lithium fluoride as received from the vendor in boule form is 38 x 38 x 13 mm thick. this block is cleaved to wafers of the desired thickness, x-ray-evaluated for ''d'' spacing and greatest intensity, bent to the required radius, and then acid-etched to remove foreign material. the diffraction and dispersion characteristics of a wafer are analyzed using well-collimated tungsten x rays that strike the crystal and are diffracted onto no-screen x-ray film. if the crystal is satisfactory, it is mounted in a spectrogoniometer and rotated through an x-ray beam while a detector is set at the optimized angle for the diffracted x rays. the average intensity across the length of the crystal is recorded by multichannel scaling. any imperfections appear as peaks or dips compared to the average intensity. the crystal next goes to a 10-channel, filter-fluorescer x-ray unit that compares zero-order intensity to diffracted k alpha and k beta intensity. counts for 100-s intervals are taken in groups of three and averaged. correction factors for instrument geometry, air, pinhole diameter at zero order, k alpha -k beta , barometric pressure, temperature, etc., are added to the efficiency calculations to obtain the crystal efficiency (epsilon) vs kev data. the crystal is mounted in the spectrograph or spectrometer and calibrated to either the detector or film plane by using direct radiation with proper x-ray filters or absorbers. the crystal is then ready for use (era citation 02:003554)