Gamma Rejection Ratio Research Articles

Investigation of neutron/gamma-ray distribution in SiPM-based pulse shape discrimination using EJ-276 plastic scintillators

The distribution of neutrons and gamma-rays in mixed fields is a main interest in pulse shape discrimination (PSD) studies, and becomes more significant when it involves techniques such as machine learning which requires true data. One of the surest methods to label neutrons and gamma-rays is neutron Time-of-Flight. This can be applied to the identification of special nuclear materials and by extension to nuclear non-proliferation. Moreover, the integration of silicon photomultiplier (SiPM) technology can make the neutron Time-of-Flight system more compact. Two EJ-276 plastic scintillators, which are capable of PSD, were coupled to SiPMs. The actual portion of neutrons from a 252Cf source intruding the gamma-rays in PSD ratio distribution, labeled by neutron Time-of-Flight, was calculated and then compared with gamma-rejection ratio (GRR) for each threshold in SiPM-based pulse shape discrimination.

Novel flexible and conformable composite neutron scintillator based on fully enriched lithium tetraborate

Thermal neutron detection is a key subject for nuclear physics research and also in a wide variety of applications from homeland security to nuclear medicine. In this work, it is proposed a novel flexible and conformable composite thermal neutron scintillator based on a fully enriched Lithium Tetraborate preparation (^{6}Li_2^{10}B_4O_7) combined with a phosphorescent inorganic scintillator powder (ZnS:Ag), and is then distributed into a polydimethylsiloxane matrix. The proposed scintillator shows a good neutron detection efficiency (max. sim 57% with respect to the commercial EJ-420), an average light output of sim 9000 ph/neutron-capture, a remarkable insensitivity to gamma-rays (Gamma Rejection Ratio <10^{-11}), and an extraordinary flexibility, so as to reach extremely small curvature radii, down to 1.5 mm, with no signs of cracking or tearing. Its characteristics make it suitable to be employed in scenarios where non-standard geometries are needed, for example, to optimize the detector performance and/or maximize the detection efficiency. Finally, the response of a hybrid detector made of a plastic scintillator, wrapped with the proposed scintillator, coupled to a silicon photomultiplier array is described, and the excellent discrimination between gamma-rays, fast and thermal neutrons resulting from data processing is demonstrated.

LiInSe2 for Semiconductor Neutron Detectors

Lithium Indium Selenide (LiInSe2) is being developed for use as a room temperature semiconductor detector for thermal neutrons The material has been studied for a number of applications including nonlinear optics such as parametric oscillators, as anode material for lithium ion batteries, piezoelectrics, as a scintillation detector material and as a semiconductor detector material. The recent advances of the crystal growth, material processing and detector fabrication have led to semiconductor neutron detectors with up to 100 mm2 active area. The thermal neutron detection sensitivity and gamma rejection ratio (GRR) are comparable to 10atm, He-3 tubes of similar size. Synthesis, crystal growth, detector fabrication, and characterization are discussed.

Large-Area, Low-Cost, High-Efficiency Neutron Detector for Vehicle-Mounted Operation

We have developed a large-area, low-cost, high-efficiency neutron detector for vehicle-mounted operation. The detector, which has overall dimensions 12.7 cm $\times 91.4$ cm $\times 102$ cm (5” $\times 36$ ” $\times 40$ ”), a sensitive area equal to $0.85~\text {m}^{{\mathbf {2}}}$ (1320 in2), and weight of 110 kg (242 lbs), employs an array of 90 boron-coated straw (BCS) detectors. PTI has also developed electronics to minimize cost and space while providing low-noise signal conditioning for both neutron and gamma detection channels, as well as low energy Bluetooth communication with handheld devices. Extremely low power consumption allows continuous use for 225 hours (~10 days) using three AAA lithium-ion rechargeable batteries. We present radiological, mechanical, and environmental tests, collected from four full-scale prototypes. Outdoor neutron-counting tests with a moderated 252Cf source 2 m away from the center of the detector face showed an average detection rate of 5.5 cps/ng with a standard deviation of 0.09 cps/ng over the four individual detector measurements. Measurements showed a gamma rejection ratio of $1.0\times 10^{{\mathbf {-8}}}$ , and gamma absolute rejection ratio (GARRn) of 0.93. The prototypes were also operated successfully onboard a moving vehicle for high-speed tests and a long-range 1433-mile, two-day road trip from Houston, TX, USA, to Laurel, MD, USA. Using auxiliary DARPA SIGMA equipment, the GPS, timestamp, gamma and neutron data were transmitted over the cellular network with 10 Hz resolution to a server and real-time tracking website. Mechanical impact and electrostatic discharge testing produced no spurious counts in either the neutron or gamma channels. Ambient environmental temperature testing showed less than ± 1% response variation over the range from −30°C to +55°C.

Pulse Shape Discrimination Algorithms, Figures of Merit, and Gamma-Rejection for Liquid and Solid Scintillators

Pulse shape discrimination (PSD) in scintillators is useful to distinguish between signals from neutrons and gamma rays. A standard algorithm is to divide the integral of the tail end of the pulse by the total pulse integral, which itself yields the deposited energy. When the PSD ratio is plotted against the energy, two bands emerge for scintillators that respond differently to neutrons and gammas. Often, a figure of merit (FOM) is defined as the distance between the two bands in a particular energy range, divided by the sum of the full-widths at half-maximum of the PSD ratio bands. A high FOM is usually interpreted as a good indicator of how well a certain scintillator device can distinguish between neutrons and gammas. This, however, ignores the actual shape of the two bands, which may not have a Gaussian shape in the direction of the PSD ratio axis, especially in the presence of pulse pileup. Thus, the FOM may not say much about the gamma rejection capability of the detector, and can therefore be a little misleading. Here, we describe the results of research and development performed on a stilbene detector borrowed from the Lawrence Livermore National Laboratory, but the algorithms described were also successfully used on a number of plastic and liquid scintillation detectors. The standard algorithm described above was compared with a newly developed wavelet-based algorithm that is also used to reject pileup, as well as a pulse shape fitting approach which is shown to be capable of significantly improving the gamma-rejection ratio.

Microchannel plate special nuclear materials sensor

Abstract Nova Scientific Inc., is developing for the Domestic Nuclear Detection Office (DNDO SBIR #HSHQDC-08-C-00190), a solid-state, high-efficiency neutron detection alternative to 3He gas tubes, using neutron-sensitive microchannel plates (MCPs) containing 10B and/or Gd. This work directly supports DNDO development of technologies designed to detect and interdict nuclear weapons or illicit nuclear materials. Neutron-sensitized MCPs have been shown theoretically and more recently experimentally, to be capable of thermal neutron detection efficiencies equivalent to 3He gas tubes. Although typical solid-state neutron detectors typically have an intrinsic gamma sensitivity orders of magnitude higher than that of 3He gas detectors, we dramatically reduce gamma sensitivity by combining a novel electronic coincidence rejection scheme, employing a separate but enveloping gamma scintillator. This has already resulted in a measured gamma rejection ratio equal to a small 3He tube, without in principle sacrificing neutron detection efficiency. Ongoing improvements to the MCP performance as well as the coincidence counting geometry will be described. Repeated testing and validation with a 252Cf source has been underway throughout the Phase II SBIR program, with ongoing comparisons to a small commercial 3He gas tube. Finally, further component improvements and efforts toward integration maturity are underway, with the goal of establishing functional prototypes for SNM field testing.

A general purpose pulse shape discriminating circuit

A new general purpose pulse shape discriminating circuit and its applications are described. The circuit is based on a risetime to pulse height converter which can measure, independently of its input pulse height, an arbitrary fraction of the risetime of the input pulses. The feature of this circuit is that this can define the risetime of the input signals in the range of 0.2 to 10μsec with an accuracy of ±2% for a pulse height range of 0.3 to 10 V. In applications, this circuit can perform n-γ discrimination with a liquid scintillator, a usual photomultiplier and a linear amplifier, and the gamma rejection ratio obtained for neutrons less than 500 keV is better than 1/500. Also a gamma rejection ratio nearly 1/50 can be obtained for neutrons less than 60 keV by using an H-filled proportional counter. When neutron capture gamma rays are measured with a 20 cm 3 coaxial type Ge(Li) detector in the reactor, the background reduction by 50% and the improvement of a photopeak width by 20% are obtained by using this circuit. Furthermore, when it is applied to (n-γ)T reaction measurement with a Si(SB) detector in the reactor, an improvement of the tritium peak width by 50% and a background reduction by a factor of ten are obtained. It is also investigated that pileup rejection and characteristics measurement of semiconductor detectors can be easily performed by using this circuit.