What does not allow X-rays to pass through?

Answer 1

Dense metals such as lead block x-rays. A much thicker layer of concrete will also work. X-rays can show internal structures because the denser materials block them, leaving a shadow on photographic film or fluoroscope screens.

Answer 2

We know that shielding attenuates X-rays. Material that is very dense (like lead) works best. By the way, different materials have what is called a half-thickness for different energy X-rays. Stopping X-rays isn't like protecting yourself from solid bits of material. X-rays will penetrate any material to some degree, but a half-thickness of a specific material will attenuate X-rays by half. Another half-thickness will attenuate the radiation by another 50%, so only a quarter of the energy gets through. You can see how things are going. With the right amount of shielding, almost all of the radiation will be stopped.