A new positron-gamma discriminating phoswich detector based on wavelength discrimination (WLD)

Abstract

The intra-operative positron probe, used in the field of nuclear medicine, has been developed to localize positron (beta+) emitted from positron emission tomography (PET) radiopharmaceuticals by selectively detecting beta+ particles in the presence of background gamma (gamma) radiation. In this research, we propose a new method for real-time discrimination of radiation types in a mixed radiation field of positron (beta+) and gamma (gamma) radiation, based on simple electronics and signal processing. Scintillators often have distinct emission wavelengths, and this method exploits the scintillator's emission wavelength, as opposed to conventional pulse shape discrimination (PSD) approach. In this study, we used a two-layer phoswich detector consisting of europium-doped calcium fluoride (CaF2(Eu)) and Ce3+ doped Lu3Al5O12 (Ce:LuAG), with dimensions of 3 mm x 3 mm x 0.5 mm and 3 mm x 3 mm x 10 mm, and emission wavelengths of 380-450 nm and 480-700 nm respectively. This phoswich detector was placed at the center of two adjacent optical filters (one short pass and one long pass filler), and each coupled to a silicon photomultiplier (SiPM). The optical filters are used for directing a specific scintillator photon wavelength to its corresponding SiPM for identification of the crystal of interaction. This proposed method allows us to realize a high sensitivity (true positive rate) positron probe, that can also double as a gamma-probe, while simplifying readout electronic and signal processing. A Na-22 positron(beta+) source and Cs-137 gamma-source were used to validate the proposed approach experimentally. Results indicate the possibility of discriminating beta+ and gamma-radiations in real time, with an excellent gamma-rejection ratio exceeding 99.9 %.