From a pure mathematical perspective, no amount of shielding material will block 100% of the beam. But because of the exponential nature of the depth function, you can practically always build a thick enough shield to reduce the intensity below the noise threshold of your detector.
Using the link you provided, I generated the following plot of the transmission of X -rays through water. If you look at the top of the graph, which is a photon energy of about 30,000 eV, this is still somewhat less than commonly used for medical X -rays.
At that distance, the attenuation length for 30,000 eV photons is about 30,000 microns, which is about 30 mm. Hence, you need the better part of one meter of water to stop 90% of the photons from penetrating.
That is photons interacting with inner shell electrons of oxygen atoms. The evidence of this can be seen with the characteristic rise in attenuation leading up to a sharp edge in the attenuation versus energy plot which corresponds to a photoelectric absorption edge for oxygen atoms at 0.54 key.
The other lighter area in the middle of the image is the heart. The muscle which makes up the heart is also quite dense and stops X -rays passing through.
So, dense parts of the body that block many of the X -rays show up as white (such as bones). Soft tissues (such as muscle and body organs) show up as various shades of gray, depending on how dense they are.
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