Comparison of Timing Resolution between Inter-Crystal Scatter and In-Crystal Events

A 511keV gamma ray interacts with scintillation crystals by photoelectric interaction or Compton scattering. In photoelectric interaction, it deposits all 511keV energy at the first interaction location. In Compton scatter, it can place its energy within a crystal or it can distribute its energy into multiple crystals. Compton scatter events between multiple crystals were expected to have worse timing resolution due to increased scintillation light spread. In this paper, timing resolution of Compton scattered events involving multiple crystals (Inter-Crystal Scatter Events) were compared to those interactions isolated within a single crystal (In-Crystal Events: Photoelectric and Compton scatter events within a single crystal). For the measurement, LYSO crystals of 3.95 × 5.3 × 25mm³ was used to make 2×3 arrays. One layer of ESR film was inserted in between crystals to have one-to-one coupling to a 2×3 array of SiPMs. Two crystal arrays were built with all surface polished crystals while two other arrays were built with crystals with all surface polished but one lapped surface of 5.3mm×25mm. First, light output and coincidence resolving time of 2×3 arrays were measured with a conventional method. Second, in-crystal and inter-crystal events were separately analyzed and compared to the conventional method result. Crystal arrays with one surface lapped crystals showed 15% more light collection and 21ps better timing resolution of 259ps compared to 280ps in all polished crystals. For both types of crystal arrays, coincidence resolving time of intercrystal events were close to in-crystal events, which indicated that light spread due to inter-crystal Compton scatter did not affect timing resolution noticeably.