[Halla_parity] PREX status -- no shifts this weekend

[rom at jlab.org]

As the RC indicated already in the HALOG, there are no shifts this weekend.
Shift takers should be on standby Monday swing, but it's most likely not
until Tuesday that we take beam again for PREX.

The techs will be attempting to repair the leak starting on Monday when
hopefully RADCON allows it.  Tungsten shielding will be added at the beam
exit area.  Actually this shielding had been in place during HAPPEX-II
but was removed because the new angle didn't allow it to fit.  This was
probably a mistake (and partly my fault) because the shielding protects
the O-ring on the vacuum flange from being damaged by radiation.  None
of us thought of this, unfortunately.  A new O-ring will be installed
and the target chamber pumped down.  We've received a ton of advice about
O-rings.  The advice has been noted by the experts who are doing the job.

We will have a 3 pm meeting Monday, 2nd floor counting room.

The silver lining on this downtime is we've had plenty of time optimize
the location of our detectors, to understand the data we've taken and make
adjustments to the apparatus and refinements to our run plan.  Hopefully
we can get into production fairly quickly.

A final note about the radiation excursion on the Tues owl shift when
the vacuum went bad: it's now seen that this was due to scraping on the
massive aluminum frame that holds the multi-foil optics target.  Nobody
violated any operational restrictions, but in retrospect we should
probably avoid large-raster high-current on that target, which is
rather tiny (normally used with unrastered beam) and too close to a
heavy frame.  We have plenty of solid targets that are easier to hit.

yours
Bob Michaels