A STANDARDIZED METHOD FOR MAKING NEUTRON FLUENCE MEASUREMENTS BY FISSION FRAGMENT TRACKS IN PLASTICS,

A neutron detector is described which consists of a fission foil 232Th, 235U, 238U, 237Np or 239Pu in contact with a plastic track detector. These detectors were exposed to reactor neutrons and to monoenergetic neutrons with energies between 1.0 - 18 MeV. Fission fragment tracks registered in the plastic were selectively etched by an hydroxide and counted in an optical microscope. For thick foils of fissionable metals the sensitivity of the system was found to be 1.16 plus or minus 3 X 10 to the minus 5th power fission fragment tracksneutron - barn which is in good agreement with theoretical calculations. This sensitivity is independent of the fissionable element used, independent of the neutron energy, fairly independent of the material chosen for track registration plastics, glass, mica and of etching conditions. Since the n,f cross sections are accurately known for most neutron energies, the above constant can also be used for standardized measurements of neutron fluences.