Table of Contents
What Is Compton Scattering?
Compton scattering is the inelastic scattering of a photon by a charged particle, usually an electron. The photon loses energy (increases in wavelength) and transfers kinetic energy to the electron. Discovered by Arthur Compton in 1923, this effect provided definitive evidence that light behaves as a particle (photon) with definite momentum, a cornerstone result in the development of quantum mechanics.
The wavelength shift depends only on the scattering angle and the mass of the target particle, not on the initial photon energy. This universal relationship, characterized by the Compton wavelength of the electron (2.426 pm), is a direct consequence of energy and momentum conservation applied to the photon-electron collision.
Compton Formula
Scattering Examples
| Angle | Δλ (pm) | 100 keV → E' (keV) | 500 keV → E' (keV) |
|---|---|---|---|
| 0° | 0 | 100.0 | 500.0 |
| 45° | 0.71 | 97.2 | 388.4 |
| 90° | 2.43 | 83.6 | 253.3 |
| 135° | 4.14 | 72.3 | 188.1 |
| 180° | 4.85 | 66.9 | 169.2 |
Historical Significance
- Compton's 1923 experiment proved the particle nature of X-rays, confirming quantum theory predictions.
- The Compton wavelength defines the scale at which quantum effects become important for a particle.
- Compton scattering is used in gamma-ray astronomy to detect and measure high-energy photons from space.
- Inverse Compton scattering (high-energy electrons boosting photon energies) is important in astrophysics and produces X-rays in many cosmic sources.
Frequently Asked Questions
Why does the wavelength shift not depend on incident energy?
The Compton formula gives the change in wavelength, not the change in energy. While the wavelength shift is constant for a given angle, the energy change depends on the initial energy. High-energy photons transfer more energy to the electron even though the wavelength shift is the same. This is because energy is inversely proportional to wavelength, making the energy-wavelength relationship nonlinear.
At what energies is Compton scattering important?
Compton scattering is the dominant photon interaction mechanism in matter for energies roughly between 100 keV and 10 MeV, depending on the material. Below this range, the photoelectric effect dominates; above it, pair production takes over. In human tissue, Compton scattering is the primary interaction for diagnostic X-rays and therapeutic radiation.
What is the maximum energy transfer?
Maximum energy transfer occurs at 180 degrees (backscattering). For this angle, the Compton edge energy is E_max = 2E²/(m_e*c² + 2E). For a 662 keV gamma ray (Cs-137), the Compton edge is at 478 keV, transferring about 72% of the photon energy to the electron.