W&M  report 07-01-2004

On Tuesday June 29 we had a special Region 3 meeting at JLab
(Allena Opper, Yongguang Liang , Roger Carlini, Dave Mack, David Armstrong, Mike Finn and Klaus Grimm)
Main purpose was the location and impact of region 3 on the wall thickness. According to the drawings
shown in my last report, the location of region 3 drift chambers is only possible with a reduced
shielding wall thickness of 70cm instead of 100cm.

Several items were discussed, here is my personal and obviously incomplete summary of this meeting =8-)

1) Can you change the tilting angle of 45 deg in respect to the nominal electron track ?
    
Tiling the VDC to higher angles (extreme: perp. to ray) gives you more space but  less wires or drift cells will be hit.
At 45 deg ~4 wires or drift cell  will be hit in the mean. Less than 3 wire hits is not advisable since the track reconstruction
is questionable. In order to get a good track reconstruction we would rather tilt the DC more towards to the nominal track ...
Changing to higher tilt angles can slightly be compensated by changing the drift cell size (smaller wire spacing, larger cell height).

However I have to insert the region 3  drift chambers into the Qweak Geant Code and figure out how many drift cell will be hit/respond
for different VDC orientations and setups.

2) Can you change the wire stringing angle ?

There are two related ideas behind it:  get a better position resolution per (double) plane, therefore we can reduce the spacing between
the two VDCs and still maintain an angular resolution of 1-2 mrad.
Remenber: the angular resolution per plane is ~1-2deg. An incident 45 deg in respect to the VDC plane will translate into a 57 deg
track seen by a plane perp. to the uv-wire plane (standard setup for Garfield)
Since we need a good angular resolution for the Q2 determination, the azimuthal track angle resolution is more important than
the polar track angle. By changing the stringing angle the effective track angle seen by the drift cell will change as well. Imaging we will string  the wires
with 0 deg and 90 deg  in respect to the frame (typical x,y wiring). An incident 45 deg  track in respect to the VDC plane  will translate  into 45 deg
in the xy-plane. The nominal trackangle is smaller compared to 57 deg with an uv-plane, the track will hit more drift cells, the track reconstruction will improve
with more cell beeing hit.
Unfortunately we can not string our VDC with this wire angle orientation: while the y-wire is ~50cm long, the x-wire is ~210cm long and we will
suffer from wire sagging, esp. with thin 25um wires with a limited wire tension.
The sagging is proportional to L^2. The x-wire sagging will be around 160um = order of what we want to resolve ...

3) Can we shift the shielding wall a bit upstream and gain more space ?

According to Roger there should be ~50cm space between the end of the main magnet and the start of the shielding wall, but this question can only be answered
by Stan Sobczynski (MIT). Mark Pitt and I asked Stan for his Solidworks drawings weeks ago but we are still  waiting for a response from Stan ...
Allena reminded us that shifting the shielding wall more upstream will lead to larger octant window openings due to the magnet (de)focusing
leading to a reduced shielding.

4) Is a shielding thickness of 70cm sufficient enough ?

According to Yongguang Liang 's Geant simulation the difference between a shielding wall thickness of 50cm and 100cm
was marginal. However we still have the possibility to use more dense material (e.g. lead sheets/bricks) at the exit of the shielding
wall  to improve the shielding.


A) Orientation of the VDCs

Since the VDCs can be tilted +-45 deg to the nominal central ray, I was asked about both
tilting directions  and their impacts. As you can see, only the tilting shown in the first pic is useful,
tilting into the other direction will take away even more space.
   top pic:  intended tilting direction
   mid pic: VDC allowed tilting direction, but space consuming and leads to collisions with the detectors
   last pic:  overlay of top and mid pic