We were asked to look at the inelastics and mollers for a few of these, so a decision could possibly be made. Below are the backgrounds for 20 and 25 radiation lengths of lead, and 20 radiation lengths of the bronze alloy.
At the meeting I showed the elastic photon background versus collimator thickness for different materials:
We were asked to look at the inelastics and mollers for a few of these, so a decision could possibly be made. Below are the backgrounds for 20 and 25 radiation lengths of lead, and 20 radiation lengths of the bronze alloy.
Table 1: Backgrounds for Pb and CuPb collimators
| FOM | Elastic Photons | Elastic Photons from coll. region | Inelastic Photons | Inelastic Electrons | Moller Photons | Moller Electrons | |
| 20 Xo Pb | 4.221% | 0.0611+/-0.0022 | 0.0532+/-0.0019 | 0.0018+/-0.0001 | 0.0141+/-0.0006 | 0.6171+/-0.0245 | 0.5788+/-0.0402 |
| 25 Xo Pb | 4.226% | 0.0533+/-0.0021 | 0.0429+/-0.0018 | 0.0014+/-0.0001 | 0.0125+/-0.0005 | 0.5376+/-0.0226 | 0.4418+/-0.0325 |
| 20 Xo CuPb | 4.231% | 0.0733+/-0.0027 | 0.0639+/-0.0025 | 0.0021+/-0.0001 | 0.0122+/-0.0005 | 0.7631+/-0.0288 | 0.5027+/-0.0360 |
Conclusions: If a Pb collimator can be machined, we still recommend a 25 Xo Pb collimator (14 cm thick). If we have to use CuPb, the 20 radiation length (21.65 cm) will be ok.
Yongguang showed that there is direct line of sight from the lower edge of the primary collimator to the Cerenkov detector. I was finally able to figure out how to reproduce these images. However, I think the images Yongguang showed underestimated the line of sight, because he was viewing along the angle of the electrons, not the photons. See the image below.
So, instead of using the angle 20.57 degrees, we need to look upstream along ~17 degrees (found by geometry).
| 20.57 degrees: angle of electrons | 17.6 degrees: angle of photons |
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I looked at how far the primary collimator would need to be moved upstream to eliminate the line of sight: 100 cm if you look upstream along 17.6 degrees.
Conclusions: Moving the collimator 100 cm upstream puts it in the middle of the minitorus. The line of sight from the primary collimator to the detector can not be eliminated by moving the collimator far enough upstream.
We added 10 mils of aluminum at the downstream face of the collimators and the upstream face of the shielding wall to determine if these more realistic conditions would negate the benefit of using helium.
- VT air everywhere
- VT hel in GLOB, air in COH
- VT hel in GLOB, air in COH, with aluminum windows
Table 2: Air versus helium
| Elastic Photons | Elastic Photons from coll. region | Inelastic Photons | Inelastic Electrons | Moller Photons | Moller Electrons | |
| orig | 0.2032+/-0.0051 | 0.0626+/-0.0024 | 0.0027+/-0.0001 | 0.0145+/-0.0006 | 0.6641+/-0.0429 | 1.3686+/-0.1511 |
| hel | 0.0614+/-0.0023 | 0.0534+/-0.0021 | 0.0016+/-0.0001 | 0.0132+/-0.0006 | 0.7013+/-0.0385 | 0.5148+/-0.0502 |
| hel w/ al | 0.0823+/-0.0029 | 0.0464+/-0.0020 | 0.0022+/-0.0001 | 0.0130+/-0.0006 | 0.6531+/-0.0420 | 0.8929+/-0.0897 |
Conclusions: Using helium would still be nice, there are still reductions in the elastic photons and, more importantly, the moller electrons (reduced by about 1/3). Although, helium was still in the global volume (so not restricted to just the area between the aluminum windows), so further simulations may be necessary.
We were told to put sides on the collimator hut, and also try extending the first collimator out to 120 cm to see if that helped reduce the moller electrons that were being created in the top of the shielding hut. Currently, I have only been running theta from 5-14 degrees and +/- 15.5 degrees in phi. So making these changes had no impact because they were outside the region we have been sampling.