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Re: Tagger energy resolution? (fwd from David)



Hi David,

Just to clarify the content in GlueX-doc-1167 (microscope talk at the 
beamline review):
Slide 4 states the very initial and lenient design requirements put 
forth for the tagger design, i.e. no worse an energy resolution than 
that of the GlueX detector. Slide 6, which begins to quote the 8MeV 
figure and repeats it in subsequent presentations on the topic is what 
we end up with in the actual segmentation scheme in the microscope. But, 
this number is given in crude terms: it is just the naive division of 
0.8GeV into 100 energy bins represented by the SciFi columns. The beam 
energy resolution and the contribution from magnets' optics are not 
taken  into account, nor is this an r.m.s. figure.

As far as the rates, even if the average time between hits seems 
manageable, the effective dead time of the detector/electronics would 
still undermine the detection efficiency significantly.

Best,
-Igor


On 5/20/2009 9:05 PM, David Lawrence wrote:
>
> 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>/
>