ABSTRACT
In contrast to light scattering experiments in diamond anvil cells, high pressure neutron scattering in the multi-GPa range uses in almost all cases opaque anvils which exclude fluorescence techniques for the pressure determination. This is often considered a major drawback of opaque anvils, but is in practice for most measurements a minor problem, for various reasons:
1) Approximate pressure values are known for the applied load and calibration curves as given in chapter 10. These are usually sufficient to guide an experiment. In many experiments the pressure coefficient of the physical property x under investigation (such as fractional atomic coordinates, phonon frequencies etc.) is such that the statistical error in x outweighs the uncertainty in pressure. In other words an increased precision in the pressure will not significantly increase the precision in the pressure coefficient of x. If a comparison with theory is sought, then the relevant parameter is anyway not pressure but volume (or volume change), which can in most cases be accurately determined.
