Compton Scattering

Experimental verification that photons scatter off of electrons like particles.

BIG DEAL!

A photons (which has a particular wavelength) hits an electron, come out with a different (longer) wavelength.

• Cannot happen in classical E&M !
• Everything in classical E&M happens at some frequency

Convinced the community that photons really exist. eg: quantized massless photons with E & p

This experiment has 3 key results

• photons exist
• Can measure m_e
• Test prediction of Classical EM vs QFT

Experiment:

• Cs(137) source in lead housing
• gamma emitter at 0.6 MeV
• Channeled into a beam, hits electrons
• Detector: NaI crystal, emits small, brief flash of light when a gamma goes into it
  • Size of flash ~ E_gamma
• Stage is movable. Make measurements at various θ
• Light detected with PMT -> DAQ - > computer