Raymond T. Klann

Design considerations for thin film coated semiconductor thermal neutron detectors-I: basics regarding alpha particle emitting neutron reactive films

Semiconductor-based thermal neutron detectors provide a compact technology for neutron detection and imaging. Such devices can be produced by externally coatingsemiconductor-charg ed-particle detectors with neutron reactive films that convert free neutrons into charged-particle reaction products. Commonly used films for such devices utilize the 10 B(n,a) 7 Li reaction or the 6 Li(n,a) 3 H reaction, which are attractive due to the relatively high energies imparted to the reaction products. Unfortunately, thin film or ‘‘foil’’ type thermal neutron detectors suffer from self-absorption effects that ultimately limit neutron detection efficiency. Design considerations that maximize the efficiency and performance of such devices are discussed. Theoretical and experimental results from front coated, back coated, and ‘‘sandwich’’ designs are presented. r 2002 Elsevier Science B.V. All rights reserved. PACS: 29.40.W

Thin-film-coated bulk GaAs detectors for thermal and fast neutron measurements

GaAs-based structures are presently under investigation as the substrate for 10 B-coated and polyethylene-coated neutron detectors. The semi-insulating (SI) GaAs-based devices operate at low bias voltages by employing the truncated electric field effect, which allows for acceptable signals to be produced with bias voltages ranging between 10 and 50 V. At this time, the 10 B-coated devices have demonstrated over 3.5% intrinsic thermal neutron detection efficiency with reactive films 1.8mm thick. Relatively high neutron/g-ray rejection ratios can be achieved with an appropriate choice of lower level discriminator setting. Polyethylene-coated GaAs devices are being studied as fast neutron detectors and have shown evidence of (n,p) reactions for 14 MeV neutrons. Theoretical neutron responses and experimental neutron detection data are presented and compared. # 2001 Elsevier Science B.V. All rights reserved.

Designs for thin-film-coated semiconductor thermal neutron detectors

Thin film coated semiconductor detectors have been studied and used as neutron detectors for decades. Unfortunately, with front-irradiated devices, the basic design limits the thermal neutron detection efficiency to only 3.95% for /sup 10/B-coated devices and only 4.3% for /sup 6/LiF-coated devices. Presented in the following work are several straightforward methods to increase the thermal neutron detection efficiency for thin-film-coated semiconductor thermal neutron detectors.

A neutron detector to monitor the intensity of transmitted neutrons for small-angle neutron scattering instruments

A semiconductor-based neutron detector was developed at Argonne National Laboratory (ANL) for use as a neutron beam monitor for small-angle neutron scattering instruments. The detector is constructed using a coating of 10 Bo n a gallium–arsenide semiconductor detector and is mounted directly within a cylindrical (2.2 cm dia. and 4.4 cm long) enriched 10 B4C beam stop in the time-of-flight Small Angle Neutron Diffractometer (SAND) instrument at the Intense Pulsed Neutron Source (IPNS) facility at ANL. The neutron beam viewed by the SAND is from a pulsed spallation source moderated by a solid methane moderator that produces useful neutrons in the wavelength range of 0.5–14 ( A. The SAND instrument uses all detected neutrons in the above wavelength range sorted by time-of-flight into 68 constant DT=T ¼ 0:05 channels. This new detector continuously monitors the transmitted neutron beam through the sample during scattering measurements and takes data concurrently with the other detectors in the instrument. The 10 B coating on the GaAs detector allows the detection ofthe cold neutron spectrum with reasonable efficiency. This paper describes the details of the detector fabrication, the beam stop monitor design, and includes a discussion of results from preliminary tests using the detector during several run cycles at the IPNS. r 2003 Elsevier Science B.V. All rights reserved.

Development of semiconductor detectors for fast neutron radiography.

A high-energy neutron detector has been developed using a semiconductor diode fabricated from bulk gallium arsenide wafers with a polyethylene neutron converter layer. Typical thickness of the diode layer is 250 to 300 μm with bias voltages of 30 to 150 volts. Converter thicknesses up to 2030 μm have been tested. GaAs neutron detectors offer many advantages over existing detectors including positional information, directional dependence, gamma discrimination, radiation hardness, and spectral tailoring. Polyethylene-coated detectors have been shown to detect 14 MeV neutrons directly from a D-T neutron generator without interference from gamma rays or scattered neutrons. An array of small diode detectors can be assembled to perform fast neutron radiography with direct digital readout and real-time display of the image produced. In addition, because the detectors are insensitive to gamma rays and low energy neutrons, highly radioactive samples (such as spent nuclear fuel or transuranic waste drums) could be ...

Tracking of weak radioactive sources in crowded venues

Monitoring for the clandestine transport of nuclear and radiological materials at large public gatherings or special events, such as the presidential inauguration, is one element in a strategy for preventing their subsequent dispersal. Being able to track these materials as they approach or move through such a venue can provide law-enforcement personnel with important information. The focus of this paper is on methods developed by the authors to meet the challenges inherent in the tracking problem. Results of laboratory experiments performed using the RadTrac prototype system are described.

Current state of commercial radiation detection equipment for homeland security applications.

Abstract With the creation of the U.S. Department of Homeland Security (DHS) came the increased concern that terrorist groups would attempt to manufacture and use an improvised nuclear device or radiological dispersal device. As such, a primary mission of DHS is to protect the public against the use of these devices and to assist state and local responders in finding, locating, and identifying these types of devices and materials used to manufacture these devices. This assistance from DHS to state and local responders comes in the form of grant money to procure radiation detection equipment. In addition to this grant program, DHS has supported the development of American National Standards Institute standards for radiation detection equipment and has conducted testing of commercially available instruments. This paper identifies the types and kinds of commercially available equipment that can be used to detect and identify radiological material—for use in traditional search applications as well as primary and secondary screening of personnel, vehicles, and cargo containers. In doing so, key considerations for the conduct of operations are described as well as critical features of the instruments for specific applications. The current state of commercial instruments is described for different categories of detection equipment including personal radiation detectors, radioisotope identifiers, man-portable detection equipment, and radiation portal monitors. In addition, emerging technologies are also discussed, such as spectroscopic detectors and advanced spectroscopic portal monitors.

Treatment of Shielding in Real-Time Source Tracking Software

Abstract Within the homeland security and emergency response communities, there is a need for a low-profile system to detect, locate, and identify radioactive sources in real time. Such a system could be deployed for area monitoring around venues for special events. A system was developed at Argonne National Laboratory, called RADTRAC, which is based on a network of radiation detectors and advanced signal-processing algorithms. The initial implementation of RADTRAC did not account for dynamically changing shielding due to crowd movements. An algorithm was developed that utilizes the gamma-ray energy spectrum from each detector to estimate the amount of attenuation and scattering that is present between the source location (a priori unknown) and the detector location in real time. The attenuation and scattering estimations are then included in the maximum likelihood model to significantly improve the source localization solution. Results are presented for several test cases showing the improvement in the real-time source localization solution. This algorithm has been implemented into the current version of RADTRAC such that it now accounts for the effects of dynamically changing shielding and scattering due to crowd movements in real time in order to accurately determine the source location in crowded venues.

Detection of nuclear sources in search survey using dynamic quantum clustering of gamma-ray spectral data

In a search scenario, nuclear background spectra are continuously measured in short acquisition intervals with a mobile detector-spectrometer. Detecting sources from measured data is difficult because of low signal-to-noise ratio (S/N of spectra, large and highly varying background due to naturally occurring radioactive material (NORM), and line broadening due to limited spectral resolution of nuclear detector. We have invented a method for detection of sources using clustering of spectral data. Our method takes advantage of the physical fact that a source not only produces counts in the region of its spectral emission, but also has the effect on the entire detector spectrum via Compton continuum. This allows characterizing the low S/N spectrum without distinct isotopic lines using multiple data features. We have shown that noisy spectra with low S/N can be grouped by overall spectral shape similarity using a data clustering technique called Dynamic Quantum Clustering (DQC). The spectra in the same cluster can then be averaged to enhance S/N of the isotopic spectral line. This would allow for increased accuracy of isotopic identification and lower false alarm rate. Our method was validated in a proof-of-principle study using a data set of spectra measured in one-second intervals with sodium iodide detector. The data set consisted of over 7000 spectra obtained in urban background measurements, and approximately 70 measurements of 137Cs and 60Co sources. Using DQC analysis, we have observed that all spectra containing 137Cs and 60Co signal cluster away from the background.

Sensitivity Improvement In Low-Profile Distributed Detector Systems For Tracking Sources In Transit

The RadTrac real-time detection and tracking software runs on a laptop computer networked to gamma-radiation detectors. A probabilistic estimate for source position is generated by combining measured count rate data with a first-principles stochastic model for the space and time dependence of count rates and knowledge of detector intrinsic efficiency. Recent development work has focused on improving RadTrac sensitivity in low-count rate situations. A method has been developed for processing count rates by energy according to that part of the energy spectrum with the greatest signal-to-noise ratio. In addition a method has been developed that places constraints on the solution that are physically appropriate when count rates approach background. In both instances experiments with a weak source confirmed the uncertainty in estimated position is reduced.