Fission fragment distribution measurements with time of flight - energy (ToF-E) spectrometers

Abstract

Neutron induced fission has been studied for over seventy years, however, there is still much to learn about the process. Evidence has been shown for energy dependence of the neutron inducing fission on the fission product yield (FPY) distribution, and there is very little existing data for the FPY of various isotopes of interest as a function of incident neutron energy. The CSM FPY time of flight - energy (ToF-E) spectrometer was developed and measurements were performed with 252Cf(sf) in support of the Spectrometer for Ion Detection in Fission Research (SPIDER) collaboration. A variety of ionization chamber (IC) entrance windows were investigated. Stretched polypropylene and a commercial option from LUXEL were found to be adequate entrance windows. Silicon nitride windows might offer the least amount of energy loss from fission products through the entrance window, however, there is a significant amount of scattering of fission products off of the frame and support structures, which has shown to impact FPY results in the symmetric fission region as well as the upper and lower ends of the FPY distribution. The impact from scattering on FPY results are greatly reduced with either the stretched polypropylene window or the LUXEL window. Single arm operation of the CSM FPY ToF-E spectrometer from 252Cf(sf) was shown to have a potential lower limit on mass resolution of 1.5 amu full width at half maximum (FWHM) for light fission products and 3.0 amu FWHM for heavy fission products, operating with a 40 cm arm length, a stretched polypropylene entrance window (92 μg/cm2 thickness), and 30 μg/cm2 thick carbon conversion foils. Several methods for extracting nuclear charge information from the Frisch grid (FG) signal of the IC were investigated. Nuclear charge sensitivity was exhibited with a potential resolution around 4 amu FWHM. Single arm analysis of SPIDER data from 235U(nth,f) with 70.0 & 67.3 cm arm lengths and 200 nm thick silicon nitride entrance windows showed a potential lower limit on mass resolution of 1.5 amu FWHM for light products and 3.0 amu FWHM for heavy products. Dual arm analysis with the double energy - double velocity (2E-2v) method revealed improvement in the mass resolution showing a potential lower limit possibly better than 1.0 amu FWHM for light products and 1.5 amu FWHM for heavy products. Single arm analysis of SPIDER data from 233U(nth,f) with the same operating conditions as the 235U(nth,f) run showed a potential lower limit on mass resolution of 2-3 amu FWHM for light products and 4-6 amu FWHM for heavy products. Again, using the 2E-2v analysis method resulted in improvement in the mass resolution achieving a potential lower limit of 1.5 amu FWHM for light products and 3.0 amu FWHM for heavy products. Further investigation into the 233U target revealed potential “crud” in the chemical com- position deposited during the production process. This “crud” caused degradation in mass resolution by a factor of almost two, indicating FPY mass resolution dependence on target quality. The goal of unit mass resolution from SPIDER has been shown to be achievable with use of clean targets, 70 & 67.3 cm arm lengths, 20 μg/cm2 thickness carbon foils, and 200 nm thick silicon nitride entrance windows.