment group in the transition region from asymmetric to symmetric fission for thermal neutron induced fission of Pu239 and Pu241 using the novel technology of calorimetric low temperature detectors in combination with the passive absorber method. Nuclear charge distributions were determined for 24 masses in the range A=89 to A=112 for Pu241(nth,f) for the first time with the LOHENGRIN mass spectrometer. Moving from asymmetric to symmetric fission, known data were supplemented for masses from A=110 to A=112 for Pu241(nth,f) and from A=109 to A=113 for Pu239(nth,f). The isotopic yields were used to evaluate the charge polarization ΔZ and the proton odd-even effect δp with emphasis on the borderline between asymmetric and symmetric fission. This highlighted a virtual shell effect near Z=44 which mirrors the effect of the Z=50 shell in the complementary heavy fragment group of the two Pu isotopes (Z=94) studied.