[Olympus] Olympus Digest, Vol 41, Issue 14
Anton Izotov
izotov at mail.desy.de
Wed Aug 24 12:33:33 EDT 2011
Hi all,
just one question. How an inch of led shield can "conflict with the
survey"? I do not see a relation.
Anton
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> Today's Topics:
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> 1. Re: rates in TOFs (Uwe Schneekloth)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Wed, 24 Aug 2011 17:14:38 +0200
> From: Uwe Schneekloth <uwe.schneekloth at desy.de>
> Subject: Re: [Olympus] rates in TOFs
> To: olympus at MIT.EDU
> Message-ID: <0DC99D37-8AAA-4EE5-A8DE-6F00E28FAF7D at desy.de>
> Content-Type: text/plain; charset=us-ascii
>
> Dear Colleagues,
>
> Just two comments concerning the discussion:
>
> We shouldn't use the conditions during the weekend after the startup as
> a reference. The vacuum pressure was still quite poor, very low beam lifetime
> without any gas flow, and therefore high background rates.
> It would be very useful to take longer runs now and look at the rates as a
> function of beam current.
>
> Presently, there is indeed no shielding upstream of the detector. We were
> planning to install a lead wall just upstream of the detector. This was not done
> because the shielding would have be in conflict with the survey and there was
> no time between the final survey and the startup.
> The shielding could be installed during the service week in October or
> more likely during the December/January shutdown.
> The lead thickness is 12.5mm in the central part and 3mm in the outer
> part. On the very top there will be gap for air ventilation and smoke
> detection.
>
> Best regards,
> Uwe
>
>
> _____________________________________________________
> |
> | Dr. Uwe Schneekloth
> | OLYMPUS
> | Deutsches Elektronensynchrotron
> | Notkestrasse 85
> | D-22603 Hamburg/Germany
> | Tel: +49-40-8998 2007 Fax: +49-40-8998 3092
> | email: uwe.schneekloth at desy.de
> _____________________________________________________
>
> On 23.08.2011, at 17:16, Brinker, Frank wrote:
>
> > 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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