David C. Stromswold

Comparison of plastic and NaI(Tl) scintillators for vehicle portal monitor applications

Experimental data and computer simulations are presented for gamma-ray detection by vehicle portal monitors for homeland security applications at international borders. The experiments and simulations use spectral processing of gamma rays from various sources (/sup 241/Am, /sup 57/Co, /sup 133/Ba, /sup 137/Cs, /sup 60/Co) and background to provide data for comparing plastic and NaI(Tl) detectors. The effects of gamma-ray scattering in cargo are also examined. Plastic scintillators are well suited for primary screening of gamma-ray sources because of their large size and low cost. Sodium iodide is preferable to plastic for applications of isotope identification based on gamma-ray spectrometry. Some applications may benefit from integrating features from both types of detectors.

Field tests of a NaI(Tl)-based vehicle portal monitor at border crossings

Radiation portal monitors are commonly used at international border crossings to detect illicit transport of radioactive material. Most monitors use plastic scintillators to detect gamma rays, but next-generation monitors may contain NaI(Tl). In order to directly compare the performance of the two types of detectors, a prototype NaI(Tl) monitor was tested at two international border crossings adjacent to a comparable plastic scintillator monitor. The NaI(Tl) monitor housed four large detectors, each 10.2 cm /spl times/ 10.2 cm /spl times/ 41 cm. The empirical data set from the two field tests contains approximately 3800 passages with known cargo loads for each vehicle. For a small subset of the vehicles, high purity germanium detector spectra were also collected. During the survey period several vehicles containing commercial products with naturally occurring radioactive material (NORM) passed through the monitor. Typical NORM cargo included pottery, large granite slabs, rock-based floor tiles, construction stone blocks, abrasive material, and fertilizer. Non-NORM sources included a large source of /sup 60/Co (200,000 GBq) and a shipment of uranium oxide, both items being legally transported. The information obtained during the tests provides a good empirical data set to compare the effectiveness of NaI(Tl) and plastic-scintillator portal monitors. The capability to be sensitive to illicit materials, but not alarm on NORM, is a key figure of merit for portal monitors.

Neutron-gamma discrimination in plastic scintillators

Direct detection of fast neutrons prior to moderation offers increased performance at lower cost for future neutron detection technologies. Neutron detection by proton recoil in plastic scintillators could provide this capability if efficient techniques for discrimination against gamma events were available. We describe two possible approaches to neutron/gamma discrimination; one based on digital pulse processing to differentiate pulse types and the other based on low density scintillators to lengthen the time interval between multiple interactions.

Detection and Location of Gamma-Ray Sources With a Modulating Coded Mask

The detection of high-energy γ-ray sources is vitally important to national security for numerous reasons, particularly nuclear materials smuggling interdiction and threat detection. This article presents two methods of detecting and locating a concealed nuclear γ-ray source by analyzing detector data of emissions that have been modulated with a coded mask. The advantages of each method, derived from a simulation study and experimental data, are discussed. Energetic γ-rays readily penetrate moderate amounts of shielding material and can be detected at distances of many meters. Coded masks are spatial configurations of shielding material (e.g., small squares formed from plates of lead or tungsten) placed in front of a detector array to modulate the radiation distribution. A coded mask system provides improved detection through an increased signal-to-noise ratio. In a search scenario it is impossible to obtain a comparison background run without the presence of a potential concealed source. The developed analysis methods simultaneously estimate background and source emissions and thus provide the capability to detect and locate a concealed high-energy radiological source in near real time. An accurate source location estimate is critically important to expedite the investigation of a high-probability γ-ray source. The experimental examples presented use a proof-of-concept coded mask system of a 4 × 4 array of NaI detectors directed at a γ-ray source in a field-of-view roughly 4 m wide × 3 m high (approximately the size of the side panel of a small freight truck). Test results demonstrate that the correct location of a radiologic source could be determined in as little as 100 seconds when the source was 6 m from the detector.

Location of neutron sources using moderator-free directional thermal neutron detectors

The use of neutron detectors designed to record only those thermalized neutrons coming from a highly specific direction can provide enhanced capability for applications such as locating nuclear weapons material or monitoring storage vaults containing special nuclear materials. Such a detection system should be as free of moderator as possible to assure effective directionality. We provide evidence from computer modeling and experimental tests that this technique allows the identification and localization of neutron sources at greater distances and narrower angular resolution than would be possible with conventional moderating neutron detectors.

Lithium Loaded Glass Fiber Neutron Detector Tests

Radiation portal monitors used for interdiction of illicit materials at borders include highly sensitive neutron detection systems. The main reason for having neutron detection capability is to detect fission neutrons from plutonium. The currently deployed radiation portal monitors (RPMs) from Ludlum and Science Applications International Corporation (SAIC) use neutron detectors based upon 3He-filled gas proportional counters, which are the most common large neutron detector. There is a declining supply of 3He in the world and, thus, methods to reduce the use of this gas in RPMs with minimal changes to the current system designs and sensitivity to cargo-borne neutrons are being investigated. Four technologies have been identified as being currently commercially available, potential alternative neutron detectors to replace the use of 3He in RPMs. Reported here are the results of tests of the lithium-loaded glass fibers option. This testing measured the neutron detection efficiency and gamma ray rejection capabilities of a small system manufactured by Nucsafe (Oak Ridge, TN).

Coated Fiber Neutron Detector Test

Radiation portal monitors used for interdiction of illicit materials at borders include highly sensitive neutron detection systems. The main reason for having neutron detection capability is to detect fission neutrons from plutonium. The currently deployed radiation portal monitors (RPMs) from Ludlum and Science Applications International Corporation (SAIC) use neutron detectors based upon 3He-filled gas proportional counters, which are the most common large neutron detector. There is a declining supply of 3He in the world, and thus, methods to reduce the use of this gas in RPMs with minimal changes to the current system designs and sensitivity to cargo-borne neutrons are being investigated. Reported here are the results of tests of the 6Li/ZnS(Ag)-coated non-scintillating plastic fibers option. This testing measured the required performance for neutron detection efficiency and gamma ray rejection capabilities of a system manufactured by Innovative American Technology (IAT).

Boron-Lined Straw-Tube Neutron Detector Test

Radiation portal monitors used for interdiction of illicit materials at borders include highly sensitive neutron detection systems. The main reason for having neutron detection capability is to detect fission neutrons from plutonium. The currently deployed radiation portal monitors (RPMs) from Ludlum and Science Applications International Corporation (SAIC) use neutron detectors based upon 3He-filled gas proportional counters, which are the most common large neutron detector. There is a declining supply of 3He in the world, and thus, methods to reduce the use of this gas in RPMs with minimal changes to the current system designs and sensitivity to cargo-borne neutrons are being investigated. Four technologies have been identified as being currently commercially available, potential alternative neutron detectors to replace the use of 3He in RPMs. Reported here are the results of tests of a boron-lined proportional counter design variation. In the testing described here, the neutron detection efficiency and gamma ray rejection capabilities of a system manufactured by Proportional Technologies, Inc, was tested.

Spectroscopic radiation portal monitor prototype

A spectroscopic radiation portal monitor (SPM) prototype consisting of four 10.16-cmtimes10.16-cmtimes40.64-cm sodium iodide (NaI) crystals has been constructed at Pacific Northwest National Laboratory (PNNL). The prototype was put through a variety of tests, including measurements of the absolute detection efficiency of unshielded sources and the detection efficiency and isotopic identification capability of the detector for shielded isotopic sources. The monitors response to various types of cargo and source configurations was also studied. The results of these tests are presented in this report

3He Neutron Detector Pressure Effect and Comparison to Models

Reported here are the results of measurements performed to determine the efficiency of 3He filled proportional counters as a function of gas pressure in the SAIC system. Motivation for these measurements was largely to validate the current model of the SAIC system. Those predictions indicated that the neutron detection efficiency plotted as a function of pressure has a simple, logarithmic shape. As for absolute performance, the model results indicated the 3He pressure in the current SAIC system could not be reduced appreciably while meeting the current required level of detection sensitivity. Thus, saving 3He by reducing its pressure was predicted not to be a viable option in the current SAIC system.