TRIAC Progress Report - KEK

CO2 (10 %) and 16 kPa, respectively, for the measurement of the 8 Li(α, n) 11 B reaction
cross sections. The gas gain on the THGEM was measured under the condition of the
operating gas. The measured gas gain was high enough (~10 3 ) in the single THGEM
configuration [2-26, -27].
Fig. 2-42. (Left) The photograph of the GEM-MSTPC. The drift space has two electrodes and is
enclosed by field-guiding wires (100 x 100 x 100 mm 3 ); ‘Top’ is a plate electrode and ‘Bottom’ is an
electrode which supports insulating pillars. ‘Pad’ indicates an anode plate, segmented for having
position sensitivity. (Right) The (vertical) configuration of the electrodes in the GEM-MSTPC is
shown, where the beam comes into the middle of the drift space (across the position of 50mm among
the vertical length of the 100 mm.)
When the GEM is incorporated in a 3-dimensional tracking detector, it is known
that the vertical position of the particle track is distorted by ion feedback [2-27]; the
electron drift time is disturbed due to the electric field induced by numerous ions
drifting away from the proportional region in gas amplification around the GEM into
the drift space. In order to suppress the ion feedback from the GEM foil to the drift
space, the double GEM configuration was finally adopted as shown in the right panel of
Fig. 2-42. Such a configuration helps reduce the operational gas-gain on the one GEM,
subsequently reducing the number of ions that back to the drift space.
We measured the time dependence of pulse height of anode output by injecting 4 He
from a radioactive source of 241 Am or the 12 C beam of 13.2 MeV provided by the
TRIAC. We observed shifts in the pulse-height with time, as reported in the previously
developed THGEM [2-25]. This phenomenon may come from the gas-gain instability
due to the electrical charge-up on the insulator between the GEM electrodes. It strongly
depends on the structure of holes punching through the electrodes. In order to avoid the
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