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Alex,<br>
I have used an electron beam with no size or divergence -- the
angles are only from (incoherent) bremsstrahlung. I wasn't
suggesting a re-design. Presumably the goniometer position and
quadrupole have been optimized for coherent bremsstrahlung and the
microscope. I was only pointing out that a simple target ladder
closer to the dipole would have advantages in the endpoint region if
we ever run with an amorphous radiator.<br>
I can put the quadrupole into my calculations, but I need values for
the position, length and gradient. Is there a document that lists
all of this?<br>
For coherent bremsstrahlung, my acceptance calculations are clearly
invalid, and I will not spend a lot of time thinking about how to
incorporate a collimated coherent angular distribution unless
someone has specific wishes. <br>
I will proceed with the counter layout calculations up to about
0.980. <br>
Dan<br>
<br>
<div class="moz-cite-prefix">On 6/28/2012 5:08 PM, Alexander Somov
wrote:<br>
</div>
<blockquote cite="mid:Pine.LNX.4.64.1206281704070.10043@jlabl1.jlab.org" type="cite">
<br>
Dear Dan,
<br>
<br>
Thanks, interesting.
<br>
<br>
What beam profile did you use for the gap acceptance
<br>
estimation (a pencil beam) ?
<br>
<br>
1. For the standard GlueX running with a 3.4 mm radiator
<br>
the acceptance for collimated photons is going to be
<br>
slightly larger. For a typical bremsstrahlung angle of about
<br>
2 mrad for 300 MeV electrons, the average radial displacement
<br>
of photons at the collimator is
<br>
(2e-3 * 0.3 / 11.7)*76 m ~ 3.9 mm,
<br>
i.e., if the electron is scattered to a large angle the photon
<br>
doesn't make it through the collimator. The bremsstrahlung angle
<br>
dominates the multiple scat. and the beam emittance. Ok, we need
<br>
to take the beam profile and the effect of the quadrupole into
<br>
account. The bg is a concern.
<br>
<br>
For a 5 mm radiator, the situation is slightly worse.
<br>
We have checked the tagging efficiencies using a Geant simulation,
<br>
with a 'realistic' beam parameters and ray tracing through the
<br>
quad and the dipole magnets, GlueX-doc-1368, Fig 15.
<br>
<br>
The quadrupole field has to be optimized for runs with a 5 mm
<br>
collimator for the best end-point efficiency. Currently we
<br>
optimized it for the best vertical resolution in the microscope.
<br>
<br>
<br>
2. It may be a good idea to move the goniometer a little bit
closer
<br>
to the dipole, though it might be too late. As the quad is
positioned
<br>
close to the goniometer, we will need to move the quadrupole as
well.
<br>
I recall that there was a discussion about this long time ago; may
be
<br>
I've missed something important regarding the correct positions.
<br>
<br>
We can check with accelerator people how critical the current
<br>
goniometer position is; they have several monitors in front
<br>
of the goniometer; the quad stands may have already been
installed.
<br>
<br>
<br>
3. Pushing to .980 would be Ok. I would also install a few more
<br>
counters at the end point region (the counters are wide in this
area)?
<br>
<br>
Ideally it would be nice to double the number of counters from the
<br>
end-point to the microscope, we will need like 80 more (seems to
be too
<br>
expensive, though). The fixed-array energy resolution is dominated
by the
<br>
counter size for Ee ~> 0.5 GeV. Any thoughts about this ?
<br>
<br>
<br>
Cheers,
<br>
Alex
<br>
<br>
<br>
<br>
<br>
<br>
On Thu, 28 Jun 2012, Daniel Sober wrote:
<br>
<br>
<blockquote type="cite">Dear Alex and Richard,
<br>
Sorry for the error. I should get a full set of magnet
dimensions so that I can do things correctly the first time.
<br>
Attached is the calculation for the 3 cm gap. If there is any
dedicated amorphous-radiator running with interest in the
endpoint region, one could improve things by putting the
radiator closer: a distance of 1.5 m (instead of 3.19 m) would
increase the gap acceptance from .858 to .948 at k/E0 = 0.980
and from .895 to 0.962 at k/E0=0.975 (11.7 GeV) without
substantially changing the energy resolution all the way down to
the microscope.
<br>
My primary goal in resuscitating these codes is to work out the
counter placement at the high energies. Should I consider
pushing to .980 (11.76 GeV)?
<br>
Dan
<br>
<br>
On 6/27/2012 9:22 PM, Richard Jones wrote:
<br>
<blockquote type="cite">Dan,
<br>
<br>
Some things to keep in mind:
<br>
<br>
1. remember the quadrupole is vertically focusing, and can be
tuned
<br>
to improve things near the endpoint when the physics
requires
<br>
endpoint energies
<br>
2. the gap is 3cm
<br>
3. there is significant scraping at 11.7 GeV under GlueX
running
<br>
conditions
<br>
4. we chose 11.7 GeV because things get impossible above
that, even
<br>
with the quad
<br>
<br>
-Richard J.
<br>
<br>
1. On 6/27/2012 4:49 PM, Daniel Sober wrote:
<br>
<br>
<blockquote type="cite">I have put the current tagger magnet
into my old codes and come up with at least one interesting
result that needs investigating: Using a realistic
bremsstrahlung calculation integrated over photon angles
(Maximon and Lepretre, 1985) for an amorphous gold radiator,
<br>
the fraction of the bremsstrahlung electron cone clearing
the 2 cm magnet gap gets bad rather quickly as k/E0 >
0.95, with only 81% transmitted at k/E0 = 0.98. See the
attached files, one for the full range and the other in fine
steps near the endpoint. Some of the numbers in the header
(especially "FULL-ENERGY ANGLE") may not make sense to you,
but they generate what we need. The second column (Gap
frac.) gives the fraction of electrons clearing the 2 cm
gap, neglecting the Rogowski chamfer which will make things
a little better -- I will need a detailed drawing of the
pole shape to account for this effect. The subsequent
columns give the fraction passing through a given detector
full width. (The "negative" fractions just flag the cases
where the magnet gap is the limiting aperture.)
<br>
<br>
I am not set up to calculate coherent bremsstrahlung or the
effect of photon collimation, but with some work I could
plug in the appropriate electron angular distributions if I
had them.
<br>
<br>
Dan
<br>
<br>
-- <br>
/Daniel Sober
<br>
Professor
<br>
Physics Department
<br>
The Catholic University of America
<br>
Washington, DC 20064
<br>
Phone: (202) 319-5856, -5315
<br>
E-mail: <a href="mailto:sober@cua.edu">sober@cua.edu</a> <a href="mailto:sober@cua.edu"><mailto:sober@cua.edu></a>/
<br>
</blockquote>
<br>
<br>
</blockquote>
<br>
-- <br>
/Daniel Sober
<br>
Professor
<br>
Physics Department
<br>
The Catholic University of America
<br>
Washington, DC 20064
<br>
Phone: (202) 319-5856, -5315
<br>
E-mail: <a href="mailto:sober@cua.edu/">sober@cua.edu/</a>
<br>
<br>
<br>
<br>
</blockquote>
</blockquote>
<br>
<div class="moz-signature">-- <br>
<font color="#ff0000"><i>Daniel Sober<br>
Professor<br>
Physics Department<br>
The Catholic University of America<br>
Washington, DC 20064<br>
Phone: (202) 319-5856, -5315<br>
E-mail: <a href="mailto:sober@cua.edu">sober@cua.edu</a></i></font><br>
</div>
<br>
<br>
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