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RE: Coherent Brem using SOS configurations
Hi Franz,
Do you have values for the analysing power of a diamond for incident
electron beam energies of 6 and 12 GeV? For GluEx, we are primarily
interested in tagged linearly polarised photons between 7 and 9 GeV.
Although we can probably estimate the degree of linear polarisation to about
5% by fitting the tagger scaler spectra - both ungated and gated with
photons that pass through the collimator or generate reaction events - I
think we should try to measure the polarisation. It would only be worthwhile
to go to the the extra expense and complexity of a goniometer mounted
diamond analyser, if it is significantly better than other techniques.
So far as high flux or high linear polarisation is concerned, I think we
need as high a degree of linear polarisation as we can obtain - which means
the highest incident electron energy - and as high a flux as the experiment
can handle. For tagged photon systems, the flux is limited by the focal
plane counting rate and the maximum allowable rate in the detector system
(eg: CLAS).
Regards,
Jim.
-----Original Message-----
From: owner-halld-jlab@jlab.org [mailto:owner-halld-jlab@jlab.org]On
Behalf Of Franz Klein
Sent: 15 March 2004 15:54
To: James Kellie
Cc: Elton Smith; Richard Jones; mecking@jlab.org; sober@cua.edu; Alex
Dzierba; Ken Livingston; halld-jlab@jlab.org; Guenther Rosner
Subject: RE: Coherent Brem using SOS configurations
Hi Jim,
I looked into that when we "designed" the Hall-B coherent beamline.
The 2nd goniometer/crystal must have considerably heigher quality
than the 1st (production goni), otherwise the analyzing power is
too low. Yerevan used such a setup 10+ years ago (although at lower
energies).
The question should be: do we want/need high degrees of photon
polarization or only high flux?
When it comes to baryon resonances, we'll NEED as much information
on polarization in the initial state - for meson spectroscopy this
will also be necessary if there are many overlapping states
(otherwise you depend too much on "perfect" calibration/understanding
of the detector).
Greetings
Franz
On Fri, 12 Mar 2004, James Kellie wrote:
> Dear Elton,
>
> When I was in Jlab recently for the CLAS meeting, Bernhard gave me a copy
of
> the draft paper from Harut, and as you say the degree of linear
polarisation
> from the SOS radiation was measured to be only around 5%.
>
> However, the most interesting aspect of the paper is I think the way in
> which the authors use a second diamond crystal mounted in its own
goniometer
> downstream of the radiator to act as an alalyser to measure the degree of
> linear polarisation of the photon flux. Furthermore the analyser is
tunable,
> since by changing the crystal orientation, the photon energy for which its
> analysing power is maximum can be changed. I think this could be
potentially
> very advantageous for hall B and certainly for Gluex.
>
> I am going to follow up the idea at Glasgow, and I think your suggestion
> that a workshop is organised at JLab, to which experts in the field are
> invited is an excellent one.
>
> Regards,
>
> Jim.
>
>
>
> -----Original Message-----
> From: Elton Smith [mailto:elton@jlab.org]
> Sent: 11 March 2004 16:39
> To: Richard Jones; mecking@jlab.org; sober@cua.edu; Jim Kellie
> Cc: halld-jlab@jlab.org
> Subject: Coherent Brem using SOS configurations
>
>
>
> Hi Richard and Jim,
>
> I wanted to make sure you were aware of some recent developments in
> producing photon beams with crystal radiators.
>
> You have probably already heard about recent work/measurements which
> produce high-energy beams using a special case of coherent bremstrahlung
> using the so-called String-or-Strings (SOS) configuration. Harut Avakian
> has made a draft paper available to Bernhard Mecking describing recent
> measurements at CERN. The draft indicates that coherent-like peaks at say
> 80% of the electron beam energy can be achieved with enhancements over
> Bethe-Heitler of around 18. For comparison, our coherent peaks (after
> collimation) show enhancements of approximately 4-5. The drawback of this
> system, if it is proven is that measurements of the polarization are quite
> small, i.e. less than 5%.
>
> Let me know if you have any information and/or opinions on this work. We
> should think about whether to pursue this as a collaboration. Bernhard is
> interested in developing some of these ideas here at JLab. The Yerevan
> Physics Institute is involved in the measurements (A. Apyan and R.O.
> Avakian are authors). Richard: Are these some of the contacts you have in
> Armenia, and has any of this work been discussed for further study?
>
> Generally, JLab clearly has an interest in developing special photon beams
> at high energy and/or polarization. Therefore, it might be useful to
> consider a workshop and/or meeting at JLab to invite some of the experts
> and pursue this further.
>
> Any comments/suggestions?
>
> --
> Elton Smith
> Jefferson Lab
> elton@jlab.org
> (757) 269-7625
>
>
>
===============================================================
Franz J. Klein, Assistant Professor
CUA, Department of Physics
Washington, DC 20064
office: Hannan Hall 210 phone: 202-319-6190
or: Jefferson Lab,TR 92/CC 123 phone: 757-269-7578 (or 5271)
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