CEA Compton /OSP /General presentation / T. Pussieux
 
 
General presentation
 
In order to measure the longitudinal polarization of the 3-6 GeV high intensity TJNAF electron beam, a Compton Polarimeter was built by CEA Saclay, LPC Clermont-Ferrand, and Jefferson Laboratory. The Compton polarimeter is now running since february 1999 and has been used by severals HALL A experiments. Two collaborators from Saclay have been awarded for this apparatus :
Christian Cavata awarded the Prix Joliot-Curie 2000 of the Société Française de Physique and Maud Baylac awarded the SURA 2000 Thesis Prize.
Compton Polarimetry principles
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 (QED), the theory of electromagnetic (EM) interactions.
During 50's and 60's, the QED theoretical developments allow Klein and Nishina to compute accuratly the so-called Compton interaction cross section. Experimental physicists performed serveral experiments which are in good agerement with the predictions. This is now 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 or Mott, 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.
 From theoretical point of view
Arthur Holly Compton This physical process is described by 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 cross sections are different not equal if the incident helicity of the electron are in opposite directions. One define the theoretical cross sections asymmetry Ath by the ratio of the difference over the sum of these two quantities. With the kinematical parameters used at JLab, the mean value of these asymmetry is of order of few %.
From experimental point of view
Using a specific sdetup, the number of Compton interactions can be measured for each incident electrons helicity state (aligned or antialigned with the propagation direction). These numbers are dependant of process cross sections, luminosity at the interaction point and time of the experiment. At first order, assuming the time and luminosity are equal for the both electron helicity states, the counting rates asymmetry is directly proportionnal to the theoretical cross section asymmetry.
From one to the other
The proportionnality factor is equal to the values of the photon circular polarization Pphoton multiplied by the electron polarization Pelectron, so that :
Aexp  =  Pelectron P photon Ath
Measuring the photons polarization and experimental asymmetry, calculating theoretical asymmetry, one can deduce the electron beam polarization. One electron over a billion is interacting with the photon beam which means 100000 interactions per second. So as only few incident electrons are interacting, these polarization measurements are completly non-invasive for the electron beam in term of positions, the orientations and the physical characterictics of the beam at the exit of the polarimeter.
Compton polarimeter principles at JLab

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 PbWO4 coming 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.

The description of the Compton Polarimeter
The Compton polarimeter consists on :

The optical setup
This is our photon target ! the optical setup is made off 4 parts :
  1. a 300mW infra-red Laser,
  2. the first optical path to make in form the laser beam in terms of size and polarization,
  3. the resonant cavity which delivers more than 1kW of circularly polarized infra-red light
  4. optical devices to measure the circularly polarization of the photons at the exit of the cavity

Basics of resonant Fabry-Perot cavity for Compton polarimetry can be found in Nuclear Instruments And Methods In Physics Research Section A412 1 (1998) pp. 1-18
http://hallaweb.jlab.org/compton/Documentation/Papers/falletto.pdf

The magnetic chicane
The Compton magnetic chicane consists of 4 dipoles (1.5 T maximum field, 1 meter magnetic length) here after called D1,2,3,4. (D1,D2) deflect the electrons vertically down to steer the beam through the Compton interaction point (CIP) located at the center of the optical cavity. After the CIP, the electron are vertically up deflected (D3,D4) to reach the Hall A target. The scattered electron are momentum analyzed by the third dipole and detected thanks to 4 planes of silicon strips. The magnetic field is scaled with the beam energy, insuring the same vertical deflection at the CIP, up to 8 GeV electrons for 1.5 T field.

The Chicane parameters

  • The distance between the geometrical axis  of the dipoles (D1,MMC1P01) and (D2,MMC1P02) in the longitudinal plane is 5400 mm
  • The distance between the beam entry axis in (D1,MMC1P01) anfd the beam exit axis in (D2,MMC1P02) in the bending plane (vertical axis) is 304 mm
  • The longitudinal magnetic length on the axis of (D1,MMC1P01) and (D2,MMC1P02) is 1000 mm.

Under these conditions :

  • the bending angle is 3.22261o
  • The radius of curvature is r=17.7887 m
  • B * r (T.m) = 3.33564 p (GeV/c)
  • At the centre of (D2,MMC1P02) : B(T) = 0.1875145 p (GeV/c).

 The photon detector
To detect Compton backscattered photons, an electromagnetic calorimeter is used. It consists of 25 PBWO4 cristals (2cmx2cmx23cm) read by XP1911 Philips photomultiplier tubes and is located in the line of sight of the optical cavity, just behind the third dipole of the chicane. Details on this calorimeter can be found in Nuclear Instruments And Methods In Physics Research Section A443 2-3 (2000) pp. 231-237 http://hallaweb.jlab.org/compton/Documentation/Papers/neyret.pdf

The electron detector
It is made of 4 planes of silicon strips composed of 48 strips each of width 650 (600 + 50) microns and 500 microns thick. The planes are staggered by 200microns to allow for better resolution and the first strip of the first plane is about 8 mm away from the beam. Distance between the CIP and the first strip is 5750 mm. We recall that between the CIP and the end of the Dipole 3 is 2150 mm. For a beam of 3.362 GeV the Compton edge is at 3.170 GeV. This corresponds to a deviation of 17 mm. Thus at this energy, only one half of the Compton spectrum is covered and it extends to the 13th strip of the first plane. The trigger logic looks for a coincidence between a given number of plane in a "road" of 2 strips. For each trigger it outputs a signal check by the Polarimeter DAQ.

The fast acquisition system
The goal of this system is to acquire for each electron helicity state the energie of the scattered photons at a rate up to 100 kHz. The energy of each Compton event can be reconstructed from the signals of the 25 PMT of the photon calorimeter with front-end electronics and ADCs. Each helicity state, given by the accelerator, is also numbered. Further information is given for each event (type of event, status of the polarimeter at event's time) and for each polarization period (duration, dead time, counting rate,...).
A specific tool, the so-called spy_acq, has been developped in Tcl/Tk to manage all acquisition system parameters. Finally, a web-based logbook is available on this site at http://hallaweb.jlab.org/compton/Logbook/index.php


Compton Polarimeter last updated : 12/14/2001
General presentation CEA 1995 - all rights reserved