A portable measurement system for subcriticality measurements by the CF-source-driven neutron noise analysis method

A portable system has been assembled that is capable of measuring the subcriticality of fissile materials using the 252CF-source-driven neutron noise analysis method. The measurement system consists of a parallel-plate ionization chamber containing 252CF, two 3He proportional counters with their associated electronics, and a small computer containing anti-aliasing filters and A/D convertors. The system Fourier analyzes the digitized data and forms the appropriate auto and cross-power spectral densities. These spectra are used to form a ratio of spectral densities, G12G13/G11G23, where 1 refers to the ionization chamber, and 2 and 3 refer to the 3He counters, from which subcriticality can be determined. The chamber and detectors are located appropriately near the fissile material. The system is capable of sampling signals at rates of up to 80 kHz and processing these data at rates of 2 kHz to form the appropriate spectra. The presently configured system is a two-channel system, hence the measurement of G12, G13, and G23 must be done sequentially before the ratio of spectral densities is obtained. Future improvements of the system will allow simultaneous measurement of all spectra and will further reduce size, thereby enhancing portability. This measurement system can provide reliable, cost effective, and convenient determination of the subcriticality of a wide variety of fissile materials and moderators