with the GIXRD (classic theta-2theta instruments) without the cradle of Eulero, it's possibile to control the penetration depth by the incident angle in solid samples. What is the equation for the depth?
Applying Beer-Lambarts law i.e., Ix = I0 exp(-[mu/rh]o. rho. x), and apply simple take-off andle correction you can calculate the thickness x of your sample.
if you're talking about XRay Diffraction in reflexion, for the "x " instead of the thinckness of the sample you use the lenght of the path traveled by the X-Ray inside the material (geometrically related to the depth of diffraction).
Then you have to fix a value Ix/I0 (for example 0.9) to calculate that x. From x you deduce the "depth of diffraction", which will be in this case the thickness of material contributing to 90% (=Ix/I0) of the diffracted intensity.
You can not calculate the thickness corresponding to 100% of the diffracted intensity: it is infinite (because of Beer Lamber Law exponential decrease..)
Penetration depth is calculated from the mass absorption coefficient and the "density". The density value ρ is expressed as a combination of the specific gravity of the material and the packing density.
First the path length L is calculated:
IL = I0 × exp -(μ/ρ)ρL
Next the penetration depth (τ) :
τ = 0.5 L×sin(θ)
This is the thickness of the sample contributing 99% of the diffracted intensity for a given incident angle θ.
If u use HSP, there's a MAC calculator where u can calculate the penetration depth. All u need is the wavelength, chemical formulas, weight fraction of each, estimated packing density and angle of incident (w).