The use of Time-over-Threshold (TOT) for the discrimination
between fast neutrons and gamma-rays is advantageous when large
number of detection channels are required due to the simplicity of
its implementation. However, the results obtained using the
standard, Constant Threshold TOT (CT-TOT) are usually inferior to
those obtained using other pulse shape discrimination (PSD) methods,
such as Charge Comparison or Zero-Crossing approaches, especially
for low amplitude neutron/gamma-ray pulses. We evaluate another TOT
approach for fast neutron/gamma-ray PSD using Constant-Fraction
Time-over-Threshold (CF-TOT) pulse shape analysis. The CT-TOT and
CF-TOT methods were compared quantitatively using digitized
waveforms from a liquid scintillator coupled to a photomultiplier
tube as well as from a stilbene scintillator coupled to a
photomultiplier tube and a silicon photomultiplier. The quality of
CF-TOT neutron/gamma-ray discrimination was evaluated using Receiver
Operator Characteristics curves and the results obtained with this
approach were compared to that of the standard CT-TOT method. The
CF-TOT PSD method results in > 99.9% rejection of gamma-rays with
> 80% neutron acceptance, much better than CT-TOT.
We describe the process of selecting a silicon photomultiplier (SiPM) as the light sensor for an ultrathin (≈2 mm) highly efficient cold neutron detector. The neutron detector consists of 6 LiF:ZnS(Ag) scintillator in which wavelength shifting (WLS) fibers have been embedded. The WLS fibers conduct the scintillation light out from the scintillator to the SiPM photosensor. In addition to the many benefits of using silicon photomultipliers as photosensors (low cost, compact size, insensitivity to magnetic fields), their selection also presents many challenges (thermally induced dark noise, delayed cross talk, afterpulsing, etc) which are not shared by traditional photomultiplier tubes. In this work, we discuss the considerations for the selection of the appropriate silicon photomultiplier to achieve the best net neutron sensitivity and gamma ray discrimination. Important characteristics for these devices include short recovery time (≈35 ns), high photodetection efficiency (>30% at the target wavelength), low thermal noise (<35 kHz mm −2 at ambient temperatures), and low crosstalk.
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