Lesson 2.2 - Interactions with Scintillator Materials
Electromagnetic radiation can interact with matter via
- Photoelectric Effect
- Compton Effect
- Pair Production
Pair Production only occurs at energies above 1.02 MeV. In practice, all effects have a chance to occur, this chance being proportional to the energy of the radiation and the atomic number (Z-value) of the absorber (the scintillation material).
Interactions with Electromagnetic Radiation
Figure 2.1 - Photoelectric effect
In the Photoelectric effect, all energy of the radiation is converted into light. This effect is important when determining the actual energy of the impinging X-ray or gamma-ray photons. The lower the energy and the higher the Z-value, the larger the chance of the photo effect.
Figure 2.2 - Photoelectric effect
In real applications, several interaction processes play a role. These interactions are illustrated below.
Figure 2.3 - Pulse Height Spectrum
Shown below is a typical pulse height spectrum measured with a 76 mm diameter by 76 mm high NaI(Tl) crystal in which the radiation emitted by a 137Cs source is detected. The Photopeak, Compton maximum (referred to as the Compton edge) and Backscatter peak are indicated in the spectrum below. The lines around 30 keV are barium X-rays also emitted by the source. The Compton scatter appears below the gamma photopeak with its edge at about 437 keV. The backscatter peak for 137Cs is typically just over 200 keV. The baseline of the spectrum is referred to as the continuum and in addition to Compton and backscatter contains terrestrial background. Terrestrial background from the three decay chains (238U, 232Th and 235U) plus 40K is referred to as Naturally Occurring Radioactive Material (NORM) background. The contribution of NORM background to the continuum is typically low (~5-10 μrem/hr in the US), however, in some areas above sea level NORM background can exceed 30 μrem/hr. Typically Asian countries (e.g., China and Japan) will have high levels of NORM even at sea level. When high levels of NORM occur it is common to see prodigy of terrestrial background forming small peaks in the continuum.
Photopeak Counting Efficiency
The total detection efficiency (counting efficiency) of a scintillator depends on the size, thickness, and density of the scintillation material. However, the Photopeak Counting Efficiency (important in gamma-ray spectroscopy), is a strong function of the scintillation material and increases with the Z of the scintillator. At energies below 100 keV, electromagnetic interactions are dominated by the Photoelectric effect.
Photoelectric absorption is the ideal process as described above. However, as the photon energy increases Compton scattering increases. Compton interaction takes place between the incident gamma-ray photon and an electron (called the recoil electron) in the absorbing material (scintillator). In Compton scattering, the incoming photon does not impart all of its energy to the electron and is deflected through an angle with respect to its original direction. Because all angles of scattering are possible, the energy transferred to the electron can vary from zero to a large fraction of the gamma-ray photon. The maximum fraction of this energy then forms what is known as the Compton Edge.
The third significant gamma-ray interaction is pair production. When the incoming gamma-ray energy exceeds twice the rest-mass of an electron (1.02 MeV), the process of pair production is energetically possible. As a practical matter, the probability of this interaction remains very low until the gamma-ray energy approaches several MeV. This interaction must also take place very close to the nucleus of an atom. When these conditions are met the gamma-ray photon disappears and is replaced by an electron-positron pair. Because the positron will annihilate after slowing down, two annihilation photons are normally produced as secondary products of the interaction (511 keV each).
Backscattering of Gamma Rays
When observing pulse height spectra from gamma-ray detectors a small peak is often seen in the vicinity of 200 – 250 keV. This is called the backscatter peak. This peak is caused by gamma rays from the source which have first interacted by Compton scattering in one of the materials surrounding the detector (usually the stainless steel housing but could also be any hard surface outside the detector). Any large scattering angle will result in scattered photons of nearly the same energy. High monoenergetic sources like 137Cs will give rise to many scattered gamma rays whose energy will form a small peak just above 200 keV as seen in Fig. 2.3 above.