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