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Half-Value Layer (Shielding)

As was discussed in the radiation theory section, the depth of penetration for a given photon energy is dependent upon the 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 is the smaller the depth of radiation penetration will be. Materials such as depleted uranium, tungsten and lead have high Z numbers, and are therefore very effective in shielding radiation. Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults.

Since different materials attenuate radiation to different degrees, a convenient method of comparing the shielding performance of materials was needed. The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level. 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 for that material. Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value.

Sometimes shielding is specified as some number of HVL. For example, if a Gamma source is producing 369 R/h at one foot and a four HVL shield is placed around it, the intensity would be reduced to 23.0 R/h.

Each material has its own specific HVL thickness. Not only is the HVL material dependent, but it is also radiation 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. Below are some HVL values for various materials commonly used in industrial radiography. As can be seen from reviewing the values, as the energy of the radiation increases the HVL value also increases.

Approximate HVL for Various Materials when Radiation is from a Gamma Source

 Half-Value Layer, mm (inch) Source Concrete Steel Lead Tungsten Uranium Iridium-192 44.5 (1.75) 12.7 (0.5) 4.8 (0.19) 3.3 (0.13) 2.8 (0.11) Cobalt-60 60.5 (2.38) 21.6 (0.85) 12.5 (0.49) 7.9 (0.31) 6.9 (0.27)

Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source

 Half-Value Layer, mm (inch) Peak Voltage (kVp) Lead Concrete 50 0.06 (0.002) 4.32 (0.170) 100 0.27 (0.010) 15.10 (0.595) 150 0.30 (0.012) 22.32 (0.879) 200 0.52 (0.021) 25.0 (0.984) 250 0.88 (0.035) 28.0 (1.102) 300 1.47 (0.055) 31.21 (1.229) 400 2.5 (0.098) 33.0 (1.299) 1000 7.9 (0.311) 44.45 (1.75)

Note: The values presented on this page are intended for educational purposes. Other sources of information should be consulted when designing shielding for radiation sources.