In the proposal of JLC/CDC, a requirement for the two-track separation
is better than 2mm over the wire length of 4.6m in the magnetic field
of 2T.
To achieve this requirement, a mini jet-cell type drift chamber with
a slow gas mixture was designed as a 0-th order approximation.
In order to verify the two-track separation capability, we should
test the expected perfirmance experimentally with the test chamber.
The drift chamber simulation program GARFIELD would give us some expectations.
The ouput signal from the sensing wires can be simulated and track analysis
program would simulate the real performance.
However, these calculation would contain lots of assumptions or input
parameters on the gas mixtures and signal handling processes.
Therefore, we think that the simulated expectations should be compared
with the experimental results.
Through these studies, our understanding of performances of the practical
detector would be obtained.
The two-track separation capability would be determined mainly by following
factors:
a) the cell structure
b) gas characteristics
c) signal wave-form deformations due to the wire length
d) readout electronics for wave form digitization.
Even with the small-sized test chamber, these items can be tested and
compared with the simulation expectations.
2.Test Chamber
The main purpose of this test is to prove that the present cell structure
and the gas mixture are suitable enough to obtain the two-track separation
of 2mm.
The length of the wire would be 30cm and the number of super-layers would
be two, which are shifted by a 1/2 unit cell.The cell structure is shown
in Fig.1.
This two-super-layer structure may be useful for the test of allover
performance of the chamber.In vicinity of the sense wire plane, the
two-track separation efficiency would be decreased, and this would be
experimentally checked.
The total number of wires are;
Shielding wires --- 123(41*3)
Potential wires --- 317
Sense wires --- 25
-----------------------
total --- 465
The mechanical structure is shown in Fig.2.
Because of difficulteies to prevent the gas leak through feedthroughs
gas volume.The gas-tight connectors will be used for the HV supply
and for the readout signals.Preamplifier cads will be placed outside
of the gas volume.
3.Electronics and Data Acquisition
We will readout 15 sense wires. The pulse shaping would be very important
to get good two-track separation.Therfore, the best preamplifiers and
post amplifiers should be used and careful pulse shaping should be made.
The output signals will be read by the FADC with a 2ns sampling speed.
The data will be collected by the CAMAC system.The CAMAC-PC system or
the CAMAC-UNIX WS system should be prepared.
4.Two-track Separation Test
A schematic setup for the two-track separation test is shown in
Fig.3.The bremsstrahlung beam from the electron synchrotron
will be used.It may be better to use the tagged-photon beam.
The electron-positron pair will be produced at thin aluminum converter.
The two-tracks will be separated by the magnetic field.The distance
between two-tracks will be adjusted by the strength of the magnetic field.
The two-track trigger will be made by use of scintillation counters.
Coincidences of two finger counters with the scintillator width of 1mm
would be good enough.
In order to get various kinds of information such as those for inclined
tracks,a rotating table for the chamber would be necessary.Also, a
linearly-movable table with the accuracy of 100um would be necessary.