https://www.researchgate.net/post/What_is_the_approximate_probing_depth_of_an_X-Ray

Dear Kuni Ulya ,

for the derivation of the estimation of the mean information depth please see for example:

Presentation Penetration Depth For General X-Ray Set-Up

But remind, that this relation is due to the exponential behavior of x-ray attenuation, and its value has to be regarded as a 'mean' depth. Actually you will still get a lot of x-rays arising from deeper sections of the sample.

So for reducing XRD signals from the sample substrate/support, you should go for sample thicknesses of twice or even 3 times of that value.

For very thin samples it is advised to go for grazing incidence XRD (GIXRD).

For x-ray depths around the so-called 'critical angle of total reflection' please see for example:

Presentation Attenuation Length and Penetration Depth of X-rays

Best regards

G.M.