[Halld-tagger] photon beam working group

[Hovanes Egiyan]

Hi Richard,

the main purpose of putting the beam profiler in front of the active 
collimator
at the commissioning stage is to unambiguously verify the presence of 
the beam
in the collimator cave at a position where the active collimator should 
provide us
with a response.

A secondary use of the profiler would be to calibrate the distance vs 
asymmetry
by moving the beam in the horizontal and vertical directions. This 
calibration can be
verified at least in the x-direction by moving the collimator with 
respect to the beam
(and the profiler if we want to keep it in). This step of the 
profiler-AC calibration can fail because
of the beam profiler response can significantly deteriorate due to 
background if the
actual collimator block is inserted behind it. In this case we could 
rotate the AC by
90 degrees and sue the x-stage again, assuming that the beam-AC 
calibration is the same
in both orientations.

During regular operations when the active collimator is in and is 
functioning properly ,
having the profiler before or after the collimator will not serve a 
significant purpose.
Although It could be helpful during the uncollimated beam tuning if it 
is behind the target
and surveyed with respect to it. It could also help to identify some 
abnormal conditions.

Hovanes.


On 11/12/2013 04:31 AM, Richard Jones wrote:
> Hovanes,
>
> In our beam test it is the detector that moved, the beam did not move. 
>  As a result, we can only "see" the material that moved with the 
> detector.  Other material may be present and distorting the signal, 
> but we only see it as a distorted beam shape, not as a distorted 
> signal.  However, in the real situation in Hall D, it is the beam that 
> moves.  Anything that is stationary in front of the active collimator 
> distorts the active collimator signal and can potentially throw off 
> the response.
>
> For example, during an earlier run in 2007 we accidentally had one of 
> the readout cables that come out from the front of the detector 
> sticking out forward instead of coming out with a 90 degree coupling 
> from the front plate.  That cable was visible in the detector response 
> as a prominent peak (10% of the maximum pulse height) in the 
> left-right signal.  These are not huge effects, but the asymmetry 
> signal is highly sensitive to the local slope of the response 
> functions for the individual wedges.
>
> I suppose one could do a special calibration with the profiler in 
> place, and then try to be careful not to move it or the active 
> collimator (small rotations are not restricted in the mount) but then 
> if we ever wanted to move the profiler it would require a 
> recalibration of the AC response.  I am wondering, what are we 
> learning from the profiler that we do not get from the active 
> collimator?  There is no fine-structure in the beam at this distance 
> from the radiator, unless it is put there by nearby material upstream. 
>  On the other hand, having an image of the beam down inside the hall 
> (just downstream of the FCal) would be potentially useful as a monitor 
> of scraping on the photon beam pipe in the hall, or on the walls of 
> the target.
>
> -Richard J.
>
>
> On Mon, Nov 11, 2013 at 11:30 PM, Hovane Egiyan 
> <hovanes.egiyan at gmail.com <mailto:hovanes.egiyan at gmail.com>> wrote:
>
>     Hi Richard,
>
>     I attached  pictures of the Hall B downstream alcove from 2011
>     beam test run in Hall B taken from
>     the preliminary report at the collaboration meeting in May of 2011
>     (page 8 in Jones et al ,  GlueX-doc-1759).
>     The black box after the big flange with red HV cables is an
>     identical twin of the beam profiler that
>     Elton's commissioning list included today. The Hall D active
>     collimator at that time was riding on top of the
>     total absorption counter's translation table within a meter
>     downstream from the beam profiler.
>     This beam profiler was present on the photon beamline just
>     upstream of the active collimator the whole time during the test.
>     And that is in addition to the main CLAS target far upstream which
>     must have been a few % of a radiation length.
>     Based on UConn analyses, the active collimator worked well in
>     these conditions. Could you explain
>     why you are sure that the active collimator worked well in Hall B
>     and it will not work in Hall D if there is
>     an identical profiler in the beamline upstream of it?
>
>     Hovanes
>
>
>     From Elton's bullets with, I believe, your underlined comments:
>     1. Require profile monitor to be installed and operational --
>     _where? (cannot be upstream of active collimator)
>     _....
>     7. Compare with response of active collimator -- _cannot be done
>     with profile monitor installed
>
>     _
>
>
>     On 11/11/2013 11:17 AM, Richard Jones wrote:
>>     Hello all,
>>
>>     The photon beam working group meeting is taking place this
>>     morning at 11:30EST.  A draft agenda has been placed on the wiki.
>>      Please change as needed.
>>
>>     -Richard J.
>>
>>
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>

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