Forward Drift Chambers

[Howard Fenker <hcf@jlab.org>]

Following the discussion that a few of us had on Monday
I've read through the report on the LASS Spectrometer with
an eye towards their cathode-strip chambers.

1. I did an estimate of the mass of the LASS "gap" chambers
seen by tracks. I then found the same number quoted in the
LASS writeup. We agree: 0.17% of a radiation length.

2. If I expand the LASS Gap Chambers to include three wire
planes as in the Hall-D plans the resulting mass is 0.35% Xo.

3. If this design is modified to make the chambers self-
supporting (i.e. no heavy frames at the edges) by including
HexCell support planes, the mass jumps to ~2.2% Xo. There is
likely no reason to do this for Hall-D -- see item 4. I just did
the calculation as an exercise.

4. In the LASS Gap Chambers the anode wires were run in on/off (PWC)
mode while cathode strips were readout and analyzed for pulse-height.
The report indicates that they got sigma_x = sigma_y = 185 um
with this design -- AFTER DISCARDING SOME UNSTATED FRACTION OF
EVENTS IN WHICH THE CHARGE DISTRIBUTIONS ON THE STRIPS WERE
PROBLEMATIC. The anode wire spacing was ~2mm, the strip pitch
was 8.1mm, and the anode-cathode spacing (half-gap) was 5.08mm.

The plan in the Hall-D Design Report is fundamentally different:
it specifies 1cm wire spacing with drift-mode readout (TDCs),
and 1cm wide cathode-strips with FADC readout (LASS used
conventional 12 bit ADCs). Resolution of 155um in both 'X' and
'Y' in each of the wire planes is indicated by the physics
simulations.

Geting 155um resolution out of an atmospheric-pressure drift plane
is demanding but attainable. Probably we want to incorporate
field wires between each pair of anode wires.

Cathode-strip charge interpolation is to provide the coordinate
along the length of each wire. Simulations reported in
NIM-A 367(1995) 285-289 (I have reprints)
indicate that the best ratio R=(strip-width / half-gap) is
about 0.9. The LASS Gap Chambers used ~1.6. It would appear that
LASS might have achieved 155um strip resolution had they chosen
a better R (see their Fig. 89, which agrees with this result).

The same NIM article suggests that analog measurements with 0.5%
accuracy are appropriate for precise charge interpolation. dE/dx
of at least x10 must be accomodated, as well. These two
considerations suggest that 11 bits of ADC are required, at least.
Are the flash ADCs capable of this? Channel-to-channel gain variations
must be below the 0.5% level, or calibrated out to this extent,
and preamps must be low-noise and well matched.

Based on the quoted strip width and wire spacing, and using the
optimum ratio R, we would have a drift cell some 18mm deep (along the
~particle direction) and 10mm wide. LASS included many spacers to
maintain anode-cathode separation, and we would need to do the same.
We should look around for examples of planar drift chambers with
similar aspect ratios and see how they perform.

Howard