Re: PS target issues

[Richard Jones <richard.t.jones@uconn.edu>]

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