A system includes a count detector, a communication medium; and a processor coupled to the count detector. The processor continuously receives a plurality of pulses from the count detector. A pulse indicates a detection of a radiation unit emitted from a source material or a background. The processor determines a first period of time based on an expected range of speed of a carrier of the source material, and integrates the plurality of pulses over the first period of time, thereby yielding an integrated count associated with a time at a midpoint of the first period of time. The processor creates a continuous time series of count profiles from a plurality of integrated counts that are computed using a plurality of windows within the first period of time, and shifts each window over a second period of time. The second period of time is shorter than the first period of time. The processor estimates a background count from a history of the count profiles, computes an adaptive threshold based on the estimated background count, and detects the source material when consecutives of the integrated counts exceed the adaptive threshold.