<html>
<head>
<meta content="text/html; charset=ISO-8859-1"
http-equiv="Content-Type">
</head>
<body text="#000000" bgcolor="#FFFFFF">
<div class="moz-cite-prefix">Hi Richard, <br>
<br>
the main purpose of putting the beam profiler in front of the
active collimator <br>
at the commissioning stage is to unambiguously verify the presence
of the beam <br>
in the collimator cave at a position where the active collimator
should provide us <br>
with a response. <br>
<br>
A secondary use of the profiler would be to calibrate the distance
vs asymmetry <br>
by moving the beam in the horizontal and vertical directions. This
calibration can be <br>
verified at least in the x-direction by moving the collimator with
respect to the beam <br>
(and the profiler if we want to keep it in). This step of the
profiler-AC calibration can fail because <br>
of the beam profiler response can significantly deteriorate due to
background if the <br>
actual collimator block is inserted behind it. In this case we
could rotate the AC by <br>
90 degrees and sue the x-stage again, assuming that the beam-AC
calibration is the same <br>
in both orientations. <br>
<br>
During regular operations when the active collimator is in and is
functioning properly , <br>
having the profiler before or after the collimator will not serve
a significant purpose. <br>
Although It could be helpful during the uncollimated beam tuning
if it is behind the target <br>
and surveyed with respect to it. It could also help to identify
some abnormal conditions. <br>
<br>
Hovanes. <br>
<br>
<br>
On 11/12/2013 04:31 AM, Richard Jones wrote:<br>
</div>
<blockquote
cite="mid:CABfxa3T--FDFZc_S6QFaGCj3PKP-XJx_t8eP4VH1NHaT4mrDBA@mail.gmail.com"
type="cite">
<div dir="ltr">Hovanes,
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>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. </div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>-Richard J.</div>
</div>
<div class="gmail_extra"><br>
<br>
<div class="gmail_quote">On Mon, Nov 11, 2013 at 11:30 PM,
Hovane Egiyan <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:hovanes.egiyan@gmail.com" target="_blank">hovanes.egiyan@gmail.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
<div>Hi Richard, <br>
<br>
I attached pictures of the Hall B downstream alcove
from 2011 beam test run in Hall B taken from <br>
the preliminary report at the collaboration meeting in
May of 2011 (page 8 in Jones et al , GlueX-doc-1759). <br>
The black box after the big flange with red HV cables is
an identical twin of the beam profiler that <br>
Elton's commissioning list included today. The Hall D
active collimator at that time was riding on top of the
<br>
total absorption counter's translation table within a
meter downstream from the beam profiler. <br>
This beam profiler was present on the photon beamline
just upstream of the active collimator the whole time
during the test. <br>
And that is in addition to the main CLAS target far
upstream which must have been a few % of a radiation
length. <br>
Based on UConn analyses, the active collimator worked
well in these conditions. Could you explain <br>
why you are sure that the active collimator worked well
in Hall B and it will not work in Hall D if there is <br>
an identical profiler in the beamline upstream of it?<br>
<br>
Hovanes <br>
<br>
<br>
From Elton's bullets with, I believe, your underlined
comments:<br>
1. Require profile monitor to be installed and
operational -- <u>where? (cannot be upstream of active
collimator)<br>
</u>....<br>
7. Compare with response of active collimator -- <u>cannot
be done with profile monitor installed<br>
<br>
</u><br>
<div>
<div class="h5"><br>
<br>
On 11/11/2013 11:17 AM, Richard Jones wrote:<br>
</div>
</div>
</div>
<blockquote type="cite">
<div>
<div class="h5">
<div dir="ltr">Hello all,
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>-Richard J.</div>
</div>
<br>
<fieldset></fieldset>
<br>
</div>
</div>
<pre>_______________________________________________
Halld-tagger mailing list
<a moz-do-not-send="true" href="mailto:Halld-tagger@jlab.org" target="_blank">Halld-tagger@jlab.org</a>
<a moz-do-not-send="true" href="https://mailman.jlab.org/mailman/listinfo/halld-tagger" target="_blank">https://mailman.jlab.org/mailman/listinfo/halld-tagger</a></pre>
</blockquote>
<br>
</div>
</blockquote>
</div>
<br>
</div>
</blockquote>
<br>
</body>
</html>