High-efficiency position resolved gamma ray detectors for 2D-measurements of the angular correlation of annihilation radiation

The measurement of the 2D-Angular Correlation of Electron Positron Annihilation Radiation (ACAR) provides unique information about the bulk electronic structure of single crystals. We set up a new prototype for 2D-ACAR measurements using two 24 x 24 (26.8 mm x 26.8 mm) pixelated LYSO scintillation crystals in combination with a glass light guide and 8 x 8 (24 mm x 24 mm) Multi Pixel Photon Counters (MPPCs). Compared to conventional Anger-cameras, typically comprising large NaI(Tl) scintillators read out with photomultiplier arrays a larger implementation of our prototype would drastically improve resolution and count rate by taking advantage of the small pixel size of the scintillator, its much higher attenuation coefficient for 511 keV γ-quanta and faster digital readout. With our prototype we achieved a detection efficiency of 45%, i.e. five times higher compared to NaI(Tl) used in our Anger cameras, leading to a 25 (!) times higher coincidence count rate in ACAR measurements. A spatial resolution of 1 mm was obtained, which is limited by the pixel size of the scintillator. We demonstrate the high performance of the setup by (i) imaging the local distribution of 22Na in a proton-irradiated aluminum target and (ii) determining the Fermi energy of Cu from 2D-ACAR spectra recorded for a polycrystalline copper sample.