A crystal for a scintillator is a single crystal of Bi.sub.4 Ge.sub.3 O.sub.12 containing up to 0.5% by weight of Bi.sub.4 Si.sub.3 O.sub.12 and/or up to 50 ppm of Gd. The light output of this crystal as a scintillator can be increased by about 20% as compared with the light output of the conventional crystal of additive-free Bi.sub.4 Ge.sub.3 O.sub.12.
A recording and readout apparatus for X-rays has a storage layer whose charge distribution which is generated and stored by means of x-radiation during the recording is read out light-optically in point-by-point fashion. An image data storage of the readout may be provided along with a display apparatus for image observation.
A scintillation detector for three-dimensionally measuring the absorption position of the gamma-rays entering the detector and a positron computed tomography apparatus employing as a gamma-rays detector plural scintillation detectors arranged in the form of a ring are provided which can prevent the degradation of the spatial resolution in the peripheral region of the visual field and uniformly detect a positron distribution over the entire visual field with high resolution. The scintillation detector comprises a bundle of pillar-shaped scintillator elements and position detecting photodetectors coupled to the end surfaces of each scintillator element, and the apparatus comprises a plurality of scintillation detectors formed in a ring, each of which includes a bundle of scintillator elements, and plural position detecting photodetectors each of which is connected to both ends of the scintillator bundle.
