When neutrons interact with nuclei, the resulting energy of the interaction can be released in the form of gamma rays, whose energy is characteristic of the nucleus involved in the reaction. The PELAN (Pulsed Elemental Analysis with Neutrons) system uses a pulsed neutron generator and an integral thermalizing shield that induce reactions that cover most of the entire neutron energy range. The neutron generator uses a d-T reaction, which releases fast 14 MeV neutrons responsible for providing information on chemical elements such as C, N, and O. During the time period between pulses, the fast neutrons undergo multiple elastic and inelastic interactions that lower their energy making them easier to be captured by chemical elements, such as H and Cl. The PELAN system has been used for a number of applications where non-intrusive, non-destructive interrogation is needed. This report illustrates examples of its performance in interrogations for unexploded ordnance (UXO), landmines, large vehicle bombs and illicit drug detection.
When neutrons interact with particular nuclei, the resulting energy of the interaction can be released in the form of gamma rays, which are characteristic of the nucleus involved in the reaction. The PELAN (Pulsed Elemental Analysis with Neutrons) system uses a pulsed neutron generator and an integral thermalizing shield that induce reactions that cover most of the entire gamma ray energy spectra. The neutron generator uses a D-T reaction, which releases fast 14MeV neutrons responsible for providing information on those nuclei that mostly respond to inelastic scattering. During the time period between pulses, the fast neutrons undergo multiple inelastic interactions that lower their energy making them easier to be captured by certain nuclei; this energy spectrum of gamma rays induced by these interactions are designated as the gamma ray thermal spectra. The PELAN system has been used for a number of applications where non-intrusive, non-destructive interrogation is needed. Although Pulsed Fast Thermal Neutron Analysis (PFTNA) has been around for approximately 30 years, the technology has never been successfully commercialized for practical applications. The following report illustrates examples of the performance of on a number of applications of interrogation of Unexploded Ordnance (UXO), mine confirmation, large vehicle bombs inspection and illicit drug smuggling detection.
KEYWORDS: Principal component analysis, Explosives, Data analysis, Gamma radiation, Sensors, Data modeling, Statistical analysis, Data processing, Calibration, Pattern recognition
Irradiating substances with pulsed neutrons results in several types of interactions which cause the emission of gamma rays. The energy of these gamma rays is characteristic of the nuclei with which the reaction occurred, and can therefore be used as an indicator of the presence of an atomic species. The PELAN system uses a pulsed neutron generator, which makes it possible to separate the gamma spectra into inelastic and capture components that are easier to interpret. Historically, the analysis of PELAN data has been based on a least squares method to extract the contribution of different elemental species present in the sample. The approach uses measured response functions for each element of interest, followed by decision rules for the identification of the materials. We have investigated an alternative approach that does not require a model and response functions. Instead, the approach determines features directly from a number of spectra of substances of interest, e.g. explosives and hazardous chemicals. The PCA method has been used to obtain indicators from the spectra. These indicators are then used for detection and identification of substances using the GLRT algorithm. The performance of the data analysis is assessed through ROC curves. A comparison of the two approaches indicates that PCA followed by GLRT technique has better performance and is more robust than the previous approach.
We report the scientific motivation for and performance measurements of a prototype detector system for SONTRAC, a solar neutron tracking experiment designed to study high- energy solar flare processes. The full SONTRAC instrument will measure the energy and direction of 20 to 200 MeV neutrons by imaging the ionization tracks of the recoil protons in a densely packed bundle of scintillating plastic fibers. The prototype detector consists of a 12.7 mm square bundle of 250 micrometer scintillating plastic fibers, 10 cm long. A photomultiplier detects scintillation light from one end of the fiber bundle and provides a detection trigger to an image intensifier/CCD camera system at the opposite end. The image of the scintillation light is recorded. By tracking the recoil protons from individual neutrons the kinematics of the scattering are determined, providing a high signal to noise measurement. The predicted energy resolution is 10% at 20 MeV, improving with energy. This energy resolution translates into an uncertainty in the production time of the neutron at the Sun of 30 s for a 20 MeV neutron, also improving with energy. A SONTRAC instrument will also be capable of detecting and measuring high-energy gamma rays greater than 20 MeV as a 'solid-state spark chamber.' The self-triggering and track imaging features of the prototype are demonstrated with cosmic ray muons and 14 MeV neutrons. Design considerations for a space flight instrument are presented.
We have been developing and testing a scintillating fiber detector (SFD) for use as a fast neutron sensor which can discriminate against neutrons entering at angles non-parallel to the fiber axis (`directionality'). The detector/convertor component is a fiber bundle constructed of plastic scintillating fibers each measuring 10 cm long and either 0.3 mm or 0.5 mm in diameter. Extensive Monte Carlo simulations were made to optimize the bundle response to a range of fast neutron energies and to intense fluxes of high-energy gamma-rays. The bundle is coupled to a set of gamma-ray insensitive electro-optic intensifiers whose output is viewed by a CCD camera directly coupled to the intensifiers. Two types of CCD cameras were utilized: (1) a standard, interline RS-170 camera with electronic shuttering and (2) a high-speed (up to 850 frame/sec), field-transfer camera. Measurements of the neutron detection efficiency and directionality were made using 14 MeV neutrons, and the response to gamma-rays was performed using intense fluxes from radioisotopic sources (up to 20 R/hr). Recently, the detector was constructed and tested using a large 10 cm by 10 cm square fiber bundle coupled to a 10 cm diameter GEN I intensifier tube. We present a description of the current detector system and report the results of experimental tests.
A scaled version of a scintillating fiber detector using a high-speed CCD camera has been constructed in a feasibility study at SAIC-San Diego for characterization of its response to fast neutrons and to high energy gamma-rays. The detector concept relies on the combination of long, thin plastic fibers and the kinematics of the neutron-proton reaction to provide an effective means to discriminate against background (non-target) neutrons entering at directions non-parallel to the fiber axis. In the present study, the detector was modified to accept a commercially available high-speed CCD camera capable of frame rates up to 850 Hz. We describe here measurements for determining the neutron detection efficiency, directionality, and gamma-ray sensitivity and discuss improvements which were made to enhance the fiber bundle performance.
A scaled version of a scintillating fiber detector (SFD) has been constructed and tested in a feasibility study at SAIC-San Diego for determining its neutron detection efficiency, directionality and gamma-ray sensitivity. This concept was supported by Monte Carlo simulations which predicted the effectiveness of the fiber bundle to discriminate against neutrons entering at directions non-parallel to the fiber axis. Fibers measuring 0.5 mm square and 10 cm long are formed into a bundle and coupled to a set of gamma-ray insensitive electro-optics intensifiers and a CCD camera. In this paper we describe the detector and present the results of the experimental tests with 14 MeV neutrons and an intense beam of 60Co and 137Cs gamma-rays.
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