[Olympus] rates in TOFs

[Alexander Winnebeck]

Hi Richard,

thanks for these questions and comments. I had a look on the count rates from Sunday August 7 until Monday August 8. Here are the results for OR_L (or of all PMTs on the left sector) and OR_R (or of all PMTs in the right sector):

*Beam off:
   	OR_L  = 320 Hz
   	OR_R = 900 Hz
   	The factor of three was also present before with beam and vanishes afterwards. I changed the voltages of 		the PMTs and afterwards it was almost equalized.
* Beam on (40..20 mA and 0.2 sccm flow):
   	OR_L / OR_R = 175..60 kHz
  	The drop of current by a factor of 2 results in a count rate decay by a factor of three. When I remember 			correctly this is an indication of domination of rest gas scattering over synchrotron radiation. This is also 		expressed in the life time which was well below 1 hour at that time.
* Beam on (40..20 mA) no gas flow:
	OR_L / OR_R = 150..50 kHz
	No large difference between gas in target or not.

I think, we can learn from these numbers, that we were absolutely dominated by rest gas scattering in the ring our target is playing only a minor role. The more than linear decay of the count rates wrt. beam current supports the picture of rest gas scattering.

There is a plan for a lead curtain upstream of Olympus, but it wasn't ready to go in during our shutdown. It might be installed during the next service week.  *Uwe or Nico should comment on that*

The change of the count rate ratio due to voltage settings of the PMT shows how important it is to have proper settings for the PMTs (HV and threshold). This is something the TOF group has to address very soonish. Without a proper HV control this is a nasty business, but we should go ahead and find the optimal settings anyway.

When we worked with the scrapers we could see only very little effect on the count rates. The major impact was in the life time, but this was still with the high rest gas scattering. What we could was, that we should avoid using the last scrapers upstream of Olympus, because they produce even higher count rates when hitting the beam as anticipated.

So, the first step should be to optimize the PMTs settings. Then we can see on Wednesday if there is a way to change the orbit such a way, that the count rates drop. After this optimization we could check on the effect of using the scrapers to clean up the beam. And then we could look for a target effect again,, where the life time is so large that rest gas scattering should be much smaller. 

Hope this makes sense,

Alexander

On Aug 23, 2011, at 1:55 PM, Richard G Milner wrote:

> Hi Alexander,
> I would like to understand better the rates in the TOFs following yesterday's
> discussion at the weekly meeting.  In particular, the rates in each TOF for the
> following conditions are of interest:
> 
> beam off:
> Here one is sensitive to HV and threshold settings, light leaks, bad connections
> etc.
> 
> beam on/no gas:
> Here one is sensitive to the tune of the beam and background (non-target elastic
> processes). Presumably, we need to develop a clean tune of the beam, involving
> adjustment of the slits, centering of the beam in the target cell, etc.  Tuning
> scintillators around the beam pipe are very useful to set this up.  We need to
> minimize the singles rates in the TOFs.  I think doing this at 4 GeV to start
> would be a very significant first step. For sure, going to 2 GeV will need some
> effort but the procedure should be quite similar.
> 
> When last at DESY, I noticed that there is no shielding in front of the
> experiment.  For both HERMES and BLAST we had a wall of lead in front of the
> experiment to minimize the beam generated background affecting the detector. 
> The TOFs are at a significant distance from the beampipe and are unprotected
> from background that could be coming down the tunnel. This should be
> considered.
> 
> I will start to look at the BLAST operational experience.  As suggested
> yesterday, tightening the trigger will also improve the situation.
> 
> I copy olympus at mit.edu to stimulate discussion and comment.
> Thanks,
> Richard

--
Dr. Alexander Winnebeck
winnebeck at mit.edu

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