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

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.

Independent isotopic fission yields of 252Cf spontaneous fission via mass measurements at the FRS Ion Catcher

“…The amount of each FP is extracted by a wellestablished fitting technique that takes into account also overlapping peaks [17]. For more details on the measurement technique, see [9].…”

“…The mass resolving power (MRP) of the MR-TOF-MS at the FRS-IC can reach up to 1,000,000 [18]. However, for measuring fission yields a broadband mode was preferred, covering approximately 10 amu/e with an MRP of 320,000, corresponding to a flight time of approximately 9 ms in the MR-TOF-MS. A typical time-of-flight spectrum from this experiment is given in Fig.…”

“…We recently demonstrated a method to measure isotopic yields of spontaneous fission (SF) [9] via direct ion counting at the FRS Ion Catcher (FRS-IC) at GSI [10], utilizing its high-resolving multiple-reflection time-of-flight mass-spectrometer (MR-TOF-MS) [11]. Measurements of SF properties are important for the basic understanding of fission, since the fission process takes place at a specific excitation energy, which enables to benchmark theories.…”

NG-TRAP: Measuring neutron capture cross-sections of short-lived fission fragments

“…They are subsequently extracted from the CSC using an RF carpet. Such a CSC has already been developed [10] by the Justus-Liebig University (JLU) Gießen, Germany, and major concepts have been tested at GSI, Darmstadt, Germany [11][12][13]. The fission fragments are then passed through a RF mass filter (selecting the mass number of interest) to an RF trap, where a selected isobar chain will be stored to form a "target".…”

“…Note that we will measure reaction rates for several nuclei of the selected isobaric-chain simultaneously, which will save on beam time and help to increase statistics. This is possible due to the high mass resolution that can be achieved in modern time-of-flight mass spectrometers [18,19]. The typical analysis time will be one minute, which is dominated by the average time it takes the ions to leave the target RF trap, much longer than the detection and identification by the MR-TOF-MS, which takes about 10 ms.…”