Minimum Detectable Activity Estimates for a Germanium-Detector Based Spectroscopic Portal Monitor

The Minimum Detectable Activity (MDA) for identification is one of several important performance measures for Advanced Spectroscopic Portal Monitors (ASP) given in ANSI N42.38 (draft) to determine the effectiveness of the ASP. In most spectroscopic applications, the samples are counted for a sufficiently long time to achieve the required minimum detectable activity or until the measured activities are within the desired statistical limits. However, in the ASP, the counting time is defined by the transit time of the vehicle or container through the portal field of view. For the specified transit speed and field of view, this gives a data collection time of a non-distributed source of 3 to 8 seconds. The performance criteria for a specified nuclide are stated in terms of the minimum detectable activity and in addition, at that stated activity limit, the number of false negatives must be less than 5 %, while the number of false positives in the same situation must be less than 1 in 1000. This low number of false negatives and false positives defines where the threshold for declaration of detection or identification should be set. This threshold must be based on some parameter derived from the spectrum, but is not necessarily only the activity of the nuclide. In addition, it is not desirable to use the concept of “blank” MDA, as used in sample counting, (which would correspond to measuring the background when the portal was unoccupied), because changes (normally suppression) to the background count-rate occur because of the presence of the vehicle and its cargo. Ideally, therefore, this parameter must be determined dynamically from the actual spectrum collected and compared to the threshold to determine if the nuclide is present. To determine the MDA for the ORTEC ASP, the detection parameter threshold was determined by modeling the portal monitor performance using measured efficiencies of the germanium detectors at the geometries specified in ANSI N42.38 and specifying the maximum number of false negatives and false positives to be as required in the standard. Then the parameter value was calculated on measured spectra for the geometries and measurement conditions (speed and distance) and for a range of nuclide activities. Using these measurements, a function can be determined relating the parameter value and the activity. From this function, the activity at the parameter threshold can be determined, which is the portal MDA for the tested geometry.