An extended theory of multiplicity counting from fission chamber signals in the current mode

Paper i proceeding, 2017

This paper concerns the derivation of the individual and joint statistics of the signals of up to threefission chambers operating in the current mode, detecting neutrons emitted from a sample containing fissioningmaterial. The purpose is to develop an alternative method to the traditional pulse detection based multiplicitycounting for the determination of the sample parameters. The underlying theory and corresponding method ofunfolding the parameters of the sample from such continuous signals was recently developed by the authors forthe case when multiple neutrons emitted simultaneously were assumed to be also detected simultaneously inthe same or different detectors. In the present paper the method is generalized by extending it to the case whenthe detection of the multiply emitted neutrons occurs with a random time delay individually for each neutron,such that the delays are independent, identically distributed random variables. It is seen that in the arisingformulas, in addition to the detector pulse shape and amplitude distribution, the properties (parameters) ofthe time delay distribution appear as well. At the same time it is also seen that, although at the expense of asomewhat more involved calibration procedure, the unfolding of the sample parameters from the three lowestorder auto- and cross-cumulants of the detector signals is still possible, in a procedure similar to using thesingles, doubles and triples count rates of traditional multiplicity counting. In contrast to this latter method, theprocedure proposed here is free from the dead time problem, and requires a somewhat simpler data processing.In particular, being free from the dead-time problem and by the relative insensitivity of the fission chambers togamma contributions, especially for the higher order cumulants, makes the method particularly suitable forthe multiplicity analysis of spent fuel.