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DEPTH OF PENETRATION OF RADIATION ENERGY

After reading this section you will be able to do the following:

  • Explain why radiation penetrates deeper into some materials than it does others.
  • Define half-value layer and how it can be used to compare the radiation absorption characteristics of a material.
  • Explain how radiation energy affects its penetrating power.

How deep will radiation penetrate into a material?

Now that we have looked at the interaction that the radiation has with matter, let's consider the radiation ability to penetrate materials. We know that one of the factors affecting ionization is the material type. We also know that radiation has a more difficult time penetrating dense materials, such as metal than it does less dense materials, such as plastic.

Radiation photons of the same energy will not penetrate a given material to the same depth. Some of the photons will collide with atoms and lose their energy before others. Some may pass completely through the material with minimal or no interaction. Also, the depth of penetration for a given photon energy is dependent upon material density (atomic structure). The more subatomic particles in a material (higher Z number), the greater the likelihood that interactions will occur and the radiation will lose its energy. Therefore, the more dense a material, the less the depth of radiation penetration will be.

When does the absorption of radiation start?

The absorption of radiation starts as soon as the radiation enters a material. The process is progressive and continues as the radiation penetrates deeper into the material. Additional energy is absorbed through the various processes of ionization. At some point in the material, there is a level at which the radiation intensity becomes one half that at the surface of the material. This depth is known as the Half Value Layer, (HVL) for that material. Each material has its own specific HVL thickness. Not only is the HVL material dependent, but it is also energy dependent. This means that for a given material, if the radiation energy changes, the point at which the intensity decreases to half its original value will also change.

How does radiation energy affect the depth of penetration?

If we raise the energy of the radiation interacting with the same material, the HVL will occur deeper in that material. X-rays and gamma rays with shorter wavelengths will have more energy that must be absorbed and, therefore, more energy will make it deeper into the material or through the material. Conversely, if we lower the radiation energy, the HVL will occur shallower in depth.

The following table shows some examples of the relationships between radiation energy and material types:

Energy (KeV)
Lead (mm)
Concrete (mm)
50
0.06
4.32
70
0.18
12.70
100
0.27
15.10
150
0.30
22.32
200
0.52
25.00
250
0.88
28.00
300
1.47
31.21
400
2.50
33.00
1000
7.90
44.45
2000
9.98
62.23

-- - - Review:

  1. The more subatomic particles in a material the more quickly radiation energy will be absorbed resulting in less depth of penetration.
  2. The half-value layer is the depth within a material where half of the radiation energy has been absorbed. The HVL is useful in making material comparisons.
  3. Higher energy radiation will penetrate deeper into a material before it is absorbed.