1. Beam Halo Considerations for Back Angle Running
There are two ways that significant beam halo could potentially be problematic
for the experiment.
Interaction of beam halo with the thick parts of the G0 target flange:
The < 1 x 10-6 outside of a 3 mm radius comes from the desire to minimize
interaction of beam with thick part of G0 target flange (radius = 5.5 mm).
During forward angle running, this was monitored continuously with our
calibrated halo monitor system (6 mm diameter hole in 2 mm thick Al target
+ downstream PMTs for monitoring). The specification was routinely
achieved for the potentially more problematic 31 MHz beam.
Interaction of the beam halo with some small upstream aperture:
Beam halo interacting with a small upstream aperture could potentially
generate background that gets detected in our scintillators. We measure
this with no G0 target and no halo target. This background was negligible
during forward angle running, but the detectors were downstream of
the magnet then. For the back angle running, the detectors are upstream
of the magnet in a potentially more exposed location. The smallest upstream
aperture appears to be the ceramic pipe (inner diameter = 22 mm) through
the fast raster magnets. We are in the process of doing some simple
rate estimates to get an upper limit on the potential detector rate due
to interaction of beam halo in the ceramic tube.