Abstract
A large area multi-crystal positron camera has been constructed for diagnostic investigations with cyclotron-produced short-lived positron emitters. The camera has two identical opposing detectors each consisting of a square array of 14×14 small NaI (Tl) crystals (20 mmφ×38 mm) viewed by an array of 112 photomultiplier tubes (38 mmφ) . The detectors have a useful area of 39×39 cm2. The electronic system is simplified by grouping the photomultiplier timing signals into each row and each column in the tube array. The fast coincidence operation between the detectors is performed with the grouped signals in row-row and column-column mode, and finally each crystal is in coincidence with 25 crystals in the opposite detector to form 4096 coincidence crystal pairs in total.
The system processes the coincidence data which are stored in one of two 4K word-18 bit IC memories, and offers a 64×64 digital image focused on a plane between the detectors on a CRT monitor during and after the data acquisition. The camera has the spatial resolution of 8 mm (FWHM) at the midplane and the uniformity of the sensitive field is ±7%. The variation of sensitivity in the direction perpendicular to the detector plane is less than 1%. The efficiency of the camera is about 18 cps/μCi for a 15×15 cm2 plane source in air with a detector separation of 50 cm. The data rate of the system reaches about 120 kcps at the maximum, but image distortion is observed at the coincidence rate more than about 30 kcps. An estimation indicates that only a few percent increase in accidental coincidence rate is resulted from the coincidence operation in the row-row and column-column mode.
