[Olympus] rates in TOFs

Tue Aug 23 11:16:01 EDT 2011

Dear colleagues,
I have some remarks to the ongoing planning:

Background:
Indeed we have an indication that the rest gas pressure in the target
region was extraordinarily high.
On Tuesday we had a break where we activated the titanium pumps in the
bypass region and the cooling of the target cell was switched off for
some time. During this time the temperature raised up to 100 K.
The lifetime (4.5 GeV) raised from 0.8 h before to 4.0 hours after the
break. From the titanium pumps alone I would have expected a rise from 3
to 4 hours. From that I would estimate that the pressure in the target
region must have been in the 1E-6 mbar range over 6 m ( or 1E-5 mbar
over 0.6 m ).

During the 2 GeV runs the temperature and pressure in the cell was
significantly lower, but I assume it's been still much higher than
normal.

If you are interested and if it's feasible we could try to repeat the
measurement on Wednesday, August 31st. The time for studies on a service
day is rather short, but if about 1 hour time with stable conditions is
sufficient for such a test we could try that.

-----------

Service weeks:

During the 2 coming service weeks there will be interlock tests at the
Petra photon beamlines. This implies pretests on Monday/Tuesday without
Petra machine itself and final tests including Petra from Wednesday
until Friday. 
Therefore Petra will do its machine studies on Monday and Tuesday.
The DESY synchrotron  will need also time for service - but probably
only one day per week.
The Linac has no longer service planned for the next service week but
would like to do installation work over 3 days in the second week ( this
might be canceled if the equipment is delayed ) 

For studies at DORIS this means that we could get beam on Monday,
Tuesday and Friday and probably also on Thursday ( say Wednesday
evening) in the week starting on Monday 12th.
We should soon make a plan about that week. 
 - what should be installed ( lead shielding ... )
 - is it worthwhile to do beam studies before that installation, or must
they follow

-----------

Energy measurement:

Would you please specify what's needed:  Do you have to rely on the beam
energy we deliver or can you extract it from the data by yourself? What
precision is needed for absolute and relative measurements?

Best regards
Frank

>-----Original Message-----
>From: olympus-bounces at mit.edu [mailto:olympus-bounces at mit.edu] On
>Behalf Of Richard G Milner
>Sent: Tuesday, August 23, 2011 4:13 PM
>To: Alexander Winnebeck
>Cc: olympus at mit.edu
>Subject: Re: [Olympus] rates in TOFs
>
>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
>> Bldg 66 / Rm 002
>> Notkestrasse 85
>> 22603 Hamburg / Germany
>> Tel: +49-40-8998-6402
>>
>>
>
>
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