I am assuming you want the x-ray attenuation coefficient that is applicable to the equation I=I0*exp(-mt) where I0 is the incoming flux, I is the exiting flux and m is the attenuation length. XRD data is not well-suited for this measurement.

Perhaps you can use the transmitted part of the beam to make some type of measurement. To do this you could mount a x-ray diode in the transmitted beam path. The ratio of the x-ray diode signal with and without the sample present yields the ratio I/I0. From this ratio and the sample thickness you can get an estimate of the attenuation length, m. Note that there are many caveats here including the spectrum of the x-ray source (coupled with the wavelength dependence of the attenuation length and the x-ray diode), the stability of the source (in order to make a comparison of the signal with and without the sample present). However, depending on how you want to use the measurement, the level of accuracy you obtain may be sufficient.

I am assuming you want the x-ray attenuation coefficient that is applicable to the equation I=I0*exp(-mt) where I0 is the incoming flux, I is the exiting flux and m is the attenuation length. XRD data is not well-suited for this measurement.

Perhaps you can use the transmitted part of the beam to make some type of measurement. To do this you could mount a x-ray diode in the transmitted beam path. The ratio of the x-ray diode signal with and without the sample present yields the ratio I/I0. From this ratio and the sample thickness you can get an estimate of the attenuation length, m. Note that there are many caveats here including the spectrum of the x-ray source (coupled with the wavelength dependence of the attenuation length and the x-ray diode), the stability of the source (in order to make a comparison of the signal with and without the sample present). However, depending on how you want to use the measurement, the level of accuracy you obtain may be sufficient.

I played with the method James Mcnaney described above in order to get experimental absorption coefficients for X-ray powder diffraction experiments in flat-plane transmission geometry. It is no problem to do these measurements using the primary beam on any laboratory powder diffractometer. You just have to apply an appropriate absorber (e.g. Cu foil) to bring down your primary beam intensity to a level your detector can handle. Too high intensity destroys the linearity of the detection curve and eventually the detector!

However, this works only for thick layers of low-absorbing samples, e.g. organic molecule structures. Otherwise the transmitted intensity will be dominated by those rays that went through the gaps between the grains of the samples.

Absorption coefficients are well know and easily calculated. You cannot get them from an XRD pattern. You must use known absorption coefficients to correct for intensities in an XRD pattern in order to get a proper structure. You must also take into account the geometry of the instrument. The shape and size of the sample and beam must be known. The most common sign that you have not correctly corrected for absorption would be finding negative thermal factors after a converged fit.