Point source detection and characterization for vehicle radiation portal monitors

Abstract

Many international border crossings presently screen cargo for illicit nuclear material using radiation portal monitors (RPMs) that measure the gamma ray and/or neutron flux emitted by vehicles. The fact that many target sources have a point-like geometry can be exploited to detect subthreshold sources and filter out benign sources that frequently possess a distributed geometry. This report describes a two-step process, which has the potential to complement other alarm algorithms, for detecting and characterizing point sources. The first step applies a matched filter whereas step two uses a weighted nonlinear least squares method. In a basecase simulation, matched filtering detected a 250-cps source injected onto a white-noise background at a 95% detection probability and a 0.003 false alarm probability. For the same simulation, the maximum likelihood estimation technique performed well at source strengths of 250 and 400 cps. These simulations provided a best-case feasibility study for this technique, which will be extended to experimental data that possess false point-source signatures resulting from background shielding caused by vehicle design and cargo distribution.

Benefits of Position-Sensitive Detectors for Radioactive Source Detection

“…so position-sensitive capability increases the noncentrality parameter η in (17) by the factor K [1,1] in (9). In this case, the inequality in Theorem 1 becomes strict.…”

“…Figure 3a shows the AUC, the position-sensitive gain factor K [1,1] from (9), and the angular uncertainty as a function of attenuation-radius product µr for a circular detector. The quantity ατ represents the expected number of photons emitted from the source during the scan and λ b τ represents the expected number of background photons recorded.…”

“…A common assumption in the field of security imaging is that sources of radiation are small, so they appear point-like to a detector [1]. There are many gamma-ray detection modalities including stationary radiation portal monitors [1], coded aperture imaging systems [2], arrays of scintillating detectors [3], and position-sensitive Compton detectors [4], [5].…”

Source Detection Performance Prediction for a CdZnTe Array

“…We model the distribution of a single recorded attribute as a mixture of and given by (4) We substitute (4) into (3) to compute the likelihood of a list of observations. Fig.…”

“…Previous work that characterized the detection performance of gamma-ray detectors relied on empirical ROC calculation, e.g., [3], [4]. Empirical ROC calculation is performed by simulating multiple scans with a detector, and using the statistics of those scans to compute the probabilities of false alarm and detection.…”