STORAGE OF HARD X-RAYS IN A CRYSTAL RESONATOR, at least for as many as 14 back-and-forth cycles, has been achieved by physicists at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The resonator consists of a pair of vertical walls (each 70% reflective) 150 mm apart carved from a single crystal of silicon, thus ensuring the perfect alignment of rows of atoms in both walls.

X rays with an energy of 15.8 keV from the ESRF machine arrive in the resonator in bursts only 10^-10 seconds in duration and with a wavelength of only 7.8 x 10^-11 m. The x rays are retained by multiple Bragg scattering between the walls. Previously lower energy "soft" x rays had been stored in resonators but this is the first time for higher energy x rays. The new resonator also serves to sharpen the range of energy of x rays transmitted as compared to the range for the incoming x rays. Plenty of x-ray optics applications are expected. (Liss et al., Nature, 23 March /pnu/2000/.)