Tagger energy resolution? (fwd from David)

[Elton Smith <elton@jlab.org>]

---------- Forwarded message ----------
Date: Wed, 20 May 2009 17:09:35 -0400
From: David Lawrence <davidl@jlab.org>
To: 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

-- 

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