The Graal of particle physics

A new particle physics experiment uses a very different setting-the European Synchrotron Radiation Facility electron ring in Grenoble. Undulators are normally composed of physical magnets, but Graal uses a micro-undulator in the $9 ESRF straight section D7, which is a beam of ultraviolet laser light moving against the electron beam. The electrons scatter the laser photons, transferring energy to them and producing narrowly collimated gamma rays. At the ESRF, a $9 laser beam in the near-ultraviolet produces gamma rays with a maximum energy of 1.47 GeV, a maximum linear polarization of 98and an intensity of a few millions of photons per second. In this way, Graal extends the ESRF resolving $9 power to nuclear and that of nucleon structure down to a spatial resolution of 0.2 fm (0.2*10/sup -13/ cm). The first advantage of this technique over normal electron bremsstrahlung synchrotron radiation is the almost flat energy $9 spectrum. Polarization is the second advantage: photons scattered in the electron direction maintain their polarization. Therefore, at the higher end of the spectrum, the polarization is very close to that of the laser light. $9 Rotating or changing the polarization of the gamma rays is easily accomplished by rotating or changing the polarization of the laser light. (0 refs).