Theory of fission detector signals in reactor measurements - detailed calculations

TL;DR

This paper extends a formalism for calculating fission detector current variance in reactor measurements to include delayed neutrons, revealing how traditional Campbell techniques are biased and how calibration depends on system parameters.

Contribution

The authors generalize previous models to account for delayed neutrons, providing a more accurate analysis of detector signals in subcritical and critical systems.

Findings

Variance remains proportional to detection intensity

Calibration factor depends on pulse shape and reactivity

Biases in traditional Campbell techniques are quantified

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 independent. Recently a formalism was developed by the present authors [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 formalism, 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 using 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.