TRIAC Progress Report - KEK

Although the gain shift of THGEM#3 was suppressed to be within 5 % under the
same 4 He injection rate, it became larger (about 8 %) in the case of the 12 C injection rate
of 10 5 pps as shown in the left panel of Fig. 2-43. Moreover, the gain shift of
THGEM#3 gradually increased in several consecutive measurements with the 4 He
injection of 10 4 pps. Actually, the shift was about 1 % at the first measurement, whereas
it changed to be 5 % at the third measurement. This phenomenon could be explained as
the ‘ageing’ of the electrodes, such as oxidation of the surface of the copper electrode.
Hence, we coated the electrode of THGEM#4 with gold. THGEM #4 showed good gain
stability as shown in the right panel of Fig. 2-43. The pulse height became stable within
3 % under the 12 C injection rate from 400 to 1.2 x 10 5 pps.
The energy resolution under the 12 C injection rate of 10 5 pps was also measured. It
was obtained from the distribution of energy-loss signals from the segmented anode
pads. The energy resolutions under the injection rate of 400 pps of the two THGEMs of
#3 and #4 were almost similar (7% in σ). However, with the higher injection rate, e.g.
10 5 pps, THGEM #4 gave better energy resolution than THGEM #3; 8 % vs. 13 %.
The observed gain stability and energy resolution of THGEM #4 under the
injection rate of 10 5 pps satisfy experimental requirements. The GEM-MSTPC using
THGEM #4 (400-µm thick, 300-µm hole-diameter, Cu electrode coated by Au without
rim) was successfully employed to measure the 8 Li(α, n) 11 B reaction cross sections
(RNB-K05).
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