[Hps] ECal position

[Nelson, Timothy Knight]

Yes, but in December there was no other reference for the beam position.  So it was where it was.  Now it has been moved down a fraction of a mm to agree with the SVT, which was centered by the JLab survey group (with my assistance) within about 100 microns of the nominal y=0 of the “ideal Hall B beamline” (whatever that means).

Tim

> On May 6, 2015, at 10:53 AM, Gabriel CHARLES <charlesg at ipno.in2p3.fr> wrote:
> 
> Hum... but as Stepan said, this difference was not present during the run in December. So the ECal is probably centered. I don't remember the beam being positionned to have the same rate in the top and bottom ECal.
> 
> Regards,
> Gabriel
> 
> Le 2015-05-06 19:12, Gabriel CHARLES a écrit :
>> That's right!
>>   When we installed the ECal the first time, the distance between top
>> ECal and vacuum chamber, and bottom ECal and vacuum chamber were very
>> similar (we moved the calorimeter mm by mm so we could see a 1 mm
>> difference), we concluded that it was due to the position of the
>> chamber. There may be 1 mm difference between the center of the chamber
>> and the "true" center, so it was hard to conclude at that time. The
>> calorimeter box could also be 1 mm down, the targets used for the survey
>> being independant of the box.
>>  Anyway, your idea is probably right, could tracking help?
>> Regards,
>>  Gabriel
>> Le 2015-05-06 18:36, Nelson, Timothy Knight a écrit :
>>> Here are the results of the ECal survey shown in January.  Am I wrong
>>> that this indicates that the bottom ECal is closer to the beam by 1 -
>>> 2 mm (and about 1.5 mm in the middle where rates are high) compared to
>>> the top?  If so, then that could explain the rate effects we see in
>>> the top vs. bottom ECal, since that’s about 1 mr change in acceptance.
>>> Tim
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> 
> -- 
> Gabriel CHARLES
> Institut de Physique Nucléaire d'Orsay