Re: PS target issues

["Hrachya.Hakobyan" <hakopian@mail.yerphi.am>]

Richard,

I was not able to reach and see the angular distribution of intensity 
spectra while can do it for polarization...

I agree with statement, based on the recent experience of Hall B and our 
own that the direct polarimetry currently reaches the level >= 5% 
in accuracy and it is too expensive. I didn't know the status of 
the direct polarimetry in the Hall D budget. So when you recently told me 
about shift of  PS converter upstream of the PS magnet I thought it was 
dictated by need to foresee the place for microstrip converter for direct 
polarimetry. Of course  the relatively low photon energies  between 1 and 
2Gev used in direct photon polarimetry by Yerphi group and JLAB-SPRING8 
collaboration were more 
favorable for pairs angular resolutions while the energy range 8-10GeV 
with lower analyzing power,lower pairs polar angles(~ m/E) and limited 
drift space before magnetic analysis  makes the task more  difficult.

What's about "beam instability within photon collimation" I think the 
meaning is clear, beam's emittance is ordinary changing due to instability 
of apparatus, leading to a short term deviations  relative the 
average position. Having 3.4mm collimation at 76m flight base that 
corresponds to app. 0.5m/E(22 microrad) one may evaluate that a  0.5mm 
shift of beam position may introduce into play the photon angles between 
22 to 29 microrad not visible when beam and collimator centroids are 
coincident. I do not know in fact how much fast and in what level of 
precision these instabilities may be prevented by active collimator 
output's  asymmetry, but they will remain and Monte Carlo simulation 
couldn't exactly reproduce them. This is why I've spoken about need in 
polarization calculations methods using measured CB shape.

Hrachya .




On Wed, 10 Sep 2008, Richard Jones wrote:

> Hrachya,
>
> Yes, the intensity profile of the collimated beam also has some azimuthal 
> structure, although the azimuthal dependence is much weaker in the intensity 
> profile than it is in the polarization profile.  See the plots below.
>
> http: //zeus.phys.uconn.edu/halld/glueXmeetings/mtg-9-2008/phi_intens.gif
> http: //zeus.phys.uconn.edu/halld/glueXmeetings/mtg-9-2008/phi_polar.gif
>
> I am not clear on what you mean by "beam nuclei instability inside of photon 
> collimation".  Could you explain?  I agree that we definitely need a direct 
> polarimetry measurement, but because of cost it is currently outside the 
> budget envelope for the Hall D beam line.  Let's talk some time about ideas 
> for this.  I am not convinced that we can do a good job using silicon 
> microstrips in the flight distance of 1-2m that we have available to us right 
> now.  We need to go back to the drawing board and consider all of the 
> options, and argue for the best one.  As you point out, the device in Hall B 
> was never shown to work convincingly, certainly not at the few-percent level 
> that we need for GlueX.  That is a topic for discussion as soon as we have a 
> finished design for the pair spectrometer.
>
> Richard Jones
>
> Hrachya.Hakobyan wrote:
>>  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
>> 
>> 
>
>