Dual Gain Multi-layer Thick GEM with high-intensity heavy-ion beams in low-pressure hydrogen gas

We are developing a gaseous active target CAT-M based on a time projection chamber with using GEMs for the study of unstable nuclei using missing mass spectroscopy [1]. CATM employ a GEM called dual-gain multi-layer thick GEM (DG-M-THGEM) [2] which has two characteristic structures. First, electrodes of the GEM is separated to control the gain at each segment independently. The GEM is called as dual-gain thick GEM (DG-THGEM). The DG-THGEM can reduce the number of secondary electrons by controlling the gain of a region which measure beams to be low even if high intensity beams is injected [3]. An active target CAT-S have been operated successfully by using the DGTHGEMs in a physics experiment of a deuteron inelastic scattering with the high intensity Sn beam of several 10 Hz [4]. Second is a structure of alternating layers of electrodes and insulators which was developed by Cortesi as multi-layer thick GEM (M-THGEM) [5]. Because the total thickness of the M-THGEM become thick, the structure is expected to result in a minimal amount of the bending even by mounting without tension and support. In this paper, performance of the prototype DG-M-THGEM with an active area of 10 × 10 cm was investigated with a high intensity heavy ion beam up to 10 Hz. Effect of space charges due to ion backflow from the GEM with the high intensity beam was also considered. The performance evaluation of the prototype DG-MTHGEM with high intensity heavy-ion beam was performed by employing the active target CAT-S filled with hydrogen gas at the pressure of 40 kPa. A Xe beam with the energy of 185 MeV/u was impinged into CAT-S at a synchrotron accelerator facility HIMAC of National Institutes for Quantum and Radiological Science and Technology. The effective gas gain for the beam was measured with the low-intensity beam of 5 k particle per pulse. The potential differences among the electrodes in the beam region were changed while those in the recoil region were fixed at the effective gain of about 2000. Figure 1 shows the result of the effective gas gain as a function of the reduced bias in the GEM holes. The effective gas gain Geff is defined as ratio of the amplified charges to the initial charge,