MEASUREMENT OF γ-RAY DIFFRACTION ANGLES TO ± 0.02 SECOND OF ARC WITH A DOUBLE FLAT CRYSTAL SPECTROMETER

An optical method is described for the precision determination of the relative angle between the diffraction planes of two crystals of a double flat crystal spectrometer. An angular precision, probable error ± 0.02 second of arc, is achieved. Measurement of relative angles are made by reference to a standard ruled scale. Absolute angles are determined by combining these measurements with measurements of the optical geometry. The factors which influence the precision measurements are discussed in detail. It is concluded that γ-ray energies between 0.5 and 2 Mev can be compared with a precision of a few parts in 105.