C. Marianno, J. Falkner, “Modeling Minimum Detectable Activity as a Function of Velocity”, 62nd Annual Meeting of the Health Physics Society, Raleigh, North Carolina, 9-13 July 2017.
Minimum detectable activity (MDA) is an important parameter of a radiation detection system’s performance. Typically, values of MDA are calculated for stationary detectors using static background conditions. However, mobile detectors are being deployed in continuously changing backgrounds, such as the search for illicit radioactive sources or in large area surveys. In cases such as these, the MDA will vary from the static value. Evaluating and quantifying the change in MDA as a function of a detector’s velocity can improve the system’s performance and decrease the cost of performing a survey. Simulations were conducted at Texas A&M University to investigate the effect of velocity on MDA. A series of 7.62 cm X 7.62 cm NaI detectors were simulated at speeds ranging from 0-100 cm s-1. The detectors were individually simulated in offsets of 5 cm intervals away from a source, up to 30 cm. Sources were simulated as point sources and line sources of various lengths corresponding to the distance the detectors would have traveled at the investigated speeds. Simulations were performed using both MCNP and Python. Results show that for the same source-background conditions, MDA decreases as both the offset distance and detector speed increase. For a given source-background condition, net detector response was compared to the detection limit as calculated from Currie’s equation. For speeds above 30 cm s-1 the net detector response was below the detection limit. A relationship between detector speed and MDA was derived. This relationship can be used to improve detector performance, increasing instrument sensitivity or reducing survey time as dictated by mission requirements. rn