Neutron Spectroscopy In A Fast Subcritical Reactor With A Proton Recoil Proportional Counter

Measurements of degraded fission-neutron spectra using recoil proportional counters are done routinely for studies involving fast reactor mockups. The same techniques are applicable to measurements of neutron spectra required for personnel dosimetry in fast neutron environments. A brief discussion of current applications of these methods together with the results of a measurement made on the LITTLE BOY assembly at Los Alamos are here described.

A methane filled proton-recoil proportional counter will be used as a fission neutron detector in the fast-neutron hodoscope. To provide meaningful fuel-motion information the proportional counter should have: a linear response over a wide range of reactor powers background ratio (the number of high energy neutrons detected must be maximized relative to low energy neutrons, and gamma ray sensitivity must be kept small); and a detector efficiency for fission neutrons above 1 MeV of approximately 1%. In addition, it is desirable that the detector and the associated amplifier/discriminator be capable of operating at counting rates in excess of 500 kHz. This paper reports on tests that were conducted on several proportional counters at the TREAT reactor.

The response to a 1/E neutron spectrum of a hydrogen-filled proportional counter in the energy region below 100 keV was studied using pulse-shape discrimination to reject gamma-induced background. The observed proton-recoil distributions permitted an evaluation of W (the energy loss per ion pair) for protons in hydrogen below 19 keV. Some problems associated with the use of 4Pi-recoil proportional counters as neutron spectrometers were considered including calibration, optimization of neutron-to-gamma ratios, and resolving power.

The response to a 1/E neutron spectrum of a hydrogen-filled proportional counter in the energy region below 100 keV was studied using pulse-shape discrimination to reject gamma-induced background. The observed proton-recoil distributions permitted an evaluation of W (the energy loss per ion pair) for protons in hydrogen below 19 keV. Some problems associated with the use of 4Pi-recoil proportional counters as neutron spectrometers were considered including calibration, optimization of neutron-to-gamma ratios, and resolving power.