Wavelength-based neutron/gamma ray discrimination in CLYC
We investigated the optical components of Cs2LiYCl6:Ce (CLYC) as a possible path to particle discrimination by measuring the effects of optical filters on the shape of the waveform and comparing filtered waveforms to unfiltered ones. Crystals packaged with two optical windows were viewed by two photomultiplier tubes operated in coincidence to be certain that the signals from the same event were analyzed. We applied long-pass optical filters with cut-off wavelengths from 280 to 455 nm, and bandpass filters with cut-off wavelengths at 313, 350, 385, and 425 nm. Waveforms from gamma rays were significantly sharpened by the 313 nm bandpass filter because this filter cut out self-trapped exciton luminescence, while neutron waveforms were largely unaffected by filtering. Cumulative distribution functions generated from average filtered waveforms indicate that the neutron/gamma assignment can be made 44 ns by applying a Kolmogorov-Smirnov test comparing an acquired waveform to the average filtered waveforms.
Characterization of Large Volume CLYC Scintillators for Nuclear Security Applications
We report on our development of large volume Cs2LiYCl6 (CLYC) detectors for nuclear security applications. Three-inch diameter boules have been grown and 3-in right cylinders have been fabricated. Crystals containing either >95% 6Li or >99% 7Li have been grown for applications specific to thermal or fast neutron detection, respectively. We evaluated their gamma and neutron detection properties and the performance is as good as small size crystals. Gamma and neutron efficiencies were measured for large crystals and compared with smaller size crystals. With their excellent performance characteristics, and the ability to detect fast neutrons, CLYC detectors are excellent triple-mode scintillators for use in handheld and backpack instruments for nuclear security applications.
Pulse shape discrimination
Abstract A method for separating the scintillation signals from particles with different specific ionization is described which is relatively insensitive to signal amplitude. It is based upon a measurement of the zero-crossing time for signals produced by RC differentiation of dynode voltage pulses. The circuit utilizes the signal from a single-photomultiplier dynode and operates satisfactorily at high counting rates. In the case of gamma-neutron discrimination in NE-213, excellent separation was observed over a range of amplitudes corresponding to recoil protons of 0.5 to 15 MeV energy. The acceptance efficiency for the recoil proton events drops to 50% at 0.3 MeV. Good experimental results are also shown for gamma-alpha discrimination in a stilbene scintillator. In addition, the pulse-shape sensing circuitry was used to separate small dynode signals due to low-intensity scintillations in NaI from phototube noise pulses of comparable amplitude. Calculations were made to select those circuit parameters which provide the optimum experimental ability to separate events due to two kinds of particles. Graphical displays of this optimization are shown for gamma-neutron discrimination in stilbene, NE-213 and NE-150 scintillators. A detailed comparison is made between the present procedure and the theoretically optimum use of the shape differences. This method of shape discrimination appears to make very efficient use of the information available in the pulse shape.