The count rate from a crystal spectrometer set-up in Johann geometry
is estimated by the relation

with being the number of incoming particles and being the yield of the X-ray transition under investigation. The quantity may be expressed by

is the fraction of solid angle
of the Bragg crystal seen from the source and is the crystal's peak
reflectivity. The quantity
is the fraction of the
source from which X-rays are reflected owing to the Bragg condition. For sources
which can be regarded as uniformly radiating disks of radius the
fraction is in good approximation given by

It is assumed that the dimensions of the source are small compared to the Rowland circle radius .