a signal processor configured to determine a high count event when a detected radioactivity level is determined to be greater than a predetermined level, and further configured to determine an alarm condition when a plurality of consecutive high count events is determined, the signal processor comprising a counter configured to determine whether at least a threshold value of radioactive counts is detected within a predetermined time period, the counter configured to be loaded with the threshold value and to be decremented on every received radioactive count, and the counter configured to determine a high count event on the counter counting at least the threshold value before the predetermined time period has expired, and in that event the counter configured to reload to an initial value and commence counting in respect of a consecutive time period.