[Prex] run without Compton

[Kent Paschke]

Since this was distributed to the full collaboration, I just want to take a moment to acknowledge the tremendous job our students and postdocs have done with our online data set. We not only keep careful track of what is collected, our cuts are well defined. When we completed PREX-2, we knew exactly what we had, and the analysis has been remarkably free of surprises.   Similarly, we know exactly what data we have and have not collected for CREX. We update these daily on web - there is no ambiguity here. Other estimates are simply misinformed.

Of course we have been discussing the use of the Compton polarimeter, given the challenges faced by the accelerator complex.  We will continue those conversations in a more productive forum. 

All my best, 
Kent


> On Aug 29, 2020, at 8:03 AM, Jay Benesch <benesch at jlab.org> wrote:
> 
> Colleagues, 
> 
> I suggest the beam be steered straight through and the Compton system turned off.  Rely on weekly Moller measurements for the remainder of the run.   The source, injector, beam transport and operations groups have not been able to find a solution which allows low Compton background in A and 30 uA to C without 0R08 BLM trips since the FY20 run began.  We are unlikely to find one in the next three weeks.  The laser swap between B and C helped but was not sufficient.  
> 
> CREX has received 444 C.  While online cuts classify only two-thirds of this as "good", my observations of all JLab parity experiments suggest that at least 90% will be made usable in final analysis.  In 20 days of running straight through, another 100+ C can be delivered.   
> 
> Data obtained to date can be examined during the cryo down and possible mitigations proposed.  Given that the source group must buy COTS lasers (commercial off the shelf) and the four lasers now installed provide quite different results, I fear that the only solution will be to buy several more systems and pick the four best for use.  
> 
> Jay 
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