Determining spontaneous fission properties by direct mass measurements with the FRS Ion Catcher

¹ Soreq Nuclear Research Center, Yavne 81800, Israel
² Tel Aviv University, Tel Aviv 69978, Israel
³ GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291 Darmstadt, Germany
⁴ Justus-Liebig-Universitat Gießen, 35392 Gießen, Germany
⁵ IFIN-HH/ELI-NP, 077126, Magurele - Bucharest, Romania
⁶ Université Laval, Québec G1V 0A6, Canada
⁷ KVI-CART/University of Groningen, 9700 AB Groningen, the Netherlands
⁸ Department of Physics and Astronomy Uppsala University, SE-751 05 Uppsala, Sweden
⁹ University of Novi Sad, 21101 Novi Sad, Serbia

^* e-mail: mardor@tauex.tau.ac.il

Published online: 30 September 2020

Abstract

We present a direct method to measure fission product yield distributions (FPY) and isomeric yield ratios (IYR) for spontaneous fission (SF) fragments. 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 a 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). We will implement the method at the FRS Ion Catcher (FRS-IC) at GSI (Germany), whose MR-TOF-MS relative mass accuracy (~ 10^-7) and resolving power (~ 600,000 FWHM) are sufficient to separate 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-ICr for SF with a 37 kBq ²⁵²Cf 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 ²⁵²Cf source and a ²⁴⁸Cm source. The method can be implemented also for neutron induced fission at appropriate facilities.