A real photon bremsstrahlung tagger has been constructed for use with the
CLAS detector; it has a large dynamic range, from 0.20 to 0.95 of the beam
energy, and a resolution of 5 MeV. Circularly polarized, tagged
photons can be generated by using polarized electron beams, but many
experiments would benefit significantly from having linearly polarized
photon beams. These beams can be generated using coherent bremsstrahlung
from a crystal, such as diamond, and one experiment using this approach has
just been approved. Coherent bremsstrahlung produces a beam with a
relatively narrow spike at about one third the energy of the electron beam,
and with linear polarizations ranging from 30 to 60%, depending upon
collimation.
An interesting alternative approach is to develop a laser backscattering capability. A scheme has been proposed to do this by pumping a high finesse optical cavity with an Argon-Ion laser, and passing the electron beam through it. The technique here is the analog of that employed at LEGS. There, electrons are stored in a magnetic ring and given multiple chances to collide with a laser photon. Here, laser photons would be stored in an optical ring and given multiple chances to collide with an electron. With such a cavity operating in the visible (515 nm), scattering against 4 GeV electrons would produce 0.5 GeV polarized photons. With a frequency-doubled cavity (257 nm) and 6 GeV electrons, the photon energy would be increased to 1.8 GeV. With 10 GeV beams from an energy-upgraded CEBAF, 4.2 GeV photons could be produced. As a first stage in the development of this capability, funds have been requested for a feasibility study of using the 45 MeV CEBAF injector.
Experiments utilizing this source would include: the determination of
helicity amplitudes in N production, multipole decomposition of
production from
He which is particularly sensitive to multi-nucleon
currents, and threshold
production which will study the origin of
OZI suppression.