Re: PS target issues (fwd from Hrachya)

[Elton Smith <elton@jlab.org>]

---------- Forwarded message ----------
Date: Wed, 10 Sep 2008 14:50:25 +0500 (AMST)
From: Hrachya.Hakobyan <hakopian@mail.yerphi.am>
To: Richard Jones <richard.t.jones@uconn.edu>
Cc: Hall D beam working group <halld-tagger@jlab.org>
Subject: Re: PS target issues

Richard,

The plot is impressve  in the sense of the quantitatively atrong
variations . Qualitatively it was clear, having expressions
for CB intensity and polarization, there is expected a noticeable azimutal
asymmetry both in CB intensity and polarization spectra.If you remember
we discussed in 2003 the  problem of beam nuclei instability inside of
photon collimation and it was one of the arguments of the need in CB
polarisation calculation methods , because Monte-Carlo simulation can not
reproduce unexpected beam flowing.
  In 2006 I told you that is possible to use an azimutal variation of CB
intensity for photon polarimetry using strip like PS converter and
alternation of CB polarization setting,oriented vertically(phi=90deg) or
horisontally(phi=0deg) to pair production plane.

Hrachya


On Mon, 8 Sep 2008, Richard Jones wrote:

> Hrachya,
>
> This is good news.  The variation in the polarization profile across the beam
> spot should be checked.
> I went ahead and computed the profile of the local polarization across the
> face of the photon beam that is accepted by the primary collimator.  I was
> surprised by how much variation there is in the local degree of polarization
> as you scan around on the surface of the beam spot.  See the plot at the link
> below.  I have not yet included the effects of beam spot size smearing, but
> it will not change things very much at the nominal setting for the virtual
> focus.
>
> http://zeus.phys.uconn.edu/halld/glueXmeetings/mtg-9-2008/phi_polar.gif
>
> The azimuthal angle is plotted in radians, whereas the polar angle is in
> units of m/E.  The outer edge of the collimator acceptance is at 0.52 m/E, so
> I only plotted it out that far.  At larger angles, the polarization actually
> reverses sign at some values of the azimuth.  Very interesting.  The ability
> to scan the beam with a wire would add to our diagnotic capability, as we
> could actually see some of this structure, in a Cartesian projection.  On the
> other hand, it would also be useful for monitoring to be able to watch the
> average polarization, which is what the experiment will be depend on.  For
> that we would probably want a converter foil that covers the entire beam
> spot.
>
> Hrachya, are you surprised by this plot?
>
> Richard Jones
>
> Hrachya.Hakobyan wrote:
>>  Richard,
>>
>>  You may easily check quantitatively that the influence of PS converter
>>  thikness,trough multiple scattering(projection angle's RMS)  onto  FSF and
>>  WSF arms is ranged from:
>>  - 0.2 to 0.62mm for thicknsses x=10^-3  10^-2 Xo and Ee=8GeV and distance
>>  3.5m
>>  - 0.45 to1.15mm for thicknesses x=10^-3 to10^-2 Xo and Ee=3.5GeV
>>
>>  Influens of mult.scat. is small and allows to increase the convererter
>>  thickness from 10^-3 to 5*10^-3 Xo, without worsening reslution. For end
>>  point energy measurement is also possible to use 1,2mm wide strips for
>>  better resolution,even thicker than 5*10^-3 Xo,while for CB peak
>>  measurements that is not allowed due to  azimutal asymmetry of CB angular
>>  distribution. As a conclusion, there is  a reserve for PS rate increse by
>>  use of thicker converter and end-point energy is possible to measure with
>>  better resolution, using 1-1.2mm wide strip and thickness up to 10^-2.
>>
>>  Hrachya
>