Randy R. Hansen

Overview of portal monitoring at border crossings

The Bureau of Customs and Border Protection has the task of interdicting illicit radioactive material at ports of entry. Items of concern include radiation dispersal devices (RDD), nuclear warheads, and special nuclear material (SNM). The preferred survey method screens all vehicles in primary and diverts questionable vehicles to secondary. This requires high detection probability in primary while not overwhelming secondary with alarms, which could include naturally occurring radioactive material (NORM) found in acceptable cargo and radionuclides used in medical procedures. Sensitive alarm algorithms must accommodate the baseline depression observed whenever a vehicle enters the portal. Energy-based algorithms can effectively use the crude energy information available from a plastic scintillator to distinguish NORM from SNM. Whenever NORM cargo limits the alarm threshold, energy-based algorithms produce significantly better detection probabilities for small SNM sources than gross-count algorithms. Algorithms can be best evaluated using a large empirical data set to 1) calculate false alarm probabilities, 2) select sigma-level thresholds for operationally acceptable false alarm rates, and 3) determine detection probabilities for marginally detectable pseudo sources of SNM.

Naturally occurring radioactive materials and medical isotopes at border crossings

Countries around the world are deploying radiation detection instrumentation to interdict the illegal shipment of radioactive material crossing international borders. Some cargo contains naturally occurring radioactive material (NORM) or technologically enhanced NORM (TENORM) that triggers alarms at border crossings. This paper discusses experience with radiation portal monitors for the interdiction of radioactive materials and the issues raised by NORM. The results of observations and computations related to NORM characteristics are discussed.

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.

Authentication of radiation measurement systems for nonproliferation

Radiation measurement systems are central to the affirmation of compliance with a variety of agreements related to arms-control and non-proliferation. Authentication is the process by which the Monitoring Party gains appropriate confidence that the information reported by a monitoring system accurately reflects the true state of the monitored item. Authentication utilizes a set of tools to provide evidence that a system performs its required and defined tasks. These tools include: functional testing using trusted unclassified calibration sources, evaluation of documentation including the software, evaluation of hardware, random selection of hardware and software, and usage of tamper indicating devices. Procedures for carrying out authentication are central to the successful implementation of the complex process of authenticating systems throughout their lifecycle. The lifecycle of a system can be divided into the elements of design, fabrication, installation, and operations. Radiation measurement systems are being built for use in the Russian Federation that will be the subject of US authentication activities.

A Large Detector Laboratory for the Development and Testing of Radiation Detection Systems

Thousands of radiation detection systems have been deployed over the last decade to domestic and international locations, primarily at points of entry or departure. An essential ingredient in supporting sensor deployment is the capability to develop and test emerging systems and replicate the behavior of deployed systems. Pacific Northwest National Laboratory recently designed and constructed a laboratory for developing and testing these large-scale radiation detection systems. The Large Detector Laboratory is capable of housing up to 30 fully integrated radiation portal monitoring systems and allows for complex testing scenarios that include background suppression, equipment shielding, and source cross-talk between systems. The laboratorys design also allows implementation of applications ranging from pedestrian screening to baggage or package inspection to vehicle and cargo inspection to standoff detection. This manuscript describes the unique attributes of this laboratory, including the ability to reproduce operational conditions faced by integrated systems, realistic source configurations, and measurements across multiple systems.

Direct Fast-Neutron Detection

Direct fast-neutron detection is the detection of fast neutrons before they are moderated to thermal energy. We have investigated two approaches for using proton-recoil in plastic scintillators to detect fast neutrons and distinguish them from gamma-ray interactions. Both approaches use the difference in travel speed between neutrons and gamma rays as the basis for separating the types of events. In the first method, we examined the pulses generated during scattering in a plastic scintillator to see if they provide a means for distinguishing fast-neutron events from gamma-ray events. The slower speed of neutrons compared to gamma rays results in the production of broader pulses when neutrons scatter several times within a plastic scintillator. In contrast, gamma-ray interactions should produce narrow pulses, even if multiple scattering takes place, because the time between successive scattering is small. Experiments using a fast scintillator confirmed the presence of broader pulses from neutrons than from gamma rays. However, the difference in pulse widths between neutrons and gamma rays using the best commercially available scintillators was not sufficiently large to provide a practical means for distinguishing fast neutrons and gamma rays on a pulse-by-pulse basis. A faster scintillator is needed, and that scintillator might become available in the literature. Results of the pulse-width studies were presented in a previous report (peurrung et al. 1998), and they are only summarized here.

A Case Study of Selected Photographic Inspection Techniques for a Transparency Regime

Photographic inspection techniques have become technically more sophisticated in recent years with the development of advanced equipment for the mass consumer market. High quality digital cameras, for example, are now available around the world. Combined with an appropriate image analysis program on a personal computer, there is now the ability to produce and analyze high quality photographs with a modest level of resources. This report is the summary of a variety of efforts, all aimed at investigating the application of commonly available, mass-market photographic and computer equipment to photographic inspection and analysis of equipment and items. It contains results of equipment test and evaluation as well as a few selected example applications.

Introduction to Methods Demonstrations for Authentication

During the Trilateral Initiative Technical Workshop on Authentication & Certification, PNNL will demonstrate some authentication technologies. This paper briefly describes the motivation for these demonstrations and provide background on them.