Pulse Shape Analysis and Discrimination for Silicon-Photomultipliers in Helium-4 Gas Scintillation Neutron Detector

Abstract

Abstract-Using natural helium (helium-4), the Arktis 180-bar pressurized gas scintillator is capable of detecting and distinguishing fast neutrons and gammas. The detector has a unique design of three optically separated segments in which 12 silicon-photomultiplier (SiPM) pairs are positioned equilaterally across the detector to allow for them to be fully immersed in the helium-4 gas volume; consequently, no additional optical interfaces are necessary. The SiPM signals were amplified, shaped, and readout by an analog board; a 250 MHz, 14-bit digitizer was used to examine the output pulses from each SiPMpair channel. The SiPM over-voltage had to be adjusted in order to reduce pulse clipping and negative overshoot, which was observed for events with high scintillation production.Pulse shaped discrimination (PSD) was conducted by evaluating three different parameters: time over threshold (TOT), pulse amplitude, and pulse integral. In order to differentiate high and low energy events, a 30ns gate window was implemented to group pulses from two SiPM channels or more for the calculation of TOT. It was demonstrated that pulses from a single SiPM channel within the 30ns window corresponded to low-energy gamma events while groups of pulses from two-channels or more were most likely neutron events. Due to gamma pulses having lower pulse amplitude, the percentage of measured gamma also depends on the threshold value in TOT calculations. Similarly, the threshold values were varied for the optimal PSD methods of using pulse amplitude and pulse area parameters. Helium-4 detectors equipped with SiPMs are excellent for in-the-field radiation measurement of nuclear spent fuel casks. With optimized PSD methods, the goal of developing a fuel cask content monitoring and inspection system based on these helium-4 detectors will be achieved.

Evaluation of the Constant Fraction Time-Over-Threshold (CF-TOT) method for neutron-gamma pulse shape discrimination

“…The TOT method has been investigated for applications such as Time-of-Flight-PET [15,16], PSD in phoswich detectors [17], neutron/gamma-ray discrimination in organic liquid and solid scintillators [18,19] and PSD in 4 He gas scintillation counters [20]. As the relationship between TOT and pulse amplitude or charge is strongly non-linear, several authors proposed various methods for obtaining a linear TOT-charge relationship, such as dynamic time-over-threshold (DToT) [21,22], time-over-linear-threshold (TOLT) [23], and multiple-thresholds (MToT) [19,24,30].…”

Nuclear recoil scintillation linearity of a high pressure 4He gas detector

“…A Phillips Scientific NIM Model 710 discriminator was used to determine when two or more of the four channels in the middle segment of the Arktis detector had pulses cross the threshold value within a 50 ns window. This two-fold coincidence requirement eliminates many electron recoil backgrounds, which are less likely to produce pulses in two or more channels at once [14]. The event was recorded if the Arktis pulse coincidence occurred within 150 ns of a pulse in the organic scintillator module, to allow for a time of flight cut in the analysis of the data.…”

“…We also show the sampled regions of the differential cross section for these five scattering angles in Figure 2. Since the elongated cell geometry contributes to smearing in the scattering angle, and each cell of the Arktis detector has slightly different optical properties [14], we restricted ourselves to using the middle cell of the Arktis detector. Neutron elastic scattering differential cross section for 4 He from the ENDF/B-VII library [21] used in Geant4 simulations.…”

Evaluation of the Constant Fraction Time-Over-Threshold (CF-TOT) method for neutron-gamma pulse shape discrimination

“…The TOT method has been investigated also for applications such as Time-of-Flight-PET [15,16], PSD in phoswich detectors [22], and PSD in 4 He gas scintillation counters [23]. Ota proposed a dual time-over-threshold method for the estimation of both energy and scintillation decay time in LYSO detectors [24], assuming a single pulse decay time constant.…”