Measurement of Post-neutron Mass Distribution in Thermal-neutron-induced Fission of 239Pu

Nuclear fission is one of the most complex physical processes. Up to now, there is still a lack of theory that can well describe the pre-fission and post-fission processes. Mass yields in the neutron-induced fission of 239Pu provide fundamental and significant data support for nuclear energy utilization, but the independent fission yields are scarce and far from precise. In addition, the current evaluation work for fission yields partially refers to the experimental data based on radiochemical methods and mass spectrometric, it will make sense to compare the evaluation data with the data obtained by direct systematic measurement. In order to determine independent mass yield distributions precisely, the fission fragments identification spectrometer (FFIS) device was developed. The axial grid ionization chamber and timing detectors based on microchannel plates were designed and tested for determining the kinetic energy and time-of-flight of fission fragments, respectively. Specifically, the intrinsic energy resolution for the ionization chamber was measured to be about 0.5% for 80 MeV 63Cu particles, and the time resolution was about 157 ps (FWHM) determined by 241Am source. By directly coincident measurement of energy and velocity of the fission fragments on their flight path, the post-neutron-emission masses can be obtained according to the classical formula for kinetic energy. The mass yield distribution in the thermal-neutron-induced fission of 239Pu was measured at the in-hospital neutron irradiator (IHNI-1). With the flight trajectory of 100.45 cm, the relative uncertainty of the flight path length is better than 0.1%, and the time resolution is about 0.2%. When using the E-v method to measure mass distribution, two vital issues need to be resolved. For one thing, the non-linear energy response of the detector affects the accuracy of the mass calculation. In this work, energy losses in the carbon foil of the stop timing detector and the silicon nitride window of the ionization chamber were corrected event-by-event based on Geant4 calculation. For another thing, the scattering of heavy ions or the inhomogeneous of the detector materials may make the recorded energy to be underestimated. These abnormal data points are hard to eliminate in the valley region, which makes the mass yields of symmetric fission fragments being overestimated, and improving statistics may relatively reduce this kind of effect. Taking into account the correction uncertainty, the energy resolution for the light fission fragment is better than 0.7%, and that is approximately 1% for heavy fragments peak. Thus, the mass resolution is estimated to be 1 amu at 99 amu, and 1.5 amu at 138 amu. © 2022, Editorial Board of Atomic Energy Science and Technology. All right reserved.