Photon-induced positron annihilation radiation (PIPAR) — A novel gamma-ray imaging technique for radiographically dense materials

A discussion is presented of some fundamental limitations of the Compton back-scattering technique for imaging radiographically dense materials. These include the low probability of Compton back-scattering for high energy (> 1 MeV) photons, the low energy (and hence high attenuation) of the scattered radiation (≤ 225 keV) at large angles of scattering and the invariable presence of a large multiple scattering background superimposed on the single scattering component. l radiographic technique—photon-induced positron annihilation radiation (PIPAR)—is described and is shown to avoid the above limitations. It thus extends significantly the depth range of materials which may be investigated with gamma rays in the reflection geometry. inary experimental results showing the feasibility of PIPAR imaging are presented here for the first time. These were obtained with megavoltage radiation from a linear accelerator exciting positrons in elemental samples through the pair production effect. The 511 keV annihilation line was extracted from the back-scattered radiation using a plastic scintillator detector and a difference filter technique. The experimental arrangement is described and potential applications are briefly discussed.