60
2 Observations of Radio and X-ray Pulsars
and with the electrons outside atomic nuclei to lose a part of the energy. In general, the
X-ray photons have very strong penetration capability, and even the soft X-rays can
also penetrate aluminum films. And thereby, usually the X-rays cannot be focused
on by using normal optical system. When the X-ray photons interact with matter,
the material atoms absorb the photons, and then the electrons outside the atomic
nuclei escape from the atomic orbits to bring up ionizing. X-ray detectors are the
devices used to measure the flux, spatial distribution, spectrum or other properties of
the X-rays. By measuring the released energy in impact effects between the incident
X-ray photons and the major materials of the detectors, the flux of the photons can
be detected.
Generally, there are three typical ways in interactions between the high-energy
photons and the major materials of the detectors: photoelectric effect, Compton
scattering effect and electron-pair effect. The three interactions are naturally different
from one another, and each of them dominates a corresponding energy range. Of
course, each interaction is also closely related with the atomic number of the matter.
2.6.1 Photoelectric Effect
When the X-ray photons interact with matter and their energy is greater than the
electrons’ binding energy in material atomic inner shell, all energy of the photons
will be transmitted to the electrons of the atomic inner shell so that the electrons
have escaped from atomic bondage. This interaction between the X-ray photons and
matter is usually known as photoelectric effect, and the electrons produced by the
photoelectric effect are called photoelectrons, to distinguish from free electrons.
The kinetic energy of photoelectrons is equal to the photons’ energy minus the
binding energy of the electrons in the atomic inner shell. The greater is the binding
energy of the electrons in the atomic inner shell, the smaller the kinetic energy of the
photoelectrons. Besides the photoelectrons, the photoelectric effect also produces an
ionized atom, in which the holes in atomic inner shell will quickly be filled with the
electrons transited from the outer layer to the inner layer, or with the captured free
electrons. And thus, the characteristic X-rays are emitted. When the characteristic
X-rays are absorbed by the matter, the photoelectrons with the same energy will be
produced once again, which are also usually called Auger electrons. If all energies
of both the photoelectrons and the Auger electrons were lost inside the matter, the
detected energy loss will represent the energy of the incident photons.
According to the momentum conservation condition, the photoelectric effect can
occur only between the photons and the bound-state electrons. By the binding force
between electrons and atoms, the momentum of photons is transformed into the
recoil momentum of the atom. The stronger is the binding force, the greater the
probability of emitting the photoelectrons. A schematic diagram of the photoelectric
effect is shown in Fig. 2.6, where the electrons in K layer have greater photoelectric
absorption probability than those in L layer. The photoelectrons from K layer usually
account for 80% of the total of photoelectrons. In general, the photoelectric cross
2 Observations of Radio and X-ray Pulsars
and with the electrons outside atomic nuclei to lose a part of the energy. In general, the
X-ray photons have very strong penetration capability, and even the soft X-rays can
also penetrate aluminum films. And thereby, usually the X-rays cannot be focused
on by using normal optical system. When the X-ray photons interact with matter,
the material atoms absorb the photons, and then the electrons outside the atomic
nuclei escape from the atomic orbits to bring up ionizing. X-ray detectors are the
devices used to measure the flux, spatial distribution, spectrum or other properties of
the X-rays. By measuring the released energy in impact effects between the incident
X-ray photons and the major materials of the detectors, the flux of the photons can
be detected.
Generally, there are three typical ways in interactions between the high-energy
photons and the major materials of the detectors: photoelectric effect, Compton
scattering effect and electron-pair effect. The three interactions are naturally different
from one another, and each of them dominates a corresponding energy range. Of
course, each interaction is also closely related with the atomic number of the matter.
2.6.1 Photoelectric Effect
When the X-ray photons interact with matter and their energy is greater than the
electrons’ binding energy in material atomic inner shell, all energy of the photons
will be transmitted to the electrons of the atomic inner shell so that the electrons
have escaped from atomic bondage. This interaction between the X-ray photons and
matter is usually known as photoelectric effect, and the electrons produced by the
photoelectric effect are called photoelectrons, to distinguish from free electrons.
The kinetic energy of photoelectrons is equal to the photons’ energy minus the
binding energy of the electrons in the atomic inner shell. The greater is the binding
energy of the electrons in the atomic inner shell, the smaller the kinetic energy of the
photoelectrons. Besides the photoelectrons, the photoelectric effect also produces an
ionized atom, in which the holes in atomic inner shell will quickly be filled with the
electrons transited from the outer layer to the inner layer, or with the captured free
electrons. And thus, the characteristic X-rays are emitted. When the characteristic
X-rays are absorbed by the matter, the photoelectrons with the same energy will be
produced once again, which are also usually called Auger electrons. If all energies
of both the photoelectrons and the Auger electrons were lost inside the matter, the
detected energy loss will represent the energy of the incident photons.
According to the momentum conservation condition, the photoelectric effect can
occur only between the photons and the bound-state electrons. By the binding force
between electrons and atoms, the momentum of photons is transformed into the
recoil momentum of the atom. The stronger is the binding force, the greater the
probability of emitting the photoelectrons. A schematic diagram of the photoelectric
effect is shown in Fig. 2.6, where the electrons in K layer have greater photoelectric
absorption probability than those in L layer. The photoelectrons from K layer usually
account for 80% of the total of photoelectrons. In general, the photoelectric cross