The mass attenuation coefficient (MAC) is a quantity used in calculations involving the penetration and energy deposition of photons (such as X-rays and gamma rays) in various materials, including biological tissues and shielding substances.
On this page:
Definition
The MAC is defined as the linear attenuation coefficient (μ) divided by the density (ρ) of the material, and it is expressed in cm2/g.
MAC = μ/ρ
This value represents the probability per unit mass that an X-ray or gamma photon will interact with a material via photoelectric absorption, Compton scattering, or pair production.
It is closely related but not the same value as mass energy-absorption coefficient (μen/ρ), which excludes energy carried away by scattered photons.
Applications
Comparability of attenuation across different photon energies and different substances
Unlike the linear attenuation coefficient (μ), which depends on density, MAC is normalized to the material's mass. This allows direct comparison of attenuation characteristics across different substances. For example, a given element (e.g., lead) will have the same MAC whether it is in pure form, alloyed, or part of a compound.
The National Institute of Standards and Technology (NIST)2 provides extensive tables of MAC values for various materials over a wide range of photon energies.
Calculations for Compounds and Mixtures
The MAC of composite materials—such as biological tissues, polymers, or alloys—can be determined by weighting the mass attenuation coefficients of individual components according to their mass fractions.
Use in Medical Imaging and Radiation Shielding
The MAC plays a key role in various medical physics calculations including:
X-ray attenuation in tissues and shielding materials
Radiation shielding design
Its standardized use makes them the gold standard for accurate radiation modeling.
MAC vs. Linear Attenuation Coefficient in Shielding
While MAC is fundamental for theoretical modeling, for practical X-ray shielding and beam hardening calculations, the half-value layer (HVL) is more straightforward.
HVL is directly related to the linear attenuation coefficient (μ), which depends on density, making it more convenient for determining the required material thickness to achieve a specific attenuation level.
Unable to process the form. Check for errors and try again.