An Experimental Study of the Time Dependence of the Beta Energy Spectrum from ²³⁵U Fission Fragments

An absolute experimental measurement has been made of the time dependence of the beta energy spectrum from fission fragments, specifically beta particles of energies greater than 0.75 MeV produced in the thermal neutron fission of uranium-235. This measurement has been made for four cases: the initiation of a constant fission rate in a cold uranium foil; shutdown after 1- and 3-h runs at a steady fission rate; and an instantaneous burst of fissions produced by a reactor pulse. The fission source was a foil coated with approximately 38 mg of ²³⁵U and placed in a thermal neutron beam from a reactor. The fission rate was measured with an ionization chamber. The beta energy spectrum was measured with a plastic scintillator, with absolute counting being determined by the known solid angle between source and detector. Background counts have been reduced to less than 10% in all cases. The total uncertainty in the analyzed data was less than 5% for the steady power runs and less than 8% for the reactor pulsing runs. The experimental results are in substantial agreement with those predicted by theory.