Re: PS target issues

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

Hrachya,

I agree with your first point completely.  My graduate student studied 
this problem briefly, and convinced me that simply copying the Hall B 
design with silicon microstrips is not going to work for us.  But we 
need to explore this more systematically at some point in the future.

Your second point is a good one.  Our design of the active collimator 
has the requirement of keeping the beam spot centered on the primary 
collimator aperture within +/- 0.2 mm, with a frequency bandwidth on the 
correction of up to 1KHz.  This requirement comes about for precisely 
the reasons you describe.

Richard Jones


Hrachya.Hakobyan wrote:
> Richard,
>
> 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 .
>