A Compton Telescope for Remote Location and Identification of Radioactive Material

The spare detectors from NASA Compton Gamma-Ray Observatory COMPTEL instrument have been reconfigured to demonstrate the capability at ground level to remotely locate and identify sources of g radiation or the movement of material that might shield gamma-ray sources. The Gamma-Ray Experimental Telescope Assembly (GRETA) employs two 28 cm diameter scintillation detectors separated by 81 cm: one 8.5 cm thick liquid scintillator detector and one 7.5 cm thick NaI(Tl) detector. The assembly electronics and real-time data acquisition system measures the energy deposits and time-of-flight for each coincident detection and compiles histograms of total energy and incident angle as computed using the kinematics of Compton scattering. The GRETA field of view is a cone with full angle approximately 120 degrees. The sensitive energy range is 0.3 to 2.6 MeV. Energy resolution is similar to 10% FWHM. The angular resolution, similar to 19 degrees in the simplified configuration tested, will improve to better than 5 with well-defined enhancements to the data acquisition hardware and data analysis routines. When operated in the mode that was used in space, the instrument is capable of measuring and imaging up to 30 MeV with an angular resolution of 1.5 degrees.The response of the instrument was mapped in the laboratory with 14 mu Ci Na-22 source 3 m from the instrument. Later, we conducted demonstrations under two measurement scenarios. In one, the remotely located instrument observed an increase of background radiation counts at 1.4 MeV when a large amount of lead was removed from a building and a corresponding decrease when the lead was replaced. In the other scenario, the location and isotope-identifying energy spectrum of a 500 mu Ci Cs-137 source 3-5 m from the instrument with two intervening walls was determined in less than one minute. We report details of the instrument design and these measurements.