When a beam of X-rays passes through matter, its intensity falls. This is called attenuation. Photons are removed from the beam either by being absorbed or by being scattered out of it.The OCR specification lists four attenuation mechanisms. Which one happens depends mainly on the photon energy:• Simple scatter: 1 to 20 keV
• Photoelectric effect: below about 100 keV
• Compton effect: about 0.5 to 5 MeV
• Pair production: above 1.02 MeVThese ranges are approximate and they overlap, so more than one mechanism can happen at a given energy.
In simple scatter, the photon does not have enough energy to remove an electron from an atom. It interacts with an electron and bounces off (is scattered) with no change in its energy, so its wavelength is unchanged. Because it changes direction it no longer travels straight to the detector, so the beam is attenuated. Photons of such low energy are no use for imaging and are removed by the aluminium filter.The other three mechanisms remove energy from the photon or remove the photon completely; the next lesson explains each of them in detail.Worked example: which mechanisms can attenuate a 3 MeV photon? 3 MeV lies inside 0.5 to 5 MeV and above 1.02 MeV, so both the Compton effect and pair production can occur. A 60 keV diagnostic photon lies below 100 keV but above 20 keV, so the photoelectric effect is the main mechanism.
• Photoelectric effect: below about 100 keV
• Compton effect: about 0.5 to 5 MeV
• Pair production: above 1.02 MeVThese ranges are approximate and they overlap, so more than one mechanism can happen at a given energy.
In simple scatter, the photon does not have enough energy to remove an electron from an atom. It interacts with an electron and bounces off (is scattered) with no change in its energy, so its wavelength is unchanged. Because it changes direction it no longer travels straight to the detector, so the beam is attenuated. Photons of such low energy are no use for imaging and are removed by the aluminium filter.The other three mechanisms remove energy from the photon or remove the photon completely; the next lesson explains each of them in detail.Worked example: which mechanisms can attenuate a 3 MeV photon? 3 MeV lies inside 0.5 to 5 MeV and above 1.02 MeV, so both the Compton effect and pair production can occur. A 60 keV diagnostic photon lies below 100 keV but above 20 keV, so the photoelectric effect is the main mechanism.