Development of a phoswich detector for a continuous blood-sampling system

We have developed a phoswich detector composed of a plastic scintillator and a BGO scintillator for a continuous blood-sampling system. The beta particle (positron) from the tube is detected by the plastic scintillator and emits 511-keV gamma photons. The BGO scintillator that is optically coupled to the plastic scintillator detects one of the gamma photons. Since the decay time of the plastic scintillator and the BGO are very different, it is possible to discriminate true (beta + gamma) events from background gamma events. First, the pulse height and pulse shape of plastic scintillator for beta particles and BGO scintillator for 511-keV gamma photons were measured and compared to estimate the possibility of the proposed method. Second, the proposed phoswich detector was fabricated and tested. Absolute sensitivity for Ga-68 (maximum energy of 1.90 MeV) and F-18 (maximum energy of 633 keV) positrons was measured and compared with conventional beta detector of similar size. The absolute sensitivity of the developed detector was 0.15 counts/Bq for Ga-68 positrons at the center of the detector. This was approximately five times higher in sensitivity than the conventional beta detector. The absolute sensitivity of the developed detector for F-18 positrons was 0.017 counts/Bq. The count rate of the developed detector was linear up to 10 kcps. The background count rate was small. These results indicate that the developed detector is useful not only for higher energy positrons such as O-15 but also for lower energy positrons such as F-18 or C-11.