Development of a prototype GEM TPC with a gating grid for an H-dibaryon search experiment at J-PARC

摘要

We have developed a prototype TPC (Time Projection Chamber) based on GEMs (Gas Electron Multipliers) with a gating grid for an H-dibaryon search experiment at J-PARC. To investigate its performance, we conducted laboratory tests and a beam test with two kinds of gases, Ar-CH_4 (90:10) and Ar-CF_4 (95:5), where the former is mainly used for the tests. The hit detection efficiency per pad row was measured to be 98% at the beam rate up to 5 × 10~5 cps cm~2 (counts per second), and 90% at the rate of 3 × 10~6 cps/cm~2. Based on the measured horizontal resolution of 0.19-0.46 mm at the drift length of 5-20 cm, the horizontal position resolution of better than 0.3 mm is estimated in 1 T magnetic field for the final experiment. The ion backflow fraction was measured to be 5% and reduced up to 3% by tuning the GEM voltage configuration, at the gain of 1.6 × 10~4. The horizontal position distortion due to ion backflow in the gate operation was suppressed within 0.2 mm at the beam rates up to 5 × 10~5 cps/cm~2, corresponding to the ion backflow up to 2.7 × 10~8 s~(-1) cm~(-1). On the other hand, the distortion with the gate open was measured to be 1.3 mm, corresponding to the backflow of 1.3 × 10~9 s~(-1) cm~(-1). The distortion in the final experiment is estimated to be 0.3 ± 0.2 mm, within the requirement of 1 mm. The GEMs have been operated continuously for a comparable period of the final experiment, with average energy deposit rates per unit GEM area of 30 times as high as that expected for the experiment.