Isotopic distributions of thermal-neutron-induced fission fragments of near-symmetric fission of Pu239,241 determined using calorimetric low-temperature detectors

Abstract

Isotopic distributions were measured for the light fragment group in the transition region from asymmetric to symmetric fission for thermal neutron induced fission of $^{239}\mathrm{Pu}$ and $^{241}\mathrm{Pu}$ 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 $^{241}\mathrm{Pu}({n}_{\mathrm{th}},f)$ 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 $^{241}\mathrm{Pu}({n}_{\mathrm{th}},f)$ and from $A=109$ to $A=113$ for $^{239}\mathrm{Pu}({n}_{\mathrm{th}},f)$. The isotopic yields were used to evaluate the charge polarization $\mathrm{\ensuremath{\Delta}}Z$ and the proton odd-even effect $\ensuremath{\delta}p$ 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.