# Photon Attenuation Coefficients

As mentioned earlier, each individual interaction has it's own attenuation coefficients, but these attenuation coefficients may also be added together to form total attenuation coefficients. The total *linear attenuation coefficient* for ionizing photons can therefore be calculated as follows:

\frac{\mu}{\rho} = \frac{\sigma_R}{\rho} + \frac{\tau}{\rho} + \frac{\sigma}{\rho} + \frac{\kappa}{\rho}

Similarily, the total *mass-energy transfer coefficient* can be calculated as a sum of its individual components:

\frac{\mu_{tr}}{\rho} = \frac{\tau_{tr}}{\rho} + \frac{\sigma_{tr}}{\rho} + \frac{\kappa_{tr}}{\rho}

Notice again here that there is no Rayleigh scattering term, as it does not contribute to energy transfer. Finally, a small fraction g of the energy transferred to charged particles will not be absorbed by matter, so the total *mass-energy absorption coefficient* is calculated as follows:

\frac{\mu_{en}}{\rho} = \frac{\mu_{tr}}{\rho}(1-g)

In the exponential attenuation tutorial, we will see how the total linear attenuation coefficient (\mu) can be used to predict how a composite beam of photons will interact with matter. The energy-related attenuation coefficients (\mu_{tr}, \mu_{en}) also play an important role in radiation dose calculation.

## Related Links

## References and Further Reading

*Introduction to Radiological Physics and Radiation Dosimetry*. Chapter 7*AAPM Summer School 2009*. Chapter 2

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