The X-ray forward scattering coefficient of water

The X-ray (1.54 Å) scattering coefficient per unit solid angle, per unit thickness of sample has been measured for water in the forward direction. It is about eight per cent greater than the value given by density fluctuation theory. Double scattering is calculated and shown to contribute a six per cent excess flux in the forward direction in our geometry. In addition, short wavelength continuum X-rays slightly increase the measured scattering coefficient. After these corrections theory and experiment are in good agreement. The double scattered flux is sensitive to certain parameters of the diffractometer geometry. It is argued that water is not a desirable reference standard for absolute intensity calibrations.