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 95 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. 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 is better than 5deg. We have previously reported measurements using a simplified readout configuration that limited GRETA´s imaging ability to a rough (~15deg) directional sensitivity. Here we report on measurements using the full (15-channel) readout that can achieve ~1.5-cm position resolution in each scintillator and permits true Compton imaging using COMPTEL software. GRETA has been refurbished to allow for ease of transport and field use with updated real time imaging and identification software. We present most recent laboratory measurements of radioactive sources of concern, ¹³⁷Cs and ⁶⁰Co, potential candidates used with radiological dispersal devices.