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Eugene,<br>
<br>
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:<br>
<br>
1. 20 micron diamond:
<blockquote>
<ul>
<li>peak polarization = 41.4 %</li>
<li>hadronic bg rate (low-energy beam flux, arb. units) = 1.9</li>
</ul>
</blockquote>
2. 50 micron diamond:<br>
<blockquote>
<ul>
<li>peak polarization = 39.4 %</li>
<li>hadronic bg rate (low-energy beam flux, arb. units) = 2.1</li>
</ul>
</blockquote>
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.<br>
<br>
-Richard Jones<br>
<br>
<br>
<br>
Eugene Chudakov wrote:
<blockquote type="cite" cite="mid:Pine.LNX.4.64.0909161656000.4641@jlabl2.jlab.org">
<pre wrap="">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
<a href="http://www.jlab.org/~gen">http://www.jlab.org/~gen</a>
phone (757) 269 6959 fax (757) 269 6331
Thomas Jefferson National Accelerator Facility
12000 Jefferson Ave,
Newport News, VA 23606 USA
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