Use of a Pulse Detector for Reactor Noise Measurements

A pulse-mode neutron detection system designed for reactor noise measurements was characterized and compared with conventional current-mode noise measurement systems. Pulses from a proportional counter with a 60-nsec electron collection time were amplified and applied to a discriminator and thence to a counting-rate circuit with a time constant of 15 µsec. Statistical fluctuations in the counting-rate voltage were frequency analyzed. Under conditions of negligible gamma flux and counting loss, the pulse system yielded frequency spectra indistinguishable from ion-chamber spectra. The results were not very sensitive to counting loss up to at least 20%, but the effect of counting loss limited the ultimate useful neutron flux for the system tested to <2 × 10⁶ n/(cm² sec). Space charge and gamma pileup in the detector controlled the performance of the pulse system in high gamma fluxes; the pulse system performed better than the best available current system over a limited range of neutron- and gamma-flux intensities. Because of its shorter time constant, the pulse-mode system can be used to measure power spectral density at much higher frequencies than the current-mode system. Thus, the pulse-mode system appears to be the more attractive for fast reactor subcriticality measurements.