Fission fragment mass and energy distributions as a function of incident neutron energy measured in a lead slowing-down spectrometer

Abstract

A new method of measuring fission fragment mass and energy distributions as a function of incident neutron energy in the range from below 0.1 eV to 1 keV has been developed. The method involves placing a double-sided Frisch-gridded fission chamber in Rensselaer Polytechnic Institute's lead slowing-down spectrometer (LSDS). The high neutron flux of the LSDS allows for the measurement of the energy-dependent, neutron-induced fission cross sections simultaneously with the mass and kinetic energy of the fission fragments of various small samples. The samples may be isotopes that are not available in large quantities (submicrograms) or with small fission cross sections (microbarns). The fission chamber consists of two anodes shielded by Frisch grids on either side of a single cathode. The sample is located in the center of the cathode and is made by depositing small amounts of actinides on very thin films. The chamber was successfully tested and calibrated using $0.41\ifmmode\pm\else\textpm\fi{}0.04$ ng of $^{252}\mathrm{Cf}$ and the resulting mass distributions were compared to those of previous work. As a proof of concept, the chamber was placed in the LSDS to measure the neutron-induced fission cross section and fragment mass and energy distributions of $25.3\ifmmode\pm\else\textpm\fi{}0.5 \ensuremath{\mu}$g of $^{235}\mathrm{U}$. Changes in the mass distributions as a function of incident neutron energy are evident and are examined using the multimodal fission mode model.