PS plots

[Alexander Somov <somov@jlab.org>]

Dear colleagues,

Here are few simple plots related to the resolution of the
pair spectrometer:


http://www.jlab.org/~somov/pair_spec/



pair_pol_ang .ps
---------------

Polar angles as function of the lepton energy

- black; production polar angle of a lepton with respect
  to the beam axis.
- red; Moliere multiple scattering angle for a converter
  thickness of 0.01 radiation length

- blue; ---- 0.001 radiation length.


As can be seen, for the L_conver range of 10-3 - 5.10-2 the
resolution is dominated by e+e- production angle (which
goes like ~1/gamma)


pair_res.ps
-----------

Position resolution (due to multiple scattering and e+e-
production angle) as function of lepton energy at a hodoscope
plane. The beam profile resolution is not included in these plots,
i.e., I set x = y == 0;  px = py\ == 0 for beam photons.

 - red plot corresponds to the converter thickness of 0.01  X0
 - blue                                               0.001 X0

Remark:
-------

The distance between the target and the hodoscope plane is
a little bit too large (~5.6 m); the target is placed about
2.20 m upstream the beam from the magnet inside the collimator
cave. The magnet length is 1.8 m (1.64 T field) and the vacuum
chamber is 1.68 m long (should be a bit shorter, i.e. 1.5 m).


---------------------


I will send around plots with counter rates/background shortly
(jobs are still running).


Cheers,
       Sascha











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
>
>