GEM-MSTPC: An active-target type detector in low-pressure He/CO2 mixed gas

We developed an active-target type gas counter operating with low pressure He/CO2 (10%) detector gas for application in studying low-energy nuclear reactions using radioactive nuclear beams (RNBs). A 400-mu m-thick gas electron multiplier (THGEM) was used as the proportional counter for high injection rate capability. We examined the gas gain stability and the influence of ion feedback on particle tracks at high beam injection rates of up to 105 particles per second (pps) using a low-energy C-12 beam. By means of modifications to the THGEM, the shift in the relative pulse height of the output signal normalized to that measured at the C-12 injection rate of several hundred pps improved to within 2% at the rate of 10(5) pps. The position distortion induced by ion feedback was suppressed to within 1.5 mm because of the relatively high drift field value of 1 kV/cm.atm and the use of the double GEM configuration. The THGEM was found to be applicable for our active target at high injection rates of up to 10(5) pps.