Instrumentation to measure the ratio of neutron capture to fission cross sections at keV neutron energies
Instrumentation for measurement of α (the ratio of the neutron capture cross section to the fission cross section) was designed, assembled, and calibrated, and measurements were made for 239Pu, 233U, and 235U at neutron energies from 10 to 600 keV. The detector consisted of eight photomultiplier tubes located around the periphery of a 210-gal tank of gadolinium-loaded liquid scintillator. The time resolution of the system was 6.5 × 10 -9 sec under operating conditions. The energy resolution at the 2.5-MeV sum peak of 60Co was 35%. Neutron energies were measured by time-of-flight techniques using a flight path of 1 meter. Fission events were distinguished from capture events by the detection of thermalized fission neutrons following the primary events. Measurement of the relative efficiency of the large scintillator tank for detecting capture and fission events was made by simultaneously compiling the background and foreground pulse-height spectra for both the capture and fission events. Typical spectra for the neutron time-of-flight for capture and fission events along with the background and foreground spectra for both types of events are shown. The techniques and adjustments used to obtain the timing resolution on the large tank are discussed. The logic necessary for distinguishing fission events and the foreground and background pulse-height spectra associated with both types of events are described.