High-intensity {gamma}-ray scattering from alkali halide crystals

We have measured the fractional change {ital F} of the resonant photon fraction due to Bragg scattered from the (200), (400), and (600) planes of NaCl and NaF, as well as the (200) and (400) planes of LiF. Unlike previous measurements of this type, we have used well-collimated beams made possible by a high-intensity {gamma}-ray source ({approx}100 Ci). The values of {ital F} reflect loss in elastic intensity and are of importance since the crystals studied here are widely used as monochromating filters. We find a remarkably large fraction of the scattered radiation remains recoilless ({gt}94%), even for the (600) reflection of NaCl, and a value of {ital F}=100.0(3%) for the (200) plane of LiF shows that this crystal is an excellent {gamma}-ray monochromator and calibration for experiments with small {ital F} values. The {ital F} values can be expressed in terms of elastic and inelastic components, and as such provide information about the Debye-Waller factors of crystals.