Improving the detecting performances of radiation portal monitors using matched filter algorithm

Full text: Introduction: Radiation Portal Monitors (RPM) are used for monitoring transportation of illicit radioactive materials. The application requires detection of radioactive materials while the vehicle is passes through a portal, thus having limited time for monitoring the passing vehicle. The pattern of the RPM detector readings, caused by measuring a moving source versus the background provides an opportunity to employ an algorithm based on matched filter technique for improving the detecting performances. Method: Matched filter is a process used for detecting a known signal in the presence of additive stochastic noise, and thereby maximizing the detected signal to noise ratio (SNR). For RPM's the matched filter is implemented by correlating a known signal, which is distance dependent, with the RPM readings. Convolving those readings, which combine the signal and the noise, with a time-reversed version of the signal, improves the RPM performances. To validate this concept a detection configuration was established according to measuring time limit and Minimal Detectable Activity (MDA) defined by ANSI 42-35 regulation. A mathematical software simulation was performed, followed by an experiment with a scale down configuration in order to confirm the benefits of the suggested algorithm. The experimental setup contained a NaI(Tl) scintillator based radiation detector, a radioactive source located on a moving toy train and a proximity sensor. Results: The detection performances obtained by both the simulation and the experiment with and without the matched filter algorithm were compared. The software simulation has shown a major improvement of up to six-fold decrease in miss alarm rate. Similar results where obtained by the experiments. Conclusions: The implementation of the matched filter in RPM detection algorithm improves its performances. The method provides either a) a higher detection reliability level, b) the ability to detect lower activity level, c) the need for fewer detectors to achieve the same MDA obtained without the matched filter. The proposed method is almost costless for implementation and requires minor hardware modifications. A future research will evaluate the advantage of the matched filter in cases of a spread source and a source which is not located at the center of the moving vehicle. (author)