Investigation of portal monitoring false alarm - a case study

Two Portal Monitoring Systems (PMS) were procured for an upcoming fast reactor fuel reprocessing facility. One of the systems was installed in a similar small scale facility to get to know the system's pros and cons. Before including any system under regular radiation monitoring programme, it is essential to do performance evaluation. Determination of false alarm rate is a part of performance evaluation and the same has to be minimum. A systematic determination of false alarm rate was done for the installed PMS and the reasons for the same were investigated and are presented in this paper. It is observable that the 3.5 σ level of 0.2 s counting time cross the alarm threshold (4660) and the calculated false alarm rate is 1 in every 230 passages against the practically recorded values of 1 in 243 passages. Hence it is inferred that the reason for higher false alarm rate is due to the software algorithm, which takes the counts obtained during occupancy also for background calculation