[Halld-tracking-hw] drift chamber positions inside Bcal

[Lubomir Pentchev]

Hi Elton,

I think the most important is to position the tracking detectors correctly 
with respect to the beam line. For the FDC, the copper-free area and the 
insensitive area are of 12 and 30mm radius resp. Due to the  sharp 
dependence of the  background on the distance to the beam, 3mm offset 
w.r.t. beam is a lot, i.e. we will see big asymmetries in the loads of the 
strips close to the beam which will eventually limit the maximum beam 
current we can work with.

As for the magnet axis, I suppose it was aligned with the beam axis, Tim 
should know what was the precision.

I'm sure there will be other considerations, pros and cons for each 
solution, but as for the FDC I think it should be aligned as precisely as 
possible with the beam line.

Lubomir

On Thu, 12 Sep 2013, Elton Smith wrote:

> Hi everyone,
>
> I wanted to bring up a discussion, which you all have probably all
> thought about, and perhaps agreed on. As we are in the process of
> installing the Bcal into the solenoid, we note that its position inside
> the magnet will not be "ideal", even as the insertion is going very
> well. (By ruler, the Bcal appears to be about 1/8" low and approximately
> centered north-south). Tim reminded me that after insertion, the Bcal
> will be surveyed and the rails for the drift chambers will be mounted
> according to "as built" locations.  But we need to decide whether the
> rails should be centered on the Bcal or the magnet. A related question
> is where to position the target. My first thought is that these
> detectors should be located relative to the actual Bcal position. Note,
> however, that this may mean the axis is not perfectly along the z-axis
> (defined by the solenoid).
>
> Comments/suggestions? We need to make this decision over the next couple
> of days.
>
> Cheers, Elton.
>
>
>
>
>
> -- 
> Elton Smith
> Jefferson Lab MS 12H5
> 12000 Jefferson Ave STE 16
> Newport News, VA 23606
> (757) 269-7625
> (757) 269-6331 fax
>
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