Schematic view of the interaction region. (enlarge)
Research Center for Nuclear Physics (RCNP) - Osaka University, Japan Synchrotron Radiation Research Institute (JASRI),
and Japan Atomic Energy Research Institute (JAERI) have constructed
a GeV photon beamline (BL33LEP) for nuclear and particle physics experiments at SPring-8.
Backward Compton scattering (BCS) of ultraviolet laser photons off circulating
8-GeV electron beam in the synchrotron radiation (SR) ring
produces a GeV energy polarized γ beam.
At usual operation, 350-nm Ar laser photons are injected from the Laser Hutch
into the straight section between the BM1 and BM2 bending magnets of the SR ring.
By transferring momentum from an 8-GeV electron to a laser photon via the
Compton scattering, the energy of the scattered photon becomes 2.4 GeV at the maximum.
The scattered electron goes through the BM2 magnet and is detected with a tagging counter
placed besides the CR2 chamber.
By measuring the position of the electron at the tagging counter, using the BM2 magnet as a
magnetic spectrometer, we can tag the energy of corresponding photon.
Due to the limitation for the distance between the tagging counter and the 8-GeV orbit,
the lowest energy of the tagged photon is 1.5 GeV.
The laser electron photon beam is transported to the
Experimental Hutch and used for the experiments.
Properties of the LEPS beam
Energy Spectra
The left figure shows the differential cross sections for the BCS process
between 8-GeV electrons and laser photons with different wavelengths, 488,
351, and 275 nm.
351-nm Ar laser is usually used to produce γ beam with the energy up to 2.4 GeV.
The BCS photons have flat energy distribution compared with photons produced by
Bremsstrahlung process.
Polarization
The linearly (circularly) polarized BCS photons can be made by using
linearly (circularly) polarized laser photons.
The degree of polarization of BCS photons is proportional to that of laser
photons and it is maximum at the highest Eγ point (Compton edge).
The figures below show the calculated polarizations of the BCS photons when the laser
is 100% polarized.
HOME T. Hotta
Last modified: Wed Nov 5 10:57:58 JST 2003
The LEPS Beamline
