The current GMu program includes the hardware and detector components as shown
in Fig. 21. The proposed detector granularity is used, but the response
function for parts of the detector are simplified,
in particular the distribution of the induced charges on the TPC cathode and
the ePC cathodes has not been implemented. (Eventually this also includes a model
of electronic center of gravity method used.) In order to demonstrate that these
deficiencies have very little impact at the present design stage, we first assumed
a hit localization at the central cathode strip. Then we reduced the granularity
of all tracking chambers (anode, cathode pitch) by a factor of 2. The observed
resolution remained nearly unchanged (see left panel of Fig. 22).
Thus the vertex resolution of the electron tracking system is dominated by
multiple scattering, consistent with the expected projected scattering angle
on 2 mm Al, which varies from 0.17 to 0.04 rad for electron momenta 10 to 50
MeV. The effect of
various other scattering objects is indicated
in the right panel of Fig. 22. The total efficiency of electron
detection in a triple coincidence
is
75 % of 4
. 72% of the accepted electron trajectories are not
obstructed by either chamber frames or magnet. Of the 28% accepted electrons
which pass through either frames or magnet, 25% hit the frame only,
35% hit the magnet only and 40% go through both frame and magnet
(the magnet is positioned such that it is shadowed by the frames).
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