A Compton Telescope for Remote Location and Identification of Radioactive Material

The spare detectors from NASA´s Compton Gamma-Ray Observatory COMPTEL instrument have been reconfigured to demonstrate the capability at ground level to remotely locate and identify sources of gamma radiation. 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 Nal(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. GRETA´s field of view is a cone with full angle approximately 120deg. The sensitive energy range is 0.3 to 2.6 MeV. Energy resolution is ~10% FWHM. The angular resolution, ~15deg in the simplified configuration tested, will improve to better than 5deg with well-defined enhancements to the data acquisition hardware and data analysis routines. We conducted a series of laboratory demonstrations to characterize the performance of the GRETA detector. Following detector characterization, we demonstrate GRETA ´s ability to locate and identify an unknown source and position within its field of view. In this scenario, the location and isotope-identifying energy spectrum of an 800 muCi¹³⁷ Cs source 10 meters from the instrument with two intervening walls was determined in less than one minute. We will present further details of the instrument design and the measurements.