Evaluation of moderately cooled pure NaI as a scintillator for position-sensitive PET detectors

A new evaluation of pure NaI has been performed to determine if moderate cooling to ≈-90°C would lead to better performance than that of existing, activated NaI(Tl) position-sensitive detectors used in PET and elsewhere, particularly at high countrates. Using a freezer, which can cool the crystal assembly to -90°C, the authors have measured the light output and energy resolution of pure NaI and compared it to NaI(Tl). At -90°C, pure NaI has a decay constant of 35 nsec, a light output which is about half that of room temperature NaI(Tl) (with pulse-clipping), and an energy resolution of 15%, compared to 10% for NaI(Tl) (again with pulse-clipping), Since the long decay of NaI(Tl) causes it to suffer more significantly than pure NaI from pre-pulse pileup, the difference in energy resolution between the two crystals at high countrates will be reduced. Also, a significantly shorter trigger deadtime with pure NaI will lead to a reduction in coincidence deadtime losses in PET. Computer simulations of large crystals operating at high countrates have been performed to quantify their trigger deadtime behavior and position resolution as a function of light output and pulse decay time. Having gained experience with the practical issues of cooling large crystals, measurements of position resolution have been performed with a NaI bar detector of similar geometry to the NaI(Tl) detectors in use in the PENN-PET scanner