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

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-CH4 (90:10) and Ar-CF4 (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 5×105 cps cm2 (counts per second), and 90% at the rate of 3×106 cps/cm2. 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 1.6×104. The horizontal position distortion due to ion backflow in the gate operation was suppressed within 0.2 mm at the beam rates up to 5×105cps/cm2, corresponding to the ion backflow up to 2.7×108s−1cm−1. On the other hand, the distortion with the gate open was measured to be 1.3 mm, corresponding to the backflow of 1.3×109s−1cm−1. The distortion in the final experiment is estimated to be 0.3±0.2mm, within the requirement of 1mm. 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.