Radiation Portal Monitor Systems Research Articles

Performance Evaluation of an Imaging Radiation Portal Monitor System

An organic scintillator-based radiation portal monitor (RPM) prototype system with imaging capabilities has been developed based on the neutron–gamma emission tomography technique. The technique enables rapid detection and precise location of small amounts of special nuclear materials, such as plutonium, using time and energy correlations between fast neutrons and gamma rays from spontaneous fission with low false-alarm rates. These capabilities, in addition to state-of-the-art detection of various gamma-emitting sources, enables the novel imaging RPM concept to efficiently address global security threats from terrorism and the proliferation of nuclear weapons. The detector approach is simple and versatile and can easily be adapted for different applications in nuclear security, public safety, nuclear emergency response, and radiological surveying. In this work, basic performance parameters of the imaging RPM prototype system developed at KTH have been evaluated.

Feasibility study of a shielding-independent radiation portal monitor system for revealing 241-Am orphan sources in radiometric surveillance of scrap metal

Orphan radioactive sources inadvertently melted in steel plants are a health and economical hazard. Thus, regulation authorities impose strict control measurements to be performed at these plants. However, a recent series of incidents related to the melting of {}^{241}hbox {Am} sources demonstrated the inadequacy of even state of the art systems in revealing this low-energy gamma ray emitter up to activities of the order of hundreds GBq. However, {}^{241}hbox {Am} sources are characterized by a weak neutron emission which can be detected by proper monitor systems. This work provides a feasibility study of a shielding-independent neutron-sensitive portal monitor system to be installed in steel plants. Measurements were performed both with a certified {}^{241}hbox {Am} shielded source and at the weighing area of a steel plant, to assess both the detector performance and the practical feasibility of the system implementation in work-place fields. A risk evaluation analysis demonstrated that with the current control systems {}^{241}hbox {Am} sources up to 185 GBq could pass undetected with a potential exposure for workers equal to 1.7 mSv, consistent with recent accidents reports. The proposed technique could reveal {}^{241}hbox {Am} activities down to 2 GBq, decreasing the worker exposure down to 24 upmu hbox {Sv}, without interfering with the standard plant operations.

Feasibility study for radionuclide identification using multi-array plastic scintillator and energy weighted algorithm of radiation portal monitors: a Monte Carlo study

Radiation portal monitor (RPM) systems have been deployed worldwide to monitor the inflow of illegal radionuclides at border crossings. Large-sized plastic scintillation detector has been widely used for RPM system, but due to the components of the plastic scintillator of low Z-number material the poor energy resolution shows the broad energy spectrum without clear photo peak identifying the radionuclides. The aim of this study is to develop an RPM based on the scintillator array for enhancing the detection efficiency and energy resolution. Using a Monte Carlo (MC) method, an RPM comprising the multi-array hexagonal scintillator was simulated with varied diameters of 8–24 cm and thicknesses of 5–25 cm, and the performance was assessed with four radionuclides of 137Cs, 60Co, 226Ra, and 40K. To determine the optimal scintillator dimensions, the generation and collection efficiency of optical photons were assessed. The performance of the multi-array scintillator was evaluated by comparing it with the conventional single scintillator in terms of the gamma-collection efficiency and the energy resolution with the full width at half maximum (FWHM) of the energy weighted spectrum. The plastic scintillator of 8 cm diameter and 15 cm thickness was determined to have the optimal dimensions out of the 25 geometry conditions. The multi-array plastic scintillator showed an increased optical photon-collection probability, up to 35% more than a conventional plastic scintillator, while the FWHM decreased by more than 20%, resulting in an improved energy resolution. Based on this MC-based design and optimization study, a proof-of-principle detection system is currently being built and an experimental study to demonstrate its superior accuracy in the identification of radionuclides will also be conducted in the future.

Radioisotope identification using an energy-weighted algorithm with a proof-of-principle radiation portal monitor based on plastic scintillators

In this study, we validated the feasibility of an energy weighted algorithm that highlights a characteristic area including the Compton edge as a single peak in a proof-of-principle radiation portal monitor system with a plastic scintillator measuring 50 × 100 × 5 cm3. We measured the energy weighted spectra with steel shielding and the dynamic movements of the 137Cs and 60Co sources. The results showed that the peak locations of each source could be identified under shielded or dynamic motion conditions, each within a maximum difference of 0.08 MeV.

Calculation of Detector Positions for a Source Localizing Radiation Portal Monitor System Using a Modified Iterative Genetic Algorithm

Calculation of Detector Positions for a Source Localizing Radiation Portal Monitor System Using a Modified Iterative Genetic Algorithm

10B+ZnS(Ag) as an alternative to 3He-based detectors for Radiation Portal Monitors

Typical radiation portal monitor systems, RPM, deployed to detect illicit trafficking of radioactive materials include a set of gamma-ray detectors and neutron detectors. Usually the employed neutron detectors are pressurized 3 He-based neutron detectors tubes. Due the shortage of 3 He reported since 2009, the amount of 3 He available for use in gas proportional counter neutron detectors has become limited, while the demand has significantly increased, especially for homeland security applications. For this reason, many different alternatives are being investigated for its use in RPM systems. The aim of this work is to study a scintillation detector ZnS(Ag) mixed with highly enriched 10 B, 10 B+ZnS(Ag). Using Monte Carlo methods, MCNPX code, the response of two neutron detectors based on 10 B+ZnS(Ag), manufactured by BridgePort Instruments LLC with different geometries, were estimated by calculating the number of 10 B(n,α)7 Li reactions for 29 monoenergetic neutron sources. Measurements and models were made, and both detectors were compared. The importance of the distance with respect to the ground was studied. The response with a 252 Cf moderated neutron source (0.5 cm lead and 2.5 cm polyethylene) was calculated in order to compare with other studied alternatives in the USA by Pacific National Northwest Laboratory, PNNL. With these results we conclude that neutron detectors using 10 B+ZnS(Ag) are an interesting alternative for replacing 3 He detectors. From the analysis with MCNPX we propose an improvement in the detector design.

3He and BF3 neutron detector pressure effect and model comparison

Radiation detection systems for homeland security applications must possess the capability of detecting both gamma rays and neutrons. The radiation portal monitor systems that are currently deployed use a plastic scintillator for detecting gamma rays and 3He gas-filled proportional counters for detecting neutrons. Proportional counters filled with 3He are the preferred neutron detectors for use in radiation portal monitor systems because 3He has a large neutron cross-section, is relatively insensitive to gamma-rays, is neither toxic nor corrosive, can withstand extreme environments, and can be operated at a lower voltage than some of the alternative proportional counters. The amount of 3He required for homeland security and science applications has depleted the world supply and there is no longer enough available to fill the demand. Thus, alternative neutron detectors are being explored. Two possible temporary solutions that could be utilized while a more permanent solution is being identified are reducing the 3He pressure in the proportional counters and using boron trifluoride gas-filled proportional counters. Reducing the amount of 3He required in each of the proportional counters would decrease the rate at which 3He is being used; not enough to solve the shortage, but perhaps enough to increase the amount of time available to find a working replacement. Boron trifluoride is not appropriate for all situations as these detectors are less sensitive than 3He, boron trifluoride gas is corrosive, and a much higher voltage is required than what is used with 3He detectors. Measurements of the neutron detection efficiency of 3He and boron trifluoride as a function of tube pressure were made. The experimental results were also used to validate models of the radiation portal monitor systems.

Designing Effective Alarms for Radiation Detection in Homeland Security Screening

In this correspondence, the human factors involved in the design of effective homeland security threat detection systems are described and illustrated for radiation portal monitor (RPM) systems deployed at U.S. ports of entry. Due to the occurrence of nuisance alarms based on naturally occurring radioactive material and the low base rate of nuclear smuggling incidents, it is shown that the probability of a true threat alarm for these systems is extremely low. Receiver operating characteristic analysis of RPM systems illustrates good simple detection capability, but threat classification performance only at the chance level. Application of the human factors concept of the threat likelihood display, based on energy spectrum and cargo commodity data fusion for signal classification, reduces nuisance alarms and increases the probability of a true threat alarm to potentially effective levels. Thus, threat likelihood displays offer an approach for enhancing the effectiveness of homeland security detection and warning systems by raising the credibility of the alerts that are provided.

The role of spectroscopy versus detection for border security

Countries around the world are deploying radiation portal monitor systems to interdict the illicit shipment of radioactive material crossing international borders. Because of their high efficiency for gamma-ray detection, most deployed systems are based on plastic scintillators and are non-spectroscopic in capability. Spectroscopic portal monitor systems are undergoing engineering development for near term deployment. The ability to identify the detected radionuclides may allow improved operational handling of radiation alarms, particularly those from the normal commerce of naturally occurring radioactive material. The goal for improved systems is to increase the sensitivity to threats while reducing the operational impact of nuisance alarms.

Gamma-ray signatures for state-of-health analysis and monitoring of widely-arrayed radiation portal monitor systems

Pacific Northwest National Laboratory (PNNL) has deployed a large array of radiation portal monitors for the Department of Homeland Security and U.S. Customs and Border Protection. These portal monitors scan incoming vehicles crossing the U.S. border and shipping containers leaving international ports for radioactive material via gamma-ray and neutron detection. Data produced and captured by these systems are recorded for every vehicle related to radiation signature, sensor/system status, local background, as well as a host of other variables. Within the Radiation Portal Monitor Project at PNNL, state-of-health observation and analysis for the whole RPM array using these data to determine functionality and performance is ongoing. Advanced state-of-health analysis and monitoring algorithms are being developed. Preparations are underway to incorporate the more difficult state-of-health monitoring of the mobile RPM and Advanced Spectroscopic Portals.

A theoretical approach to calibrate radiation portal monitor (RPM) systems

Radiation portal monitor (RPM) systems are widely used at international border crossings, where they are applied to the task of detecting nuclear devices, special nuclear material, and radiation dispersal device materials that could appear at borders. The requirements and constraints on RPM systems deployed at high-volume border crossings are significantly different from those at weapons facilities or steel recycling plants, the former being required to rapidly detect localized sources of radiation with a very high detection probability and low false-alarm rate, while screening all of the traffic without impeding the flow of commerce [Chambers, W.H., Atwater, H.F., Fehlau, P.E., Hastings, R.D., Henry, C.N., Kunz, W.E., Sampson, T.E., Whittlesey, T.H., Worth, G.M., 1974. Portal Monitor for Diversion Safeguards. LA-5681, Los Alamos Scientific Laboratory, Los Alamos, NM]. In the present work, compact analytical formulae are derived and used to calibrate two RPM systems with isotropic radiating sources: (i) polyvinyltoluene (PVT) or plastic and (ii) thallium-doped crystalline sodium iodide, NaI(Tl), gamma-ray detector materials. The calculated efficiencies are compared to measured values reported in the literatures, showing very good agreement.

Passive neutron detection for interdiction of nuclear material at borders

Radiation portal monitor systems based upon polyvinyl toluene scintillator gamma-ray detectors and pressurized 3He-based neutron detector tubes have been deployed to detect illicit trafficking in radioactive materials at international border crossings. This paper reviews the neutron detection requirements and capabilities of passive, as opposed to active interrogation, detection systems used for screening of high-volume commerce for illicit sources of radiation at international border crossings. Computational results are given for the impact of cargo materials on neutron spectra, for the response of various detector geometries, the effects of backgrounds including “ship effect” neutrons, and for simulation of a large neutron detection array.

Baseline suppression of vehicle portal monitor gamma count profiles: A characterization study

Radiation portal monitor (RPM) systems based upon polyvinyl toluene scintillator (PVT) gamma-ray detectors have been deployed to detect illicit trafficking in radioactive materials at border crossings. This report sets forth a characterization of the baseline suppression effect in gross-count gamma-ray profiles due to shadow shielding by vehicles entering RPMs. Shadow shielding is of interest because it reduces the alarm sensitivity of RPMs. This observational study investigated three types of PVT-based commercial RPM systems currently deployed at selected ports of entry in terms of spatial effects relative to detector panel positioning. Radiation portal monitor sites were characterized by driver versus passenger side, top versus bottom panel, and narrow lanes versus wide lanes as observed for a large number of vehicles. Each portal site appears to have a distinctive baseline suppression signature, based on percent maximum suppression relative to measured background. Results suggest that alarm algorithms based on gross-counts may be further refined through attention to individual site characteristics. In addition, longer vehicle transit times were often correlated with stronger baseline suppression, suggesting that baseline suppression studies should take transit time into account.

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

The demand for radiation portal monitor (RPM) systems has increased, and their capabilities are being further scrutinized as they are being applied to the task of detecting nuclear weapons, special nuclear material, and radiation dispersal device materials that could appear at borders. The requirements and constraints on RPM systems deployed at high-volume border crossings are significantly different from those at weapons facilities or steel recycling plants, where RPMs have been historically employed. In this new homeland security application, RPM systems must rapidly detect localized sources of radiation with a very high detection probability and low false-alarm rate, while screening all of the traffic without impeding the flow of commerce. In light of this new Department of Homeland Security application, the capabilities of two popular gamma-ray-detector materials as applied to these needs are re-examined. Both experimental data and computer simulations, together with practical deployment experience, are used to assess currently available polyvinyltoluene and NaI(Tl) gamma-ray detectors for border applications.