Half-Value Layer – X-rays

X-rays, also known as X-radiation, refer to electromagnetic radiation (no rest mass, no charge) of high energies. X-rays are high-energy photons with short wavelengths and thus very high frequency. The radiation frequency is the key parameter of all photons because it determines the energy of a photon. Photons are categorized according to their energies, from low-energy radio waves and infrared radiation, through visible light, to high-energy X-rays and gamma rays.

Most X-rays have a wavelength ranging from 0.01 to 10 nanometers (3×10¹⁶ Hz to 3×10¹⁹ Hz), corresponding to energies in the range of 100 eV to 100 keV. X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays. The distinction between X-rays and gamma rays is not so simple and has changed in recent decades. According to the currently valid definition, X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.

Half-Value Layer

The half-value layer expresses the thickness of absorbing material needed to reduce 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 X-ray beam by a factor of two, whereas just 0.12 mm of lead can do the same.
• The half-value layer for all materials increases with the energy of the X-rays. For example, from 0.26 cm for iron at 100 keV to about 0.64 cm at 200 keV.

Half Value Layer – Example:

How much water shielding do you require if you want to reduce the intensity of a 100 keV monoenergetic X-ray beam (narrow beam) to 1% of its incident intensity? The half-value layer for 100 keV X-rays in water is 4.15 cm, and the linear attenuation coefficient for 100 keV X-rays in water is 0.167 cm^-1. The problem is quite simple and can be described by the following equation:

If the half-value layer for water is 4.15 cm, the linear attenuation coefficient is:Now, we can use the exponential attenuation equation:

So the required thickness of water is about 27.58 cm. This is a relatively large thickness, and it is caused by small atomic numbers of hydrogen and oxygen. If we calculate the same problem for lead (Pb), we obtain the thickness x=0.077 cm.

Half Value Layers

Table of Half Value Layers (in cm) for different materials at photon energies of 100, 200, and 500 keV.

See above:

X-rays