Gr.A.A.L. photon beam

The high energy photon beam line of the GR.A.A.L. project is installed at a bending magnet port with a modified front-end that allows the tagging of the gamma-rays and the monitoring of the laser beam.

The interaction region is situated in the vacuum tube of the straight section of the storage ring between two bending magnets. Here the photons of an 8 W argon laser of energy 3.53 eV (351.2 nm) are scattered on the 6 GeV electrons stored in the ESRF. The result of this backscattering is an high energy gamma-ray beam with maximum energy of 1.5 GeV.

The energy spectrum of the GRAAL beam
with the UV laser light measured with the
microsptrips of the tagging counter.
The maximum intensity is:
 5x10^6 gamma/s

This Compton scattering process can be described easily because of the extremely relativistic nature of the participating electrons. The incident laser photon beam makes an angle phi with the velocity vector of the initial electron and is scattered back with an angle theta refered to the electron's initial direction. Two successive Lorentz transformations (from the laboratory frame to the rest frame of the electron and back to the laboratory frame) boost the photon energy each time by a factor of 2 *gamma. This relativistic factor
gamma= Ee/mc^2
is so large that the angle of the scattered photon is compressed to an extremely narrow cone ( 1 mrad) while phi looses almost all its influence on the final *gamma-ray.

The kinematic description is also facilitated as only terms of order 1/gamma are retained. So to an excellent approximation the energy of the backscattered photon is given by
where EL is the energy of the laser photon.

A maximum photon energy is produced when the laser photon is scattered through 180 degrees (theta= 0).

One of the most interesting features of this high energy photon beam is its polarisation that is obtained very easily: the laser produces completely polarized photons whose polarisation is kept during the backscattering process. For an energy near to the maximum circular or linear polarizations of nearly 100 per cent are obtained.

degree o linear (red line) and circular (blue line) polarisation
as a funcion of the photon energy when using UV laser beam

At the exit of the bending magnet one finds the tagging region where the distance of the trajectory of those electrons having interacted with the photons and having lost a part of their energy, to the normal trajectory is measured by silicon microstrips detectors providing information about energy of the gamma-rays produced. An internal tagging system where the scattered electrons will be momentum analysed by the magnets of the storage ring has been chosen.