The optimum choice of gate width for neutron coincidence counting

Abstract

In the measurement field of international nuclear safeguards, passive neutron coincidence counting is used to quantify the spontaneous fission rate of certain special nuclear materials. The shift register autocorrelation analysis method is the most commonly used approach. However, the Feynman-Y technique, which is more commonly applied in reactor noise analysis, provides an alternative means to extract the correlation information from a pulse train. In this work we consider how to select the optimum gate width for each of these two time-correlation analysis techniques. The optimum is considered to be that which gives the lowest fractional precision on the net doublets rate. Our theoretical approach is approximate but is instructional in terms of revealing the key functional dependence. We show that in both cases the same performance figure of merit applies so that common design criteria apply to the neutron detector head. Our prediction is that near optimal results, suitable for most practical applications, can be obtained from both techniques using a common gate width setting. The estimated precision is also comparable in the two cases. The theoretical expressions are tested experimentally using 252 Cf spontaneous fission sources measured in two thermal well counters representative of the type in common use by international inspectorates. Fast accidental sampling was the favored method of acquiring the Feynman-Y data. Our experimental study confirmed the basic functional dependences predicted although experimental results when available are preferred. With an appropriate gate setting Feynman-Y analysis provides an alternative to shift register analysis for safeguards applications which opening up new avenues of data collection and data reduction to explore.

The impact of gate width setting and gate utilization factors on plutonium assay in passive correlated neutron counting

“…In standard MSR electronics this expression corresponds to T g $ 1.27τ as shown in [1,10]. Extension of these considerations towards RTI approach was performed in [9], where it was shown that for purely random gates, this relationship is modified to T g $ 2.14τ. Wider optimum gate width setting is thus favored by the RTI analysis.…”

“…Graphical representation of relative standard deviation (RSD) versus coincidence gate width then exhibits a shallow minimum corresponding to the optimum gate width setting. This approach was applied in [9] to a series of 252 Cf sources to provide a comparison with the above theoretical estimates. It was shown that the optimum coincidence gate width varies strongly with the 252 Cf neutron emission rate with longer gates being favored for weaker sources, although it is common practice to use a universal coincidence gate width value for a given detector type.…”

“…Consequently, the recorded pulse train can be re-analyzed using an arbitrary number of coincidence gate width settings allowing detailed analysis and assessment of impact of these settings on the assay results. The variation of optimum coincidence gate width was studied for a range of 252 Cf sources in [9]. In this paper we will expand this work to a range of plutonium items representative of Pu items of operational interest and evaluate its impacts on accuracy of Pu assay.…”

Development and testing of neutron pulse time stamping data acquisition system for neutron noise experiment

“…Depending on the method used for the analysis, various combinations of gate generators, delay lines, shift register coincidence circuits and variable dead time circuits [6,7,9,10] are used to acquire the data in the experiment. Work related to gate width optimisation and gate generation technique was reported recently [11,12]. Thus the configuration of electronics modules for acquisition of data is specific to the theoretical method used for the data analysis.…”

Variance estimation in neutron coincidence counting using the bootstrap method

“…system. Naturally, the measurement variance has been studied before, and there are several publications regarding both the estimation [1,6,7] and the optimization [8,9] of the measurement variance. The emphasis in previous works is mainly on analytic expressions for the random variance, and the bootstrap method (or any other re-sampling method) was somewhat neglected.…”