Interaction of Radiation and Matter

In this section we will discuss how radiation interacts with matter, and conversely how matter affects radiation. We already know that radiation is capable of penetrating matter. We also know that X and gamma radiation varies in energy with respect to wavelength. As mentioned in the introduction, radiography relies on the principles of absorption and transmitted intensity to the film. So now let's consider another factor that determines how the radiation penetrates matter. Let's consider the material being radiographed.

How does matter affect radiation?

You may not realize it, but the air around you is made up of matter. X and gamma radiation will penetrate air to a considerable depth, but as with any material, air will eventually absorb the radiation. What if we were radiographing the hand of a human being? Would the radiation penetrate the tissue the same as it would with air? The answer is yes, but with less depth of penetration. This is because the human body is more dense than air. If we were radiographing a piece of metal, the depth of penetration would be even less than that of the human tissue for the same reasons.

The principle concept here is that radiation will penetrate light materials better than it will heavy (dense) materials. Heavier, more dense materials offer greater resistance to radiation penetration because they absorb more of the energy. This seems logical if you consider the number of atoms that make up air versus the number of atoms that make up steel and that each atom has the potential to absorb some the energy of the radiation.

Remember our discussion on the atomic structure. Atoms with more subatomic particles will be harder for the radiation to travel through without interaction with the particles. Think about when you go to the dentist, and you get the x-ray taken of your teeth. What happens to you before they take the picture? Normally, you have a lead apron draped over your chest, this is a protective measure to shield your internal organs from the radiation. Lead is often used as a radiation shielding material because it has a high number of subatomic particles and it is a relatively common element making it affordable to use. Look up lead on the Periodic Table of Elements and you will find that it has a high atomic number (Z number).

When radiation penetrates a material what happens?

Now we know that in addition to the energy of the radiation, the depth of penetration is also dependent on the density of the material being penetrated. But what happens to the radiation as it penetrates and interacts with the material? Remember, radiation is electromagnetic and composed of energy moving at the speed of light. When the radiation is stopped and absorbed we know that something else must happen. One of the laws of classical physics states that energy can neither be created nor destroyed, only converted from one form to another. Energy is converted in many different ways, but the energy is always there. Therefore, we know that when radiation is absorbed by a material, it must transfer its energy to the material.