User name cates
Log entry time 22:59:55 on March 23, 2006
Entry number 168013
keyword= The alignment of target cell DollyThe alignment of the target cell Dolly was monitored before and after data taking with by obtaining photographs that show the position of Dolly with respect to upstream helium bag. The position of the cell with respect to the target ladder can also be determined at some level, but in this case parallax limits the accuracy.
In the photo shown below, it can be seen that there was a small shift in Dolly's position with
respect to the helium bag of roughly 1.5mm during data taking. There are at least two
plausible explanations. The beam pipe on which the upstream berylium window is mounted
sits on the same mount as does the upstream helium bag. When the berylium window was
replaced, the helium bag certainly could have moved by 1.5 mm. Another possibility is that
the cell moved slightly. In the photo, a thin copper wire is visible that was used to pull the cell
to the left and downwards. This was necessary to improve the alignment that was observed
when the cell was first installed. The copper wire had considerable tension, and it is plausible
that over the course of data taking the wire slowly moved the cell. The wire was fixed to the
ceramic back plate of the target ladder.
Figure 1 Dolly alignment before and after data takiing
