The optimal crystal geometry for small-field-of-view gamma cameras: arrays or disks?

This work presents performance characteristics of crystal disks and needles such as energy spectrum, energy resolution and number of photoelectrons. Basic differences in the presented geometries are the discrete (arrays) or the continuous (disks) incrementation of the scintillation. Most of the low-energy animal cameras use crystal needles arranged in arrays to guarantee both linearity and optimal spatial resolution. Further reduction of the needle size to improve spatial resolution leads to internal reflection losses and detection efficiency. On the other hand crystal disks offer a high light output, but show nonlinear effects close to the edges. The authors compared different kind of geometries of CsI(Na), CsI(Tl) and NaI(Tl) disks with CsI(Na) and CsI(Tl) needles of different geometries and coatings. To determine the energy resolution as well as the number of photoelectrons the authors used a single electron PMT (RCA8850) and a 140 keV gamma source. Crystal disks always generate a greater number of photoelectrons and had a better energy resolution (<10%) compared to optimized needle geometries, which were worse than 16%. Calculations based on the: presented data help to recognize the limitations of spatial resolution for low energy camera designs.