[Halld-tagger] Info for the IPR 2009 review

[Eugene Chudakov]

Gents,

I have a few naive questions. Is the multiple scattering
for those particles which produce coherent Bermsstahlung the same as 
the scattering in amorphous materials of the same thickness? Another question - 
how long is the coherent path responsible for this Bremsstahlung? In other words, does the
ratio of the coherent to regular radiation depend on the crystal
thickness?

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

On Fri, 18 Sep 2009, Hrachya.Hakobyan wrote:

> 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
>>
>>
>>
>>
>>
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