1 is a grey-scale pixel map of explosive probabilities for a suitcase scanned by our neutron spectrometer. Grey-scale pixel map of explosive probabilities for a scanned suitcase containing an elongated piece of C4. To date we have scanned about 300 suitcases with our neutron spectrometer, of which about half contained a real or a mock explosive. From the results for about half of these scanned suitcases, a criterion for assigning suitcase probabilities was constructed as follows. For each of twelve pixel-probability thresholds the largest group of contiguous pixels exceeding that threshold is found for a given suitcase.
Pixels are contiguous if they are connected either on their sides or diagonally.
The probability thresholds we are currently using are 0.996, 0.99, 0.98, 0.95, 0.9, 0.84, 0.77, 0.7, 0.6, 0.5, 0.4, and 0.3. The rationale for contiguous-pixel tests is that explosives are expected to occur in a single piece or a small number of pieces. We use twelve thresholds in order to anticipate explosives of a variety of sizes and shapes. This is due primarily to the small number of samples, and also due to the fact that explosive sizes and shapes were not chosen systematically. Consequently, we have developed a program for simulating suitcases, described in the next section.