Transparency Measurements Using A Gamma-Ray Imager

One of the issues encountered by the United States and Russia as they strive to reduce their respective nuclear stockpiles is the ability to identify and verify the location of weapons components throughout the demolition process. The inherent difficulty in this task arises from the classified nature of components which are involved. During inspections, a balance must be drawn between revealing sufficient information to ascertain the authenticity of a part and not revealing critical (classified) design information. The use of a collimated inorganic scintillator detector scanned across a part's storage container to provide both size and isotope information is one possible technique. Both techniques may be performed without visual inspection of the actual object, and with predetermined spatial and energy resolutions. The time required for such inspections can be significantly decreased through the use of gamma-ray imaging. The Gamma-Ray Imaging Spectrometer (GRIS,) which is described elsewhere in these proceedings [1], allows one to obtain an image in the light of gamma-rays emitted by SNM. It is based on the same inorganic scintillator technology as conventional gamma-radiation detectors, and combines the scan and isotope identification in a single step. Further, the gamma-ray image may be formed from the light of particular line emissions, verifying, not only that SNM is present, but also that its spatial dimensions correspond to those expected. The position-resolution at the source may be easily determined from three simple, dimensional measurements of the imager to verify that too much information is not obtained.[l] In a simple demonstration, we positioned a plutonium disk inside a standard weapons component storage container. An inspection of this vessel undertaken using the scan method required a full morning. The results of a measurement on the same sample obtained with one of the four GRIS detectors in - 1/2 an hour provides the same information (see Figure 1.) That a distributed Pu object is in the container can be easily verified from the combined false-color gammaray/ video images and by comparison of the face and edge views obtained.