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In order to measure the longitudinal
polarization of the 3-6 GeV high intensity
TJNAF
electron beam, a Compton Polarimeter was built
by Saclay,
Clermont,
and
Jlab.
The Compton was used for the
HAPPEX
parity
violation experiment in 1999 and for the
N-Delta experiment
in 2000.
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| The Compton effect, light scattering
off electrons, discovered by Arthur Holly Compton (1892-1962), Nobel
prize in Physics, 1927, is one of the cornerstone of the wave-particle
duality. Compton scattering is a basic process of Quantum Electro-Dynamic,
the theory of electromagnetic (EM) interactions. This is a well established
theory, and is thus natural to use the EM interaction, such as Compton
scattering, to measure experimental quantities, such as polarization of an
electron beam. |
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Many of the Hall A experiments of Jefferson
Laboratory using a polarized electrons beam require a measurement of this
polarisation as fast and accurate as possible. Unfortunately the standard polarimeters,
like Möller, require the installation of a target in the
beam. Therefore, the polarisation measurement can not to be performed at
the same time than the data taking because the beam, after the interaction
with the target, is misdefined in terms of polarization, momentum
and position. Another physical solution has to be found in order
to permit a non-invasive polarisation measurement of the beam. This is the
principle of the Compton Polarimetry.
The Jefferson Lab electron beam,
which polarisation is flipped 30 times per second, is interacting with
a laser beam of measured circular polarisation. This physical
process is described by the Quantum ElectroDynamics (the so-called QED)
which allows to calculate the cross sections of the polarized electrons
scattering off polarized photons as a function of their energies and scattering
angle. The counting rates assymetry is directly proportionnal to the laser
and electrons beam polarizations and cross sections assymetry.
The main constraints consist on to
preserve the positions, the orientations and the physical characterictics
of the beam at the exit of the polarimeter. The backward scattering angle
of the Compton photons being very small, the first priority is to separate
these particles from the beam using a magnetic chicane. The energy of the
backward photons will be measured by an electromagnetic calorimeter, the
so-called PbWO4coming from the LHC's R & D. The third dipole
of the chicane, coupled to the electrons detector, will be used as a spectrometer
in order to measure the scattered electron momentum. To perform a quick
polarization measurement, the photon flux has to be as high as possible.
A Fabry-Pérot Cavity, made of 2 multi-layers concave mirrors with very
high reflectivity, will amplify this flux to a factor greater than 7000.
The 15 meters long Compton Polarimeter has been installed
in the last linear section of the arc tunnel, at the entrance of the Hall
A at spring 98. The complete setup, including the optical cavity was
installed in February 99 and is running successfully since then.
Last polarisation measurements have been performed in 2000 for the N-Delta et
GeP experiments representing more
than 450 measurements within 2% total error in 80 effective days. Next measurements are foreseen for
the HAPPEX II experiments early 2003.
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The Saclay laboratories involved in this project are :
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