Isotopic Distributions Of Thermal-Neutron-Induced Fission Fragments Of Near-Symmetric Fission Of (Pu)-P-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 Pu-239 and Pu-241 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 Pu-241(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 Pu-241(nth , f) and from A = 109 to A = 113 for Pu-239(nth, f). The isotopic yields were used to evaluate the charge polarization Delta Z and the proton odd-even effect delta p 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.