Development of a 2-dim. position-sensitive detector for high-energy x rays from SR

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Being based on the drift chamber principle, we have made basic and applied studies on the development of a 2-dim. position sensitive detector for high-energy x rays from the synchrotron radiation (SR). The specifications of the counter we have fabricated in this research are as follows: 1) A large sensitive volume of 110 x 100 x 24 mm ィイD13ィエD1 which is divided into two parts of 55 x 100 x 24 mmィイD13ィエD1. Each one has a maximum drift length of 55 mm, a sensitive area of 55 x 100 mm ィイD12ィエD1, and a sensitive depth of 24 mm. Total number of wires is 228. 2) The 24-mm depth is divided in two 12-mm thick layers in the one part and in four 6-mm thick layers in the other part, so that six separate detectors can be operated. This feature ensures high counting efficiency and high counting tolerance. The performance of the counter has been examined by using 6-keV x rays and 8-MeV protons. From the various studies on the energy response, we have confirmed that the loss of the primary electrons during the drift process can be ignored and as a result a fairly well potential distribution is realized in the drift regions. The observation of fast timing pulses with a digital oscilloscope allowed us to decide the use of 10-μm thick anode wires. The use of such a thin wire inevitably gives rise to unfamiliar electron avalanche mode peculiar in the region of the limited proportionality. In this connection, we have investigated this avalanche mode in the high-precision measurement of Tb Lx rays and have proved that the mode is quite feasible. From these studies above, we conclude that the position resolution less than 0.2 mm can be obtained for 20-40 keV x rays, and the realization of this new type detector based on the drift method is highly promising.