Figure 1.

Schematic of model geometry and coordinate system. The view is a vertical cut through the CsI crystal looking from the side with the air interface on the top and the detector layer at the bottom of the schematic. The infinitely thin incident x-ray beam enters at an angle of 90° minus θ from the x axis (ϕ, 90° minus the angle between the x-ray beam and the y axis, is assumed to be zero in this diagram). The y axis is going out of the page in this view.