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

Abstract

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 5×105cpscm2 (counts per second), and 90% at the rate of 3×106cps/cm2. Based on the measured horizontal resolution of 0.19–0.46mm at the drift length of 5–20cm, the horizontal position resolution of better than 0.3mm is estimated in 1T 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 0.2mm 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 1.3mm, 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.