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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>
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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>
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Eugene</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Mark-Macrae Dalton <dalton@jlab.org><br>
<b>Sent:</b> Wednesday, October 14, 2020 8:58 AM<br>
<b>To:</b> Richard Jones <richard.t.jones@uconn.edu><br>
<b>Cc:</b> Eugene Chudakov <gen@jlab.org>; Hall D beam working group <halld-tagger@jlab.org><br>
<b>Subject:</b> Re: [Halld-tagger] [EXTERNAL] Re: About monitoring of the photon circular polarization</font>
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<div class="">Hi all,</div>
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<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>
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<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>
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<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>
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<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>
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<div class="">Best,</div>
<div class="">Mark</div>
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<div class="">On Oct 14, 2020, at 7:19 AM, Richard Jones <<a href="mailto:richard.t.jones@uconn.edu" class="">richard.t.jones@uconn.edu</a>> wrote:</div>
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<div dir="ltr" class="">Eugene and all,
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<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>
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<div class="">-Richard</div>
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<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="">rjones30@gmail.com</a>> wrote:<br class="">
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<div dir="ltr" class="">Eugene and all,
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<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>
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<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="">gen@jlab.org</a>> wrote:<br class="">
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<span class="" style="color:rgb(156,101,0)"></span>*Message sent from a system outside of UConn.*</div>
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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>
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Eugene</div>
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