Analytical expressions for the gate utilization factors of passive multiplicity counters including signal build-up

摘要

In the realm of nuclear safeguards, passive neutron multiplicity counting using shift register pulsetrain analysis to nondestructively quantify Pu in product materials is a familiar and widely appliedtechnique. The approach most commonly taken is to construct a neutron detector consisting of ~3Hefilled cylindrical proportional counters embedded in a high density polyethylene moderator. Fastneutrons from the item enter the moderator and are quickly slowed down, on timescales of the orderof 1-2 μs, creating a thermal population which then persists typically for several 10’s μs and issampled by the ~3He detectors. Because the initial transient is of comparatively short duration it hasbeen traditional to treat it as instantaneous and furthermore to approximate the subsequent capturetime distribution as exponential in shape. With these approximations simple expressions for thevarious Gate Utilization Factors (GUFs) can be obtained. These factors represent the proportion oftime correlated events i.e. Doubles and Triples signal present in the pulse train that is detected bythe coincidence gate structure chosen (predelay and gate width settings of the multiplicity shiftregister). More complicated expressions can be derived by generalizing the capture timedistribution to multiple time components or harmonics typically present in real systems.When it comes to applying passive neutron multiplicity methods to extremely intense (i.e. highemission rate and highly multiplying) neutron sources there is a drive to use detector types withvery fast response characteristics in order to cope with the high rates. In addition to short pulsewidth, detectors with a short capture time profile are also desirable so that a short coincidence gatewidth can be set in order to reduce the chance or Accidental coincidence signal. In extreme cases,such as might be realized using boron loaded scintillators, the dieaway time may be so short that thebuild-up (thermalization transient) within the detector cannot be ignored. Another example wheresignal build-up might be observed is when a ~3He based system is used to track the evolution of thetime correlated signal created by a higher multiplying item within a reflective configuration such asthe measurement of a spent fuel assembly. In this work we develop expressions for the GUFswhich include signal build-up.