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Re: Tagger energy resolution? (fwd from David)
Hi Dan and Sascha,
I saw the 2.5 MeV number in Jim's tagger note (GlueX-doc-1127 table 4.1
pg. 42)
The only place I saw the 8MeV number was in Igor's talk from the
beamline review
(GlueX-doc-1167 slide 6) where it has a bullet that says "resulting ~8
MeV resolution more
than satisfies GlueX requirement". However, on slide 4 of the same
document, it gives
60MeV as the "GlueX detector capability". I'm guessing this is a
mis-statement and the
detector capability is ~8MeV (or 8MeV/sqrt(12)) but the tagged photon
energy resolution
is 60MeV, driven by the beam energy resolution.
Thanks Sascha for the correction on the microscope range. I will include
the correct range
in my talk. I'm not getting something about what the 2% of 12GeV
(=240MeV) represents.
Is this the actual tagged photon resolution? If so, what happened to the
60MeV?
I'm not sure though that I follow the fixed array argument. It would
seem that
the limitation of the electronics would be applied on a per channel
basis. If the single
channel rate is 5MHz, then that would be a hit once every 200ns which
the scintillator
and our electronics should easily handle. Is the 20 counters coming from
the solid angle
subtended by a bundle of 20 fiber scintillators into the magnet being
about the same as
that of a single fixed array scintillator? I guess that would make sense
(5 scintillators per
energy bin times 4 (8MeV) energy bins would cover 32 ~= 30MeV). However,
I'm not
sure how well the argument holds even then since more than half of the
rate is contained
in the center fiber of a row of 5 in the microscope (see slide 6 of
Igor's talk GlueX-doc-1167).
Sorry if I'm being a pest
Regards,
-David
Alexander Somov wrote:
>
> Hi David,
>
> Here are a few tiny remarks:
> Be sure that you use the right microscope range of 0.8 GeV
> in your slide, rather than 0.6 GeV which you have mentioned
> in your mail. As Richard has explained, the microscope
> counter resolution should be less than 6 MeV:
> ~ 8 MeV / Sqrt(12) (counter size) + 2% from 12 GeV (magnet optics)
> + multiple scattering. Counters granularity is driven by the rate.
>
> The fixed-array counters are ( most likely ) useless for high-lumi
> runs even for the endpoint energy region. The singke counter rate
> in this region is about 5 MHz. As an example, for 20 counters the
> rate is ~ 100 MHz (on average 1 hit every 10 ns). There will be a
> lot of multiple hits in any reasonble electronics time integration
> window. I doubt that it will be possible to resolve multiple hits
> even in the offline analyses with our detector resolution.
>
> Cheers,
> Sascha
Daniel Sober wrote:
> David,
> There must be a major misprint somewhere. The numbers as I understand
> them (see microscope and hodoscope contributions to last week's
> Collaboration Meeting) are
>
> Microscope channel width: 8 MeV
> Fixed array channel width: 30 MeV
> Beam energy contribution to resolution: 2.5 MeV (see my talk at
> November 2008 Tagger/Beamline review)
> The finer segmentation of the microscope is driven more by counting
> rate than by desired energy resolution.
>
> The fixed array can not be used for the coherent peak at normal
> data-taking rates, because the individual counters would run at 9-14
> MHz (see table in hodoscope talk at May 2009 Collaboration meeting).
> Only the photon energy region above the coherent peak (9-11.7 GeV),
> where counting rates are lower, is fully covered by detectors Below 9
> GeV, there is only 50% sampling for crystal alignment purposes (at low
> rate), and during full-rate running, these counters can be read out
> only in current mode.
>
> Dan Sober
>
> Elton Smith wrote:
>> ---------- Forwarded message ----------
>> Date: Wed, 20 May 2009 17:09:35 -0400
>> From: David Lawrence <davidl@jlab.org> <mailto:davidl@jlab.org>
>> To: halld-tagger@jlab.org <mailto:halld-tagger@jlab.org>
>> Subject: Tagger energy resolution?
>>
>> Hi Photon-ists,
>>
>> I'm getting my talk put together for CIPANP next week and I'm a bit
>> confused about the tagger energy resolution numbers I'm seeing. I've
>> spoken with Eugene and Elton and I think I understand them better now,
>> but I would like to get confirmation from the group.
>>
>> I've seen a couple of places (GlueX-doc-1167, GlueX-doc-1127) where
>> the microscope resolution is quoted as 0.5% of the electron beam energy
>> or 60MeV. However, the microscope has 100 detectors covering a 600MeV
>> range so the tagger itself is capable of something closer to 6MeV
>> resolution. As I understand it, the 60MeV comes from the uncertainty in
>> the electron beam energy and not due to any limitation of the tagger
>> design itself. The "over-design" of the resolution is due to rate
>> considerations.
>>
>> What raised a flag for me was the fixed array which seems to always
>> be quoted as having detectors spanning a 30MeV bite which is half as big
>> as the quoted microscope resolution. Though I don't actually see it
>> anywhere, I'm assuming that the fixed array also has an energy
>> resolution of 60MeV limited by the electron beam energy resolution.
>>
>> Can someone confirm that all of this is correct?
>>
>> If this all is correct and the fixed array and the microscope both
>> have the same energy resolution and they both can handle the rate at
>> full luminosity and they both fully cover the same energy range (8.4-9.0
>> GeV) then why do we need both?
>>
>> Sorry if this is a stupid question, but I've always assumed that the
>> purpose of the microscope was to give much finer energy resolution.
>>
>> Regards,
>> -David
>>
>>
>
> --
> /Daniel Sober
> Professor and Chair
> Physics Department
> The Catholic University of America
> Washington, DC 20064
> Phone: (202) 319-5856, -5315
> E-mail: sober@cua.edu <mailto:sober@cua.edu>/
--
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