- To reduce the photon background generated from the shower leakage of the primary collimator, we move the VPI "final" primary collimator 25 cm upstream (made of 5.25 cm thick Tungsten) without changing its acceptance, and add a 20 radiation length (11.2 cm thick) non-defined lead shielding behind the primary collimator. From the x-y projection plots of the photons and e-p elastic electrons at different Z positions inside the main torus region (-200 cm < Z < 160 cm), we found, at Z = -80 cm, it had the best photon/electron separation for the current setting. To obtain a better photon/electron separation, we try to trim the theta_max 0, 0.25, 0.5 degree respectively. Below are the simulation results of these three cases. It shows that adequate e-/photon separation is obtained by triming the theta_max 0.5 degree.
x-y plot of photons (red) and electrons (blue) at Z = -80 cm, for theta_max = 10.53, 10.28, 10.03 degree.
Theta_max = 10.53 degree Theta_max = 10.28 degree Theta_max = 10.03 degree
Based on the simulation, a lintel of 16 cm (X) by 56 cm (Y) by 4 cm (Z), made of Lead, is placed at Z = -82 cm and X = 127.5 cm (center), to block the photon background while let the elastic electrons passing through. Several simulations were done to study the photon and electron background from Moller scattering and inelastic reactions, with/without the lintel. The theta_max of these simulations is 10.03 degree (trimming 0.5 degree comparing to the VT "final" primary collimator design). In all simulations, the minitorus is turn off. The results are listed in the following table and figures. It show that significant secondary electron background is generated at the top concrete shielding of the collimator stands. We may need to used Lead, instead of concrete here.
Table 1. Background from different inelastic reactions and Moller scattering.
Reaction Photon Background (deposited energy > 5 MeV) Electron Background (deposited energy > 0 MeV) Lintel ep elastic 0.021% Yes Moller scattering 0.06% 1.29% Yes Moller scattering 0.32% 1.18% No e + p -> e + n + (pi+) 0.0006% 0.008% Yes e + p -> e + p + (pi+) + (pi-) 0 0 Yes e + p -> e + p + (pi0) 0 Yes
1. Photon background from ep elastic reaction with lintel (the blue lines show the edge of the primary collimator opening).
Photon Background 
2. Moller scattering with lintel
Photon Background Electron Background 
3. Moller scattering WITHOUT lintel
Photon Background Electron Background 
4. Inelastic reaction (e + p -> e + n + pi+) with lintel
Photon Background Electron Background 
Two side views of the new spectrometer (5.25 cm thick tungsten primary collimator, 11.2 cm thick non-defined lead shielding, lintel), one with the tracks of electron (red) and photon (blue).
