  The ability to formulate maps of minimum detectable activities (MDAs) that
describe the sensitivity of an ad hoc measurement that used one or more freely
moving radiation detector systems would be significantly beneficial for the
conduct and understanding of many radiological search activities. In a
real-time scenario with a free-moving detector system, an MDA map can provide
useful feedback to the operator about which areas have not been searched as
thoroughly as others, thereby allowing the operator to prioritize future
actions. Similarly, such a calculation could be used to inform subsequent
navigation decisions of autonomous platforms. Here we describe a near real-time
MDA mapping approach that can be applied when searching for point sources using
detected events in a spectral region of interest while assuming a constant,
unknown background rate. We show the application of this MDA mapping method to
a real scenario, a survey of the interior of a small building using a handheld
detector system. Repeated measurements with no sources and with Cs-137 sources
of different strengths yield results consistent with the estimated thresholds
and MDA values; namely, that for background-only measurements no sources are
seen above threshold anywhere in the scene, while when sources are present they
are detected above the thresholds calculated for their locations.
