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GWU Status Report: 11-1-06

GWU Status Report: 11-1-06




Last time, I showed results that suggested 14 cm of lead works just as well as 21.66 cm (the original thickness) for the VT collimator design: (Reminder: having helium in GLOB instead of air reduces the background from all space by a factor of ~3)

Table 1: VT collimator
Error Elastic Photons Elastic Photons from coll. region
1) VT21.66 10.53 4.223% 0.061351% +/- 0.002304% 0.052265% +/- 0.002087%
2) VT14 10.53 4.223% 0.053337% +/- 0.002146% 0.042849% +/- 0.001820%

This new 14 cm thick design is now compared for different geometries changing the location or theta_max (theta_norm).

Lower Edge: changing theta_min


First, for moving the collimator upstream I wanted to make sure the electron profile on the bottom and sides was unchanged (the top will change if theta_max is reduced).

I did 4 trials to compare to the original (black line), which used theta_extreme_min=5.84:

Conclusion: theta_extreme_min=5.8 was used to best match the lower edge.


Upper Edge: changing theta_norm or theta_extreme_max

Next, there are two major groups of geometries to compare:
1) Keeping the extreme angle the same (11.84) to mirror the original VT design.
2) Using theta_max=10.25 to mirror what Yongguang did.

Thus, there are the following geometries for VT14 at both standard and 25 cm upstream locations

Note, the geometries have no wall, and have helium in GLOB and air in COH (collimator region).

Table 2: VT14 geometries
Standard Location Upstream (25 cm)
theta_ex 11.84 Error 4.223% 4.228%
Elastic Photons 0.0533% +/- 0.0021% 0.0543% +/- 0.0021%
" " from coll. region 0.0428% +/- 0.0018% 0.0475% +/- 0.0020%
theta_ex 11.84 Error - 4.224%
with lintel Elastic Photons - 0.0133% +/- 0.0011%
" " from coll. region - 0.0055% +/- 0.0006%
theta_norm 10.25 Error 4.266% 4.254%
Elastic Photons 0.0637% +/- 0.0024% 0.0585% +/- 0.0023%
" " from coll. region 0.0475% +/- 0.0020% 0.0482% +/- 0.0020%
theta_norm 10.25 Error - 4.255%
with lintel Elastic Photons - 0.0128% +/- 0.0011%
" " from coll. region - 0.0061% +/- 0.0007%

Conclusion: Geometry for theta_ex=11.84 at the upstream location with a lintel is best so far.


Next question:

Can we achieve the same results without moving the collimator?

Answer: At the standard location we have the following X profiles for theta_ex 11.84, and theta_norm 10.25:

Table 3: VT14 separation profiles
theta_ex 11.84 theta_norm 10.25

Compare this to the separation for VT14_25cm (theta_ex 11.84):

Conclusion: It may be possible to get the same reduction in background by optimizing the lintel position for the theta_norm=10.25 case at the standard location. The separation is much better than theta_ex=11.84.