Email from Jim:

From jimb@mts.net Thu Aug 26 10:45:17 2004
Date: Thu, 26 Aug 2004 09:38:29 -0500
From: Jim Birchall 
To: Juliette Mammei , Roger Carlini ,
     Klaus Grimm , Mark Pitt ,
     Neven Simicevic , Greg Smith ,
     Dave Mack , Norman Morgan ,
     Mike Finn , David Armstrong ,
     Yongguang Liang ,
     Allena Opper ,
     Shelley Page , Tony Forest 
Subject: Primary collimator

Looking at the definition of collimator 1, the inner and outer edges 
are parallel at 8 degrees to the beam axis and trace back to the beam 
axis at 19.5 cm upstream of the entrance window of the target and 4.5 
cm downstream of the exit window. So the downstream end of the 
collimator defines the solid angle - electrons inside the target can 
never see along the line of a collimator edge.

I calculate the solid angle seen by electrons scattered at 8 degrees 
from the centre of the target to be 2.2 msr, compared with about 3.7 
msr from Greg's spreadsheet, so 620 MHz per bar would go to 1 GHz using 
Greg's solid angle.

Mark and Juliette have verified that GEANT is working correctly and I 
have verified that the cross-section calculated by GEANT agrees with 
Greg's numbers (weight_n is sigma * sin(theta)), so we need to open up 
the collimator. We can gain figure of merit - both rate and Q^2 - by 
opening up at large angle. Perhaps we could also move the point at 
which the line of the upper edge meets the beam axis a little closer to 
the target entrance window.

Jim