Proportional neutron counters, such as He3-filled tubes, are the most efficient and widely used thermal neutron sensors. These detectors are based on the reaction mechanism He3(n,p)H3 with a Q-value of 764 keV. For fast neutron counting applications, proportional counters are typically embedded in neutron moderator materials. Neutron proportional counters have a relatively low gamma sensitivity, thus being well suited for counting applications in mixed neutron/gamma radiation fields. Dead time and pileup effect can be limiting factors in high neutron counting rate experiments with proportional counters. Pileup takes place when two or more detected events occur close enough in time that the measurement system responds as if it is a single event. The characteristic rise time of the pulse signal in a He3 proportional counter is fairly slow compared with other detectors (~ 4-8 us). For this reason, integration times of several microseconds should be used in the counting electronics. In practice, for these kinds of detectors, corrections due to dead time and pileup are required at counting rates ~5 kcps. The present work is motivated by measurements of secondary neutrons produced in proton therapy facilities at clinical conditions, where the proper assessment of dead time and pileup corrections is required at counting rates in the range typically from 10 – 50 kcps.
In this work, we have studied dead time and pileup corrections when using a self-triggered Digital Acquisition System (DAQ). In particular, it is used the GASIFIC DAQ  which is based on the SIS3316 digitizer module from Struck . The GASIFIC DAQ relies on a trapezoidal digital filter which is implemented in the firmware of the SIS3316 digitizer. The GASIFIC DAQ is a non-paralizable system  by construction.
For this study, an algorithm that emulates the DAQ firmware has been developed. The algorithm has been validated with experimental waveform samples from low and high counting rate measurements. Finally, a simulation code for the reconstruction of the full data stream has been developed. The simulation reconstructs the observed trigger rate and energy deposited spectrum, in the He3 neutron counter, from experimental waveforms from single events. The reconstruction enables the estimation of the real neutron counting rate producing the observable information in the measurement. The application of this technique for measurements of stray secondary neutrons in proton therapy at clinical conditions (~30 kcps) is discussed.
 J. Agramunt et al. NIM A 807 (2016) 69–78
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