physicsThomson scattering refers to the elastic scattering of electromagnetic radiation and a free charged particle.The electric field of the incident electromagnetic wave accelerates the particles, which excites the particles to produce radiation (scattered wave) with the same frequency as the incident wave.Thomson scattering is the approximation of Compton scattering in the low energy region.Thomson scattering is an important phenomenon in plasma physics, which was first explained by British physicist Joseph Thomson.As long as the motion of particles is non relativistic (that is, the speed is far less than the speed of light), the main reason for particle acceleration is the electric field component of the incident wave, while the role of the magnetic field can be ignored.The particles will start to move in the direction of electric field vibration, resulting in electromagnetic dipole radiation.The radiation of moving particles is strongest in the direction perpendicular to the moving direction, and the radiation is polarized along the moving direction of particles.Thus, depending on the position of the observer, the electromagnetic waves scattered from a small body element have different degrees of polarization.
Chinese name
Thomson scattering
Foreign name
Thomson scattering
Discipline
physics
Definition
Approximation of Compton Scattering in Low Energy Region
In Thomson scattering, both the incident wave and the observed scattered wave electric field can be decomposed into components located in and perpendicular to the observation plane (the plane consisting of the propagation direction of the incident wave and the propagation direction of the scattered wave).Traditionally, the components in the plane are called "radial", while the components perpendicular to the plane are called "tangential", which is for the observer[1]。
Figure 1
Figure 1 shows the scattering in the observation plane. It shows that the radial component of the incident electric field is the reason for the movement of charged particles at the scattering point in this direction, and this movement is also in the observation plane.In addition, it can be seen that the amplitude of the scattered wave is proportional to the cosine of the angle χ between the incident wave and the scattered wave, and the light intensity of the scattered wave is proportional to the square of the amplitude, thus containing the factor cos 2 (χ).The tangential component perpendicular to the observation plane will not have a similar effect.
The best way to describe scattering is to introduce an emission coefficient
, but the object element in the time interval dt
Scatter to solid angle
In this direction, and the wavelength is between
and
The incident wave energy between.From an observer's point of view, Thomson scattering has two emission coefficients, one corresponding to the radially polarized wave
, and the other is the emission coefficient corresponding to the tangential polarized wave
。They are given by the following relations:
，
Where n is the density of charged particles at the scattering point, and I is the flux of the incident wave (the energy radiated to the unit area within the unit wavelength range per unit time).σ is the differential cross section (area/solid angle) of Thomson scattering of charged particles, and its expression is
The unit system of the first expression is centimeter gram second system, and the unit system of the second expression isInternational System of Units；Q is the charge of a single particle, m is the mass of a single particle,
Note that this is just the classical radius of a point particle with mass m and charge q.For electrons, the scattering differential cross section is
Here { displaystyle lambda _ {e}} is the Compton wavelength of the electron.
The total energy radiated by the scattered wave can be given by summing the emission coefficient and integrating all directions in space:
Here σ T is the total scattering cross section.
For electrons, this scattering cross section is
Example of Thomson scattering
Announce
edit
In the first few days of the birth of the universe, photons generated in the universe were continuously scattered by free electrons, which led to the opacity of the early universe. This scattering process is called Thomson scattering.The cosmic microwave background radiation is the product of the final evolution of this scattering,Wilkinson Microwave Anisotropy DetectorAnd Planck satellite are trying to observe its linear polarization.
The photons emitted by the sun are scattered by the free electrons in the corona, which forms the K-corona. This scattering process is also Thomson scattering.NASA launchedSun Earth Relations ObservatoryBy using two independent satellites to measure K-crowns, we can getFree electron density3D image of.Inverse Compton scatteringIt can also be regarded asRelativistic particleThomson scattering in self reference frame.
Compton scattering
Announce
edit
In atomic physics, Compton scattering, or Compton effect, refers to the phenomenon that when X-rays or gamma ray photons interact with matter, the wavelength becomes longer due to the loss of energy.CorrespondingInverse compton effect——Photons gain energy, resulting in shorter wavelengths.The magnitude of this wavelength change is called the Compton shift.Compton effect usually refers to the interaction between matter electron cloud and photon, but there is also the interaction between matter nucleus and photon - nuclear Compton effect.The Compton effect was first introduced byUniversity of WashingtonCompton, a physicist, observed this, which was further confirmed by his graduate student Wu Youxun in the following years.Compton obtainedThe nobel prize in physics。
This effect reflects that light is not just undulating.The classical wave theory of Thomson scattering before cannot explain the cause of wavelength shift here, and the particle nature of light must be introduced.This experiment persuaded many physicists at that time to believe that light is particle in some cases, and the beam is similar to a stream of particles, and the energy of the stream of particles is proportional to the light frequency.
After the introduction of the concept of photon, Compton scattering can be explained as follows: electrons collide with photons elastically (inelastic scattering caused by elastic collision), electrons bounce back after obtaining part of the energy of photons, and photons that lose part of their energy fly out from the other direction. During the whole process, the total momentum is conserved. If the residual energy of photons is enough,The second or even the third elastic collision will occur.
Compton scattering can occur in any substance.When photons emit from the photon source and shoot into the scattering material (usually metal), they mainly interact with electrons.If the photon energy is quite low (the same order of magnitude as the electron binding energy), it mainly produces the photoelectric effect, and the atom absorbs the photon and produces ionization.If the energy of the photon is quite large (far more than the binding energy of the electron), we can think that the photon scatters the free electron and produces the Compton effect.If the photon energy is extremely large (>1.022 million electron volts), it is enough to bombard the atomic nucleus to generate a pair of particles: electrons and positrons. This phenomenon is called paired generation.
Because photons haveWave particle dualityTherefore, it should be possible to use the wave theory to interpret this effect.Erwin Schrodinger gave semi classical theory in 1927.This theory uses classical electrodynamics to describe photons and quantum mechanics to describe electrons.
