RadICAL stack: A localisation method for dynamic gamma/neutron fields

A variation of the RadICAL (Radiation Imaging Cylinder Activity Locator) system capable of operating in a dynamic environment, such as that created by active interrogation techniques, has been developed. RadICAL is a novel method for locating a radiological source using a rotating detector element. The detector geometry is that of a thin sheet and is rotated to present a constantly changing surface area to the source; it therefore generates a characteristic temporal response which can be used to determine the source direction. The time required to determine the direction of a source make it unsuitable for dynamic environments and so an alternative method is presented that uses a stack of identical scintillator slabs positioned at fixed horizontal angles around a central axis. By comparing count rates from each slab to a standard response curve, using a specially developed algorithm, the direction of a source can be determined without the need to rotate the detector. EJ-299-33 plastic scintillator was used to allow detection of separate neutron and gamma events in a mixed field through pulse shape discrimination. A four element detector was built and shown to achieve a positional accuracy of approximately 4.4 degrees when exposed to a 1.44MBq 137 Cs source at distances of up to 2m. The same detector was used to discriminate separate neutron and gamma events in a mixed field, which allows for the possibility of locating a neutron source within a gamma rich environment.