[Olympus] rates in TOFs

[Inti Lehmann]

Dear Alexander and Richard,

Let me add one comment here re the count-rate effect and the PMT settings.

I understand that the TOF HVs and thresholds need to be properly 
optimised. Vlad had done this carefully for all TOFs in the preparatory 
phase, but I agree we seem to need an iteration of re-tuning due to the 
high hardware thresholds at the experiment.

It, however, seems to me that the count-rate effect discussed here has 
nothing to do with this tuning. This is because 1) the count rates 
remain low without beam and 2) the count rates seem not to change 
dramatically when requiring a top-bottom coincidence. (As pointed out by 
John this coincidence is what we should be looking at normally.) To me 
that means that we are seeing real particles, right?When hearing about 
the missing lead wall, it doesn't seem that surprising.

Cheers,
Inti


On 23/08/11 15:13, Richard G Milner wrote:
> Hi Alexander,
> Thanks for the fast reply and information.  The point that in the August test
> run we were dominated by scattering from rest gas is a good one.  In normal
> running conditions, the background should be predominantly from the fixed
> collimator, which shouldn't trouble the TOFs much. As you point out, once we
> have long lifetime, the scrapers should be more effective.
>
> It is good to hear that we have a lead `curtain' to be installed.
>
> I will think some more about this.
> best regards,
> Richard
>
>
> Quoting Alexander Winnebeck<winnebec at MIT.EDU>:
>
>> 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
>>
>> Massachusetts Institute of Technology (MIT)
>> 77 Massachusetts Avenue
>> Bldg 26 / Rm 441
>> 02139 Cambridge, MA, USA
>> Tel: +1-617-253-3680
>>
>> DESY
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>> 22603 Hamburg / Germany
>> Tel: +49-40-8998-6402
>>
>>
>
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