Optimizing light collection from thin scintillators used in a beta-ray camera for surgical use

The authors are developing a 1-2 cm² area camera for imaging the distribution of beta-emitting radiopharmaceuticals at the surface of tissue exposed during surgery. The front end consists of a very thin continuous or segmented scintillator sensitive to betas (positrons or electrons) of a few hundred keV, yet insensitive to gamma rays. The light from the scintillator is piped through clear fibers to the photon detector (PD). This approach requires that a sufficient number of scintillation photons be transported from the scintillator, through the fibers to the PD. The scintillator, reflector, surface treatments, geometry, fiber light guides, and optical couplings must be optimized. The authors report here on efforts made to optimize the light collection from <3 mm thick plastic and CaF₂(Eu) scintillators into clear fibers using experimental measurements and computer simulations. The authors measured that with a 1.25 cm diameter, 0.5 mm thick optimized CaF₂(Eu) disk coupled to a 5 cm long bundle of clear optical fibers, on average, ~250 photoelectrons are produced at a PMT photocathode for a ²⁰⁴Tl beta flood source (E_max=763 keV). This corresponds to a sufficient number of photoelectrons for <1 mm resolution imaging capabilities for the proposed camera