After
reading this section you will be able to do the following:
Discuss
the effect that matter has on radiation
Explain
what happens to radiation as it interacts with 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.
Review:
When radiation
encounters a material, some of the energy will be absorbed through
interactions subatomic particles.
More radiation
will be absorbed by materials with high atomic numbers (generally
more dense materials) because there are more subatomic particles
to interact with the radiation.
Energy
can never be created or destroyed; therefore, the energy does
not disappear but is converted into something other form.
