There are two main features of the 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. The linear attenuation coefficient for all materials decreases with the energy of the X-rays.The linear attenuation coefficient increases as the atomic number of the absorber increases.There are two main features of the linear attenuation coefficient: The materials listed in the table are air, water, and different elements from carbon ( Z=6) through to lead ( Z=82), and their linear attenuation coefficients are given for two X-ray energies. Dependence of gamma radiation intensity on absorber thickness Where I is intensity after attenuation, I o is incident intensity, μ is the linear attenuation coefficient (cm -1), and the physical thickness of absorber (cm). The attenuation of X-rays can then be described by the following equation. The relative importance of various processes of gamma radiation interaction with matter. The sum of these probabilities is called the linear attenuation coefficient: Therefore the interactions can be characterized by a fixed probability of occurrence per unit path length in the absorber. Each of these interactions removes the photon from the beam either by absorption or scattering away from the detector direction. Then the dependence should be simple exponential attenuation of X-rays. Exponential AttenuationĪssuming that monoenergetic X-rays are collimated into a narrow beam, the detector behind the material only detects the X-rays that passed through that material without any kind of interaction with this material. Compton scattering is about constant for different energies, although it slowly decreases at higher energies. For higher energies, Compton scattering becomes dominant. As E gets larger, the likelihood of interaction drops rapidly. The probability of photoelectric absorption is approximately proportional to (Z/E) 3, where Z is the atomic number of the tissue atom and E is the photon energy. Much of this effect is related to the photoelectric effect. It turns out that higher energy photons (hard X-rays) travel through tissue more easily than low-energy photons (i.e., the higher energy photons are less likely to interact with matter). The attenuation theory is valid for X-rays and gamma rays as well. X-ray Attenuation Total photon cross-sections.Īs the high-energy photons pass through a material, their energy decreases, which is known as attenuation. According to the currently valid definition, X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus. The distinction between X-rays and gamma rays is not so simple and has changed in recent decades. X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays. Most X-rays have a wavelength ranging from 0.01 to 10 nanometers (3×10 16 Hz to 3×10 19 Hz), corresponding to energies in the range of 100 eV to 100 keV. 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. The radiation frequency is the key parameter of all photons because it determines the energy of a photon. X-rays are high-energy photons with short wavelengths and thus very high frequency. X-rays, also known as X-radiation, refer to electromagnetic radiation (no rest mass, no charge) of high energies.
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