Extraction of correlated count rates using various gate generation techniques: Part I theory

This paper presents an overview of different gate generation techniques that can be used to extract correlated counting rates from neutron pulse trains in the context of Passive Neutron Multiplicity Counting (PNMC). PNMC based on shift register pulse train time autocorrelation analyzers is an important Non-Destructive Assay (NDA) method used in the quantification of plutonium and other spontaneously fissile materials across the nuclear fuel cycle. Traditionally PNMC employs signal-triggered gate generation followed by a random gate, separated from the trigger pulse by a long delay, to extract the totals rate (gross or singles), the pairs (coincidences or doubles) rate, and the triplets (or triples) rate of correlated neutron pulse trains. In this paper we provide expressions for singles, doubles and triples rates using the information available in both, the random and signal-triggered gates (traditional shift register analysis), in the randomly triggered gates only, and introduce a third approach to extract the correlated rates using signal-triggered gates only. In addition, we expand the formalism for randomly triggered gate generation to include Fast Accidental Sampling (FAS) and consecutive gate generation.