Stochastic Theory of the Fission Chamber Current Generated by Non-Poissonian Neutrons

Pál, Lénárd and Pazsit, I. (2016) Stochastic Theory of the Fission Chamber Current Generated by Non-Poissonian Neutrons. NUCLEAR SCIENCE AND ENGINEERING, 184 (4). pp. 537-550. ISSN 0029-5639

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Abstract

The Campbell theorem, relating the variance of the current of a fission chamber (a “filtered Poisson process”) to the intensity of the detection events and to the detector pulse shape, becomes invalid when the neutrons generating the fission chamber current are not indepen- dent. Recently a formalism was developed [1] by which the variance of the detector current could be calculated for detecting neutrons in a subcritical multiplying system, where the detection events are obviously not independent. In the present paper, the previous formal- ism, which only accounted for prompt neutrons, is generalised to account also for delayed neutrons. A rigorous probabilistic analysis of the detector current was performed by us- ing the same simple, but realistic detector model as in the previous work. The results of the present analysis made it possible to determine the bias of the traditional Campbelling techniques both qualitatively and quantitatively. The results show that the variance still remains proportional to the detection intensity, and is thus suitable for the monitoring of the mean flux, but the calibration factor between the variance and the detection intensity is an involved function of the detector pulse shape and the subcritical reactivity of the system, which diverges for critical systems.

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