[Halld-tagger] Info for the IPR 2009 review

[Hrachya.Hakobyan]

Richard,

Nice to hear you too and thanks for comments. As I understand you
have already investigated this subject quantitatively. I believe  the 
case of 50micron is preferable also  in the sense of better and more
reproducible quality of crystal production,with longer life time due to a
consequences of higher rigidity of the thicker radiator.

Hrachya

On Thu, 17 Sep 2009, Richard Jones wrote:

> Hrachya, it is good to hear from you again.  Find my responses below.
> -Richard J.
> 
> Hrachya.Hakobyan wrote:
>
>  Hello Richard,
> 
> In the presence of the beam divergence and multiple coulomb scattering
> the collimation dependence of polarisation seems has a saturation 
> toward the collimation decrease. 
> 
> Eventually it does saturate.  The particular choice we have made for Hall
> D is not in the saturated region because we want to maintain a high
> tagging efficiency.  But if we were willing to go to low tagging
> efficiency (level of 10%) then the polarization would saturate at the
> value given by the pure coherent component.
>
>  If so the 50 micron case probably may be 
> used with a wider collimation so with gain in FOM, For the precise  study the 
> Coulomb scattering probably has to be simulated by decomposing the crystal
> into thin layers(5x10micron f.e.). What do you think about?
> 
> 
> More multiple scattering does mean we must open up the collimator for the
> sake of tagging efficiency, yes, but that does not increase the figure of
> merit.  More multiple scattering decreases the figure of merit.  In the
> analytical calculation, the multiple scattering in the target is treated
> continuously (i.e. with N layers of 20/N microns thickness, where
> N->infinity).  This analytical model is what we are using to compute the
> performance parameters of the source (e.g. polarization, beam rates, beam
> profile, tagging efficiency, etc.)  The simulation uses the analytical
> model to generate the bremsstrahlung events inside the target.  All of
> this is to say that the many-thin-layer approach to simulating coherent
> bremsstrahlung is what we are already doing.
> 
> 
> 
>
>  On Wed, 16 Sep 2009, Richard Jones wrote:
>
> 
>
>  Eugene,
> 
> Several things change at the same time, so it takes some thought to make
> a true comparison.  Under fixed collimation conditions, the polarization
> is not very sensitive to the crystal thickness.  However, it is really
> the tagging efficiency that determines what polarization we run at, and
> the tagging efficiency is somewhat more sensitive.  If we were not
> concerned with tagging efficiency then we could narrow the collimator
> arbitrarily small and compensate with higher e-beam current, such that
> the polarization attains that of the pure coherent component.  So to make
> a fair comparison, I fix the tagging efficiency at its nominal value for
> the standard configuraration (3.4mm collimator, 20 micron diamond) and
> when I change the diamond thickness I vary the collimator diameter to
> keep the tagging efficiency the same at the coherent peak.  When I do
> that, I get the following results:
>
>     1. 20 micron diamond:
>        o  peak polarization = 41.4 %
>        o  hadronic bg rate (low-energy beam flux, arb. units) = 1.9
>
>     2. 50 micron diamond:
>        o  peak polarization = 39.4 %
>        o  hadronic bg rate (low-energy beam flux, arb. units) = 2.1
> 
> The figure-of-merit for a polarization observable is rate *
> polarization^2.  Here I am going to assume that we are bg limited (at the
> trigger level) so the hadronic bg sets the running rate.  Under these
> conditions, going from a 20 micron to 50 micron diamond costs a FOM
> factor of 20%.  If errors are purely statistical then this means 20%
> longer run time to achieve the same level of precision.  In our case,
> errors are more likely to be systematics dominated, in which case the
> higher polarization and lower bg with a 20 micron diamond will result in
> increased sensitivity to small signals.
> 
> -Richard Jones
> 
>
> 
>
>  k
> 
> 
> 
> 
>