- As shown in last status report (09/29/2005), significant photon background that deposited energy > 5 MeV at Quartz bar is generated from the shower leakage of the primary collimator. We tried to reduce this background by modify the Qweak spectrometer from a "one-bounce system" to a "two-bounce system". As the first step, we move the VPI "final" primary collimator 25 cm upstream (thickness=5.25 cm, instead of 21.66 cm) without changing its acceptance and run several GEANT simulations. From the x-y projection plots of the photons and e-p elastic electrons at different Z positions in the main torus region (-200 cm < Z < 160 cm), we can find a Z position at whcih it has the adequate photon/electron separation. A lintel, which attachs to the downstream cleanup collimator, will be placed at that Z position to block the photon background while let the elastic electrons passing through. Based on our simulation, the photon/electron separation at Z=-80 cm is the largest. Below are figures showing the simulation results (on the right) and a comparison to the results based on the VPI "final" primary collimator design (on the left).
In order to achieve a better photon/electron separation, we will try to increase the field of main torus by 5%, reduce the theta_max by 0.5 degree (?), and add a lead shielding behind the primary collimator to block some shower leakage form the primary collimator.
1. Side view of the spectrometer.
2. View of the spectrometer along the central ray from the Quartz bar.
3. x-y plot of photons and electrons at Z=-80 cm.
VT primary collimator, thickness=21.66 cm, B field=1.00 Primary collimator 25 cm upstream, thickness=5.25 cm, B field=1.00 Primary collimator 25 cm upstream, thickness=5.25 cm, B field=1.05