The energy of X-rays emitted from the target region is analyzed
by Bragg reflection from spherically bent single crystals. These are mounted
in the so-called Johann geometry .
The energy analysis relies on the Bragg reflection condition
The angular width of reflected monoenergetic X-rays determines the intrinsic energy resolution. It depends on the crystal properties for a given wavelength. More information about the imaging properties of bent crystals and the facts limiting the resolution of bent crystal spectroscopy are given in Appendix 1.
The single crystals with best known properties for our energy region are and (quartz) crystals. For the energy region below , which is about the series limit for the pionic hydrogen transitions, only the 111 lattice plane for and the 100 and 10.1 lattice planes for quartz can be used. Some properties of these crystals are listed in Table 4.
With quartz 100 it is also possible to measure the muonic transition. All these crystals cut to the appropiate planes are at the disposal of the experiment. Their geometrical form is a circular disk with a thickness of and a diameter of . In order to achieve spatial focussing they are spherically bent by mounting them on a concave glass lens. The radius of curvature is for three available (111) crystals and three (100) crystals. The two available (10.1) crystals are presently still mounted with which is foreseen to be changed. As the measurement aims at a final accuracy in the energy determination of about the Bragg crystals have to be temperature stabilized.