Development of gas electron multiplier foils with a laser etching technique

Fine-pitch gas electron multiplier (GEM) foils have been produced for cosmic X-ray polarimeters using a carbon dioxide laser etching technique. The finest hole pitch of the foil which can be produced repeatedly is 50 μ m and the smallest hole diameter is 30 μ m . The electron amplification factor was measured as a function of applied voltage. The behavior of the factor is almost the same as the 140 μ m -pitch standard foil fabricated by CERN. Our GEMs had no rate-dependent gain instability, which is expected of the GEMs having holes of good cylindrical geometry. The amplification factor of the 50 μ m foil in a mixture of 70% argon and 30% carbon dioxide reaches about 5000 without any micro-discharge at a voltage of 570 V between foil electrodes.