Automatic Identification Of NORM By Portal Monitors -- What Is Really Possible?

The increasing threat of the illicit proliferation of nuclear and radioactive materials in the territories of different states and across their borders with the possible use of these materials for terrorist attacks has within the world community necessitated the creation of effective counteractions. Of various radiation detection equipment used for the control of illicit traffic, the fixed installation gamma-neutron monitor is most common because of its superior sensitivity for detection of radiation above natural background. At present, many suppliers have achieved detection probability in their monitors to detect a small percentage of radiation above natural background, and greater sensitivity is possible. However, the increase in sensitivity is hindered by the increase of so-called “innocent” alarms caused by the presence of NORM in the objects being monitored. Considering the numerous problems which occur at the border and customs check points in the event of any alarm, equipment suppliers are trying to develop monitors capable of identifying “innocent” radiation materials and reducing unnecessary alarms. However, most fixed-installation monitors use large volume plastic scintillators based on Polystyrene (?8?8)n or Polyvinyltoluene (?7?8)n. The main type of interaction of gamma radiation with this kind of material is Compton Effect. For this type of interaction the spectra of gamma radionuclides are the distributions without clear maximum peaks. Unlike the normal inorganic scintillators where the photo effect is the result of interaction, the Compton distribution is not informative enough to conduct real spectroscopy with these detectors. The difference between the types of detectors is well seen in the following spectra, which illustrates response to a mixture of K-40 and Co-60: