CEA Compton / T. Pussieux
The Hall A Compton Polarimeter
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.
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. Arthur Holly Compton
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.

The Saclay laboratories involved in this project are :


Compton Polarimeter last updated : 06/11/2004
The Hall A Compton Polarimeter CEA 1995 - all rights reserved