Demonstration of imaging via backscattering of annihilation gamma rays

Imaging via backscattering of annihilation gamma rays (IBAG) is a technique for three-dimensional gamma ray imaging of objects when only one side is accessible, such as for objects behind a wall or other barrier. This technique utilizes a positron source to produce nearly collinear, 511-keV gamma rays in opposite directions. One of these provides timing and directionality tagging, while the other backscatters off the object to be imaged. The scattering location can be reconstructed using the relative timing of the two gamma ray signals and the positions at which they are detected. Excellent timing resolution is required, since the time-of-flight information is essential for reconstruction. Three-dimensional imaging requires two arrays of detectors, one for the 511-keV tagging gamma ray and one for the backscattered gamma ray. We are studying the feasibility of an IBAG system using LaBr3 to give good timing resolution and energy-based event selection. This paper presents one-dimensional position reconstruction measurements from our laboratory system for a variety of objects and barriers.