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Large surface gamma cameras for medical imaging: characterization of the bismuth germanate blocks

The transmission of protons in planar channeling or in the regime of crystal reflection in bent crystals is now routinely used at high energy. We used the property that channeling critical angle increases as the incident particle momentum decreases, to explore the region of moderate energies (100 MeV–1 GeV). Indeed, such energies are particularly interesting since medical applications such as particle therapy have to face the constraints of being compatible with hospital-based accelerators. Therefore, replacing tens- or even hundreds-tons gantries by bent crystals would – if feasible – meet societal applications. We used binary-encounter simulations of trajectories inside crystals oriented along planar directions. The Molière potential with thermally vibrating lattice atoms was used, and additional transverse heating was introduced to account for multiple elastic scattering by close-collisions on electrons, which depends on the transverse energy of the channeled ions. The survival yield (i.e. the fraction of ions keeping trajectories within plus-minus one critical planar channeling angle with respect to lattice planes) was simulated for protons and carbon ions, as a function of crystal nature (silicon or germanium), crystal temperature, thickness and curvature. Although the transmitted yields are far from the necessary yields required to treat patients, significant survival yields were found through cm-thick crystals at angles beyond 10°. We will discuss possible experimental verification of these findings, and in particular practical aspects of such very large bending angles.