Neutron Monitors— Self-Indication of Shielding

Neutron monitoring is extensively used in safeguards to detect the passage of nuclear material. In many of these applications, neutron monitors are coupled with camera surveillance systems. In addition to recording movement of items of interest, the camera system has also been traditionally used to confirm that no neutron shielding has been placed around the monitors and that therefore they are still effectively monitoring the area. Using cameras for this purpose means that the neutron monitoring system cannot be considered a single layer of containment and surveillance by itself, because it needs the camera system to ensure that it is still operational. However, the potential diverter would need to apply a significant amount of shielding to mask the movement of a typical item. This shielding would affect the ‘background’ counting rate of each neutron monitor, due to cosmic rays or nuclear material in the vicinity. This change in counting rate can be used to determine if shielding has been applied to the monitor. Thus, the neutron monitor provides a self-indication that shielding has been applied and the dependence on the camera data is removed. This paper gives numerical examples for the case of a nuclear material storage area and proposes that neutron monitors can be used as a stand-alone layer for containment and surveillance purposes.