Geometry and surface treatment dependence of the light collection from LSO crystals

We study the relative light collection efficiency for narrow LSO crystals as a function of surface finish and geometry. We explore both a specular and diffuse surface reflector finish, using LSO crystals that are either polished or etched. The crystals have a square cross-section with widths of 1.8, 2.2, 2.5, or 2.6 mm and lengths of 10, 20 or 30 mm. When optically coupled to a PMT on a square end and wrapped with Teflon on 5 sides, we excite them at 5 mm incremental depths with 511 keV photons and measure the photopeak position. The light collection is characterized by a maximum output (based on the photopeak position when excited at the PMT end) and a depth-dependent loss factor. Both the maximum output and loss factor are effectively independent of width, and the maximum output is only weakly dependent on length (6696±612, 5796±432, and 5328±288 photons for 10, 20, and 30 mm lengths). The loss factor is effectively independent of length, with a 0, 18, 27, and 22% reduction at excitation depths of 0, 10, 20, and 30 mm. The light collection is only weakly dependent on surface finish if it is polished or etched beyond a minimum time.