X-ray Interactions

This animation shows the two most common interactions between X-ray photons and matter (in this case a Neon atom) in diagnostic radiology. These interactions are Photo Electric Absorption and Compton Scatter.

Notice that Photo Electric interactions occur when the X-ray photons hit an inner shell (tightly bound) electron depositing all their energy in the Neon atom. This ejects the inner shell electron causing an outer shell electron to move into the inner shell. When the outer shell electron jumps to the inner shell a relatively low energy characteristic X-ray is emitted but is not being displayed in the animation.

Compton Scatter, on the other hand, occurs when the X-ray photons interact with outer shell electrons. In this, case the photons deposit a fraction of their energy and cause ejection of the electron (recoil electron) from their shell. The photon will then change their path and have a longer wavelength (i.e., less energy). The wavelength change of the scattered photon can be determined by 0.024 (1- cos θ), where θ is the scattered photon angle.

The slider on the top can be used to change the wavelength of the X-ray photon from short (far left) to long (far right). Notice how the frequency (cycles per second) increases with short wavelengths.

Finally, the rotating atom and the movement of photons as waves was included to depict the probabilistic nature of these interactions. As in many cases the X-rays might not interact with the Neon atom. The real mechanisms behind these processes are more complex than this simplified illustration.

Are you an educator in Oral Radiology, or X-ray physics at the University Level? Educational resources require further testing and evaluation of teaching impact – currently looking for collaborations in scholarly research (email scobos@uwo.ca)

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