Half Value Layer
The half value layer expresses the thickness of absorbing material needed for reduction of the incident radiation intensity by a factor of two. There are two main features of the half value layer:
- The half value layer decreases as the atomic number of the absorber increases. For example 35 m of air is needed to reduce the intensity of a 100 keV gamma ray beam by a factor of two whereas just 0.12 mm of lead can do the same thing.
- The half value layer for all materials increases with the energy of the gamma rays. For example from 0.26 cm for iron at 100 keV to about 1.06 cm at 500 keV.
The half value layer expresses the thickness of absorbing material needed for reduction of the incident radiation intensity by a factor of two. With half value layer it is easy to perform simple calculations.
Source: www.nde-ed.org
Table of Half Value Layers (in cm) for a different materials at gamma ray energies of 100, 200 and 500 keV.
| Absorber | 100 keV | 200 keV | 500 keV |
| Air | 3555 cm | 4359 cm | 6189 cm |
| Water | 4.15 cm | 5.1 cm | 7.15 cm |
| Carbon | 2.07 cm | 2.53 cm | 3.54 cm |
| Aluminium | 1.59 cm | 2.14 cm | 3.05 cm |
| Iron | 0.26 cm | 0.64 cm | 1.06 cm |
| Copper | 0.18 cm | 0.53 cm | 0.95 cm |
| Lead | 0.012 cm | 0.068 cm | 0.42 cm |
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