Optimization of the Clipping Time Constant for BF3 Neutron Detectors Operated in a High Level Gamma Environment

Radiation-hardened BF3 neutron detectors (proportional counters) have been selected for use as delayed neutron detectors in the failed fuel monitoring system of the Fast Flux Test Facility (FFTF) being constructed by the Westinghouse Hanford Company at Richland, Washington. These detectors must exhibit fpproximately 6 to 7 counts/sec (cps) per neutron/cm2-sec (nv) for neutrons while operating in a gamma environment of a about 1000 R/hr; thus, electronic counting systems capable of providing the best possible sensitivity must be used. In addition to exhibiting appropriate charge gain and sufficiently low noise, the preamplifier (or amplifier) must include at least one stage of differentiation to reduce gamma interference (pulse pileup) effects. The selection of the time constant depends on the rise time of the neutron-caused pulse from the BF3 tube and on the maximum gamma level in which the tube must operate. The conflicting requirements of large time constant to maintain pulse amplitude and small time constant to reduce gamma effects lead to an optimum value to achieve the best possible neutron detection sensitivity.