[Halld-tagger] Info for the IPR 2009 review

[Eugene Chudakov]

Richard,

thanks for the explanation.
What happens if the collimator is not changed?

Thanks again,
Eugene

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
> 
> 
> 
> Eugene Chudakov wrote:
>
>  Richard,
> 
> please remind me what would be the impact of using a 50um diamond crystal
> instead of 20um one. What will be the polarization in the peak?
> 
> Thanks,
> Eugene
> 
> ------------------------------------------------------
> Eugene Chudakov
> http://www.jlab.org/~gen
> phone (757) 269 6959  fax (757) 269 6331
> Thomas Jefferson National Accelerator Facility
> 12000 Jefferson Ave,
> Newport News, VA 23606 USA
> 
> 
> 
> 
>