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<font size="+1"><tt>The electrons need to be polarized
longitudinally w.r.t. beam. This means the iron foil has to be
in a magnetic field<br>
to provide polarized electrons in the F?-shell. the clean way to
do it is using a 2T solenoid field to brute force electron<br>
polarization and the iron foil can be oriented perfectly
perpendicular to the beam. For the solenoid this means a
superconducting magnet<br>
like used initially in HallC-moller (later on replaced by a
better compatible solenoid). A low field option with in-plane
polarization<br>
is probably not the way to go. The radiator becomes too thick
ect.<br>
Having said this, such a solenoid has to be either all the way
downstream of any detector or upstream in the alcove<br>
preferably before the static collimator and the sweeping magnet.<br>
In order to minimize background it is probably important to
detect both the electron and the scattered photon,<br>
as Mark pointed out, since the initial photon energy is not
known apriori.<br>
<br>
Beni<br>
</tt></font><br>
<div class="moz-cite-prefix">On 10/14/20 11:45 AM, Eugene Chudakov
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:BLAPR09MB6657B9A3AE023CA23908D48FC7050@BLAPR09MB6657.namprd09.prod.outlook.com">
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<div style="font-family: Calibri, Arial, Helvetica, sans-serif;
font-size: 12pt; color: rgb(0, 0, 0);">
The electron beam polarimetry with backscattering of the laser
light is quite different kinematically. At JLab energies in the
electron rest frame it would be only about 10keV for the photon.
The angular distribution is very wide and the analyzing power is
small. Then it is boosted forward. The Compton analyzing power
flips the sign at pi/2 scattering angle in the e- rest frame.
Therefore, such polarimeters have the opposite analyzing power
at small k/k0. At high energies the analyzing power is high and
large scattering angles are killed by the angular dependence of
the cross section.</div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif;
font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif;
font-size: 12pt; color: rgb(0, 0, 0);">
PRIMEX detects Compton events using FCAL+CCAL. I would consider
a symmetric scattering (k/k0 ~ 0.5) with both particles going to
CCAL (~8cm from the beam). The foil can be located at the exit
of the solenoid. The initial photon energy is known from the
tagger. </div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif;
font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif;
font-size: 12pt; color: rgb(0, 0, 0);">
Eugene</div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif;
font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<hr style="display:inline-block;width:98%" tabindex="-1">
<div id="divRplyFwdMsg" dir="ltr"><font style="font-size:11pt"
face="Calibri, sans-serif" color="#000000"><b>From:</b>
Mark-Macrae Dalton <a class="moz-txt-link-rfc2396E" href="mailto:dalton@jlab.org"><dalton@jlab.org></a><br>
<b>Sent:</b> Wednesday, October 14, 2020 8:58 AM<br>
<b>To:</b> Richard Jones <a class="moz-txt-link-rfc2396E" href="mailto:richard.t.jones@uconn.edu"><richard.t.jones@uconn.edu></a><br>
<b>Cc:</b> Eugene Chudakov <a class="moz-txt-link-rfc2396E" href="mailto:gen@jlab.org"><gen@jlab.org></a>; Hall D beam
working group <a class="moz-txt-link-rfc2396E" href="mailto:halld-tagger@jlab.org"><halld-tagger@jlab.org></a><br>
<b>Subject:</b> Re: [Halld-tagger] [EXTERNAL] Re: About
monitoring of the photon circular polarization</font>
<div> </div>
</div>
<div class="" style="word-wrap:break-word;
line-break:after-white-space">
<div class="">Hi all,</div>
<div class=""><br class="">
</div>
<div class="">I agree that this is potentially promising and I
am also impressed by how large the analyzing power is in this
configuration. However, this is something that would need to
be studied in detail.</div>
<div class=""><br class="">
</div>
<div class="">In the Compton polarimetry that we are used to in
Halls A and C, the energy of both incoming particles is known.
The tricks that are employed are to either integrate over the
outgoing photon energies or to measure the shape of the
electron spectrum. Very seldom are both outgoing particles
detected together even for diagnostics. I’m not aware of an
asymmetry ever being done using both particles in coincidence,
anywhere in the world.</div>
<div class=""><br class="">
</div>
<div class="">
<div class="">In Hall D the energy of the incoming photon
would be unknown. We would need to detect both the outgoing
particles and with enough resolution to reconstruct both the
energy of the incoming photon and the kinematics of the
outgoing particles so that the expected analyzing power
would be known. Note how quickly it varies with photon
angle before the rate disappears. The products are produced
with such small angles that for PRIMEX most of the length of
the hall is used to detect them. We would need to find a
way to detect the photons upstream of the target.</div>
</div>
<div class=""><br class="">
</div>
<div class="">So, it’s not an insignificant challenge, although
not having to produce a complicated laser setup and thread an
electron beam through it would certainly be easier.</div>
<div class=""><br class="">
</div>
<div class="">Best,</div>
<div class="">Mark</div>
<div class=""><br class="">
<div><br class="">
<blockquote type="cite" class="">
<div class="">On Oct 14, 2020, at 7:19 AM, Richard Jones
<<a href="mailto:richard.t.jones@uconn.edu" class=""
moz-do-not-send="true">richard.t.jones@uconn.edu</a>>
wrote:</div>
<br class="x_Apple-interchange-newline">
<div class="">
<div dir="ltr" class="">Eugene and all,
<div class=""><br class="">
</div>
<div class="">Yes, that could work, I was not aware
the Compton analyzing power is so high for 3 GeV
photons. I think what they have in Hall A is
different. The Hall A Compton is using a circularly
polarized laser beam amplified in a cavity to
measure the electron beam circular polarization. In
the Hall A Compton, the analyzing power for electron
beam circular polarization is a few percent, much
lower than what Eugene showed for a 3 GeV photon
beam. So measuring directly the photon beam circular
polarization is potentially much easier than what
they do in Hall A/C. We would probably do it using a
setup similar to what Primex has for their Compton
measurement, and we would just need to be able to
insert a polarized foil into the beam.
Interesting, sounds relatively simple and cheap.</div>
<div class=""><br class="">
</div>
<div class="">-Richard</div>
</div>
<br class="">
<div class="x_gmail_quote">
<div dir="ltr" class="x_gmail_attr">On Wed, Oct 14,
2020 at 7:16 AM Richard Jones <<a
href="mailto:rjones30@gmail.com" class=""
moz-do-not-send="true">rjones30@gmail.com</a>>
wrote:<br class="">
</div>
<blockquote class="x_gmail_quote" style="margin:0px
0px 0px 0.8ex; border-left:1px solid
rgb(204,204,204); padding-left:1ex">
<div dir="ltr" class="">Eugene and all,
<div class=""><br class="">
</div>
<div class="">Yes, that could work, I was not
aware the Compton analyzing power is so high for
3 GeV photons. I think what they have in Hall A
is different. The Hall A Compton is using a
circularly polarized laser beam amplified in a
cavity to measure the electron beam circular
polarization. In the Hall A Compton, the
analyzing power for electron beam circular
polarization is a few percent, much lower than
what Eugene showed for a 3 GeV photon beam. So
measuring directly the photon beam circular
polarization is potentially much easier than
what they do in Hall A/C. We would probably do
it using a setup similar to what Primex has for
their Compton measurement, and we would just
need to be able to insert a polarized foil into
the beam. Interesting, sounds relatively simple
and cheap.</div>
<div class=""><br class="">
</div>
<div class="">-Richard</div>
</div>
<br class="">
<div class="x_gmail_quote">
<div dir="ltr" class="x_gmail_attr">On Wed, Oct
14, 2020 at 3:06 AM Eugene Chudakov <<a
href="mailto:gen@jlab.org" target="_blank"
class="" moz-do-not-send="true">gen@jlab.org</a>>
wrote:<br class="">
</div>
<blockquote class="x_gmail_quote"
style="margin:0px 0px 0px 0.8ex; border-left:1px
solid rgb(204,204,204); padding-left:1ex">
<div dir="ltr" class="">
<div class=""><br
class="x_webkit-block-placeholder">
</div>
<div class=""
style="background-color:rgb(255,235,156);
width:100%; border:1pt none
rgb(250,235,204); padding:10pt;
font-size:11pt; line-height:12pt;
font-family:Calibri; text-align:left">
<span class="" style="color:rgb(156,101,0)"></span>*Message
sent from a system outside of UConn.*</div>
<br class="">
<div class=""><br
class="x_webkit-block-placeholder">
</div>
<div class="">
<div class=""
style="font-family:Calibri,Arial,Helvetica,sans-serif;
font-size:12pt">
I mentioned at the beam meeting that one
can potentially detect/monitore the
circular polarization of a photon beam,
but did not remember the details.
Meanwhile, I recalled it - one can use the
Compton scattering on longitudinally
polarized electrons (in iron). I attach a
few plots showing the kinematics, the
cross section and the analyzing power. I
do not know at this time whether it is
practical or not for the GDH project.</div>
<div class=""
style="font-family:Calibri,Arial,Helvetica,sans-serif;
font-size:12pt">
<br class="">
</div>
<div class=""
style="font-family:Calibri,Arial,Helvetica,sans-serif;
font-size:12pt">
Eugene</div>
<div class=""
style="font-family:Calibri,Arial,Helvetica,sans-serif;
font-size:12pt">
<br class="">
</div>
</div>
</div>
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<pre class="moz-quote-pre" wrap="">_______________________________________________
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