A SYSTEM FOR THE RAPID ANALYSIS AND INTERPRETATION OF THE GAMMA RAYS FROM NUCLEAR MATERIALS AFTER THEIR INITIAL DETECTION

The interdiction of nuclear materials which may be intended for various malevolent purposes involves not only the initial detection of radiation but the subsequent analysis and interpretation of the gamma-ray spectra to obtain the maximum possible information on the nature of the material as rapidly as possible. This is absolutely essential in order to mount an appropriate response to a possible threat. Experience has shown that there is an extremely high background of innocent alarms that produce a response from existing radiation detection systems. These alarms often interfere with legitimate activities and place a heavy and unnecessary burden upon responders. The answer to this problem is to identify the material promptly, and in the event of a malevolent act, to obtain complete information on the nature and source of the material. Our approach to this problem is to create a deployable, high resolution spectrometry system with the maximum achievable sensitivity and discrimination. The system consists of a HPGe spectrometer surrounded by a bismuth germanate (BGO) scintillator and the requisite circuitry for the detection and analysis of the radiation. The BGO shield has two functions: 1. To serve as an active shield and collimator to reduce background from external sources of radiation in the environment, and 2. To eliminate background events which arise from the Compton scattering of gamma rays in the HPGe. These enhancements, in addition to the high resolution of the HPGe provide optimum sensitivity and discrimination. The HPGe detector is cooled with liquid nitrogen (LN2). On the basis of extensive experience in the field, it is our judgment that this approach is totally feasible and preferable to other cooling systems or the use of detectors with far lower energy resolution. We are also investigating, with this system, the use of a germanium well counter as the detector. This will provide a deployable system with maximum sensitivity and analyzing power for the prompt analysis of very small samples of radioactive materials. Results on the performance of these systems will be presented.