Measurements of Equilibrium and Time-Dependent Energy Spectra of Beta Rays from Californium-252 Fission Fragments

Absolute measurements were made for the equilibrium and time-dependent beta-ray energy spectra from fission fragments produced by spontaneous fission of ²⁵²Cf. The beta rays were detected by a plastic scintillator and the fission rate was monitored by a semiconductor detector. The equilibrium spectrum was measured from a ²⁵² Cf source permanently encapsulated between two Mylar foils thick enough to stop the fragments. The time-dependent measurements used a ²⁵²Cf source supported on a thin nickel foil and covered with a thin collodion foil. Virtually all fragments from this second source escaped the foils. By use of a special transport system, the time dependence of the ²⁵²Cf beta-ray energy spectrum was measured for two cases: (a) buildup of the spectrum following initiation of fragment collection on clean catcher foils, and (b) decay of the spectrum following a period of fragment collection. The measured spectra were corrected for energy resolution effects by unfolding them with the use of a measured system response function. The measured data were used to generate empirical expressions for the equilibrium and time-dependent beta-ray spectra from ²⁵² Cf fission fragments. The total number of beta rays from one fission was estimated to be 6.7 ± 0.3, while the total beta-ray energy was estimated to be 6.1 ± 0.5 MeV. The results of the measurements were compared with the analogous measurements for thermal-neutron fission of ²³⁵U. High-energy transitions (5 to 10 MeV) found for ²³⁵U fission were not found for ²⁵²Cf fission.