TESTING AND IMPROVING THE DETECTION CAPABILITY OF PORTAL MONITORING SYSTEMS AT HIGHER TRANSIT SPEEDS

摘要

The aim of the paper is to validate the sensitivity of a typical portal monitoring system at high transit speed up to 44 m/s (160 km/h) and to enhance the detection capability of a prototype system by improvement of the signal processing. It has been demonstrated that speed dependent signal processing can improve the detection capability of portal monitoring at higher speeds. At 50 km/h, the minimum detectable activity (MDA) could be reduced by a factor of two and at 90 km/h by a factor of three compared to a fixed integration time of 1 s. Subsequently, these alterations can be introduced into series production to obtain special monitors to be used at higher transit speed. The signal processing has also been optimized for the scanning of long objects, such as railway trains. For the field test, six pillars, providing a total detector volume of about 138 L were installed at a railroad track near Vienna. The speed dependent data processing and decision making was carried out externally by a personal computer, processing only the raw data (count rate) received in 40 ms intervals. The system was tested at various speeds to evaluate the detection capability as a function of the transit speed. The quantum efficiency was experimentally measured, to characterize the dependence on the distance to the detector for an object moving in a straight line, passing the portal. Based on the shape of the detection efficiency, a new algorithm for the signal processing was developed. The algorithm uses speed dependent adaptation of the scan interval, based on optimization of the signal to noise ratio. Finally, the performance (detection capability), at various speeds, of the original and the new signal processing algorithm are compared. This comparison also includes consideration of the false alarm rate, to allow realistic comparison of the results.