[Halld-tagger] [EXTERNAL] Re: About monitoring of the photon circular polarization

[Richard Jones]

Eugene and all,

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.

-Richard

On Wed, Oct 14, 2020 at 7:16 AM Richard Jones <rjones30 at gmail.com> wrote:

> Eugene and all,
>
> 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.
>
> -Richard
>
> On Wed, Oct 14, 2020 at 3:06 AM Eugene Chudakov <gen at jlab.org> wrote:
>
>> *Message sent from a system outside of UConn.*
>>
>> 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.
>>
>> Eugene
>>
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