# Spontaneous Fission

A direct method to measure fission product yield distributions (FPY) and isomeric yield ratios (IYR) for spontaneous fission (SF) fragments can be implemented with the FRS Ion Catcher. These physical properties are of utmost importance to the understanding of basic nuclear physics, the astrophysical rapid neutron capture process (‘r process’) of nucleosynthesis, neutron star composition and nuclear reactor safety. With this method, fission fragments are produced by spontaneous fission from a source that is mounted in the cryogenic stopping cell (CSC), thermalized and stopped within it and then extracted and transported to a multiple-reflection time-of-flight mass-spectrometer (MR-TOF-MS) to separate and identify by mass all isobars and numerous isomers in the fission fragment realm. The system’s essential element independence and its fast simultaneous mass measurement provide a new direct way to measure isotopic FPY distributions, which is complementary to existing methods. It will enable nuclide FPY measurements in the high fission peak, which is hardly accessible by current techniques. The extraction time of the CSC, tens of milliseconds, enables a direct measurement of independent fission yields, and a first study of the temporal dependence of FPY distributions in this duration range. The ability to resolve isomers will further enable direct extraction of numerous IYRs while performing the FPY measurements.

The method has been recently demonstrated at the FRS Ion Catcher for with a 37 kBq $^{252}$Cf fission source, where about 70 different fission fragments have been identified and counted. In the near future, it will be used for systematic studies of SF with a higher-activity $^{252}$Cf source and a $^{248}$Cm source. This method can be implemented also for neutron induced fission at appropriate facilities.