Abstract
A two-dimensional positron annihilation angular correlation apparatus based on 256 bismuth germanate scintillation detectors for solid state research has been developed. The apparatus is distinguished by the use of the bismuth germanate crystals as a scintillator and two position detectors assembled of individual scintillation detectors.
In many respects, bismuth germanate crystal (Bi4Ge3O12: BGO) is one of the best materials available for compact individual detectors of positron annihilation gamma rays (511keV) . Though BGO crystals as detector materials have some disadvantages, expensive, small light yield (21% as much as NaI), small crystal grown, they possess excellent properties, high stopping power (density: 7.13g/cm3), high photopeak to Compton ratio (effective atomic number: 74), nonhygroscopic, lack of cleavage. Owing to those attractive properties, we selected BGO crystal as scintillation material of the individual detectors.
Each position detector developed is one type of large multicounters, and consists of the assembly of 128 BGO individual detectors and a crossed-slits collimator. The multicounter as a position detector has manifested distinguished properties as compared with the others (anger camera, multiwire-proportional chamber) . Especially, resolving time is very short (17 ns) because that each individual detector is independently operated, and position resolution is definitely decided by the use of the crossed-slits collimator.
Each BGO individual detector is constructed of a BGO crystal with the size of 10×10×20 mm3 and a photomultiplier tube with the diameter of a half inch (Hamamatsu R 647-01) . The crossed-slits collimator consists of double layers of two sets of long-slits, one is a set of 12 vertical long-slits and another 12 horizontal long-slits. Both sets of the long-slits have the open width of 6 mm, the length of 182 mm and the depth of 100 mm. The following is the specifications of the apparatus; the angular resolution was 0.75 mrad by 0.75 mrad over the angular field of 22.57×22.57 mrad2, the active-area to the dead-area ratio of the position detector was 0. 19, and the efficiency for counting coincidence events was about 50 cps/GBq. In order to examine the performance of our apparatus, we applied the apparatus to measurements of two dimensional positron-electron momentum space density distributions integrated in fixed directions (‹100›, ‹110›) of niobium and we could recognize many hollows in the momentum space density distributions corresponding to the holes at the Γ point and twelve N points in the Brillouin zone.
