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As with using the Half Life to describe the Radioactive Decay Law an indicator is usually derived from the exponential attenuation equation above which helps us think more clearly about what is going on. This indicator is called the Half Value Layer and it expresses the thickness of absorbing material which is needed to reduce the incident radiation intensity by a factor of two. From a graphical point of view we can say that when:
the thickness of absorber is the Half Value Layer:
The Half Value Layer for a range of absorbers is listed in the following table for three gamma-ray energies:
| Absorber | 100 keV | 200 keV | 500 keV |
|---|---|---|---|
| Air | 3555 | 4359 | 6189 |
| Water | 4.15 | 5.1 | 7.15 |
| Carbon | 2.07 | 2.53 | 3.54 |
| Aluminium | 1.59 | 2.14 | 3.05 |
| Iron | 0.26 | 0.64 | 1.06 |
| Copper | 0.18 | 0.53 | 0.95 |
| Lead | 0.012 | 0.068 | 0.42 |
The first point to note is that the Half Value Layer decreases as the atomic number increases. For example the value for air at 100 keV is about 35 meters and it decreases to just 0.12 mm for lead at this energy. In other words 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 second thing to note is that the Half Value Layer increases with increasing gamma-ray energy. For example from 0.18 cm for copper at 100 keV to about 1 cm at 500 keV. Thirdly note that relative to the data in the previous table there is a reciprocal relationship between the Half Value Layer and the Linear Attenuation Coefficient, which we will now investigate.
