2.6 Mechanism of X-ray Detection
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2.6.3 Electron–Positron Pair Effect
The so-called positron is the antiparticle of the electron. Generally, the electron–
positron pair production is a phenomenon of nature where energy is directly converted
to matter. The phenomenon of pair production can be viewed in two different ways:
one way is as a particle and antiparticle, and another is as a particle and hole, in
which the first way can be represented by formation of electron and positron, from
a packet of electromagnetic energy traveling through matter.
When the mass of the X-ray photon incoming matter is greater than two times that
of the rest electron, a pair of positive and negative electrons will be produced by the
disappearance of the incident photon in a nucleon Coulomb field. This phenomenon
is referred to as electron–positron pair effect. Typically, the threshold energy of the
pair effect is 1.022 MeV. With the incident photon energy increasing, the probability
generating the positive and negative electron pairs will gradually increase. According
to law of the conversation of energy and momentum, the positive and negative electrons commonly share the energy of the incident photons, and certainly the recoil
nuclei also share a part of the energy, but their momentums are very small. When a
positron comes to rest, it interacts with another electron, resulting in the annihilation
of the both particles and the complete conversion of their rest mass back to pure
energy in the form of two oppositely directed 511 keV gamma-ray photons. That is
to say, the positive and negative electrons are annihilated to produce two gamma-ray
photons with the energy of 511 keV. Therefore, the electron–positron pair effects
are mostly used to detect the photons from the higher energy spectrum bands, like
hard X-rays and gamma rays. The probability of pair production, characterized by
the cross section, is a very complicated function based on quantum mechanics. In
general, the cross section increases approximately with the square of the atomic
number and with the photon’s energy, but this dependence is much more complex.
2.7 Classification of X-ray Detectors
In application of the XPNAV, the X-ray detectors are designed and made on the
basis of the interaction ways between the X-ray photons and the detecting materials. Two interaction ways, the photoelectric effect and the Compton scattering
effect, are mostly utilized in the detector design for the XPNAV. By measuring
the secondary electrons produced in the interaction of the X-ray photons with the
detecting materials, the incident photons can be detected indirectly.
63
2.6.3 Electron–Positron Pair Effect
The so-called positron is the antiparticle of the electron. Generally, the electron–
positron pair production is a phenomenon of nature where energy is directly converted
to matter. The phenomenon of pair production can be viewed in two different ways:
one way is as a particle and antiparticle, and another is as a particle and hole, in
which the first way can be represented by formation of electron and positron, from
a packet of electromagnetic energy traveling through matter.
When the mass of the X-ray photon incoming matter is greater than two times that
of the rest electron, a pair of positive and negative electrons will be produced by the
disappearance of the incident photon in a nucleon Coulomb field. This phenomenon
is referred to as electron–positron pair effect. Typically, the threshold energy of the
pair effect is 1.022 MeV. With the incident photon energy increasing, the probability
generating the positive and negative electron pairs will gradually increase. According
to law of the conversation of energy and momentum, the positive and negative electrons commonly share the energy of the incident photons, and certainly the recoil
nuclei also share a part of the energy, but their momentums are very small. When a
positron comes to rest, it interacts with another electron, resulting in the annihilation
of the both particles and the complete conversion of their rest mass back to pure
energy in the form of two oppositely directed 511 keV gamma-ray photons. That is
to say, the positive and negative electrons are annihilated to produce two gamma-ray
photons with the energy of 511 keV. Therefore, the electron–positron pair effects
are mostly used to detect the photons from the higher energy spectrum bands, like
hard X-rays and gamma rays. The probability of pair production, characterized by
the cross section, is a very complicated function based on quantum mechanics. In
general, the cross section increases approximately with the square of the atomic
number and with the photon’s energy, but this dependence is much more complex.
2.7 Classification of X-ray Detectors
In application of the XPNAV, the X-ray detectors are designed and made on the
basis of the interaction ways between the X-ray photons and the detecting materials. Two interaction ways, the photoelectric effect and the Compton scattering
effect, are mostly utilized in the detector design for the XPNAV. By measuring
the secondary electrons produced in the interaction of the X-ray photons with the
detecting materials, the incident photons can be detected indirectly.