From jmichaelfinn@cox.net Thu Sep  9 07:30:33 2004
Date: Tue, 07 Sep 2004 23:22:41 -0400
From: John M. Finn 
Reply-To: finn@physics.wm.edu
To: carlini@jlab.org, 'Neven Simicevic' ,
     'Jim Birchall' ,
     'Dave Mack' , 'Juliette Mammei' ,
     'Klaus Grimm' , armd@jlab.org, 'Mark Pitt' ,
     'Allena Opper' , 'Greg Smith' ,
     'Norman Morgan' , 'Mike Finn' 
Cc: 'Yongguang Liang' ,
     'Shelley Page' ,
     'Tony Forest' 
Subject: RE: Qweak working group - next meeting

Dear Roger et al.

Overlapping bars do not work for several reasons.

1) There has to be support structure to support the shielding walls and
detector packages. The beam can't be allowed to interact with this support
structure.

2) The detector readouts (PMTs and wire chamber frames) must be shielded
from direct view of the beam.

3) It is highly desirable that the measurements in each sector be truly
independent.

4) The Region 3 chambers become impossibly large, as does the angles of
incidence (we are already close to +/-30 deg exit range for a =/- 11 deg
entrance range). We can't build them and they won't work at the extreme
angles (this is also true for HDC's, which also have a limited field of view
in any practical design). The practical limits of both the Cerenkov detector
design of the Los Alamos Group which was predicated on good shielding and no
overlap in regions, and our own Region 3 needs are nearly identical and lead
to bars of about 2m in length. I don't doubt that a few more degrees in
might be available in phi, but that is all.

Regards,
Mike

-----Original Message-----
From: Roger D. Carlini [mailto:carlini@jlab.org] 
Sent: Tuesday, September 07, 2004 4:36 PM
To: Neven Simicevic; Jim Birchall; Dave Mack; Juliette Mammei; Klaus Grimm;
armd@jlab.org; Mark Pitt; Allena Opper; Greg Smith; Norman Morgan; Mike Finn
Cc: Yongguang Liang; Shelley Page; Tony Forest
Subject: Re: Qweak working group - next meeting

Hi Folks:

If you recall the figure I sent around before the last meeting showing long
(end overlapping) bars that allow for full 
phi coverage. Such an arangement would work nicely with Jim's options
12-8-22, 12-8-23 and 12-8-24. There would still be 
enough space for supports between the inner and outer concrete shielding.

Another possible modification would be to use 2 bars/octant (which would
probably be required anyway due to the total 
lengths), but rather than glue them together - mirror and overlap the ends
in the center of each octant. This would 
result in a 16 fold symmetric detector. The 2 signals could be added
together electronically if we wish. A small 
correction to the statistical error would need to be made to account for the
overlap.

The bars could then be tilted slightly reducing their required width. This
arrangement might allow the use of "narrow 
bars" readout from the end and no glue joints. The hours would now be ~1500
(per Jim's table) for a 4% measurement 
assuming the "B" term is under control.

The "slick" feature of this configuration is that we can pretty much select
any "phi1-phi2-phiphi" (ie. average Q**2) 
selection from Jim's table by constructing addition Cu-Pb (4" to 6") single
collimators at ~1.4m from TGT. As in Jim's 
model the front disk would become (like the clean up collimator) just a
water jet cut non-acceptance defining "shield" 
for the GEMS.

This all ignores issues about the chamber dimensions, but our existing
chamber designs should work for Jim's nominal 
"phi1-phi2-phiphi" configurations. We migh be able to get coverage for the
larger acceptances by taking two measurements 
per octant instead on one.

Best Regards

Roger