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Chapter 2 Photons-particlelike properties of radiation. 2.1 Introduction. radiation with matter: the photoelectric effect, the Compton effect, and pair production. scattering or absorption of radiation with matter : bremsstrahlung (X ray) and
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Chapter 2 Photons-particlelike properties of radiation 2.1 Introduction • radiation with matter: the photoelectric effect, the Compton effect, and pair production. • scattering or absorption of radiation with matter : bremsstrahlung (X ray) and pair annihilation. 2.2 The photoelectric effect • The ejection of electrons from a surface by the action of light on the matter. • Hertz: ultraviolet light enhances discharge • Lenard: ultraviolet light causes electron emitted from cathode surface • solar cell: thermal energy is converted into electrical energy
Chapter 2 Photons-particlelike properties of radiation Results: saturation cut-off • stopping potential : at which the photoelectric current drops to zero, its value is independent of radiation intensity. • the kinetic energy of the fastest ejected photoelectron is • Three major features cannot be explained by classical wave theoty: (1)classical theory: light intensity increasing, increasing experiment: is independent of the light intensity
Chapter 2 Photons-particlelike properties of radiation (2)classical theory: for any frequency, the light can always eject the photoelectron by gradually increasing its intensity experiment: the cut-off frequency exists (3)classical theory: a time lag can be measured experiment: no detectable time lag has ever been measured 2.3 Einstein’s quantum theory of the photoelectric effect Einstein’s photon hypothesis: • photon: a energy bundle, velocity is C, and with energy • the kinetic energy of emitted electrons is
Chapter 2 Photons-particlelike properties of radiation • the prediction of Einstein’s photon hypothesis: (1) is independent of the intensity of illumination (2) the existence of a cut-off frequency (3) the absence of time lag (4) stopping potential :
Chapter 2 Photons-particlelike properties of radiation 2.4 the Compton effect energy transfer causes wave length shift
Chapter 2 Photons-particlelike properties of radiation • photon :
Chapter 2 Photons-particlelike properties of radiation Compton wavelength • Conclusions: • Wave length shift depends only on the scattering angle , and not on the initial wavelength • The first peak with unchanged wavelength is due to • The duality of EM radiation: wave ( ) and particle ( )
Chapter 2 Photons-particlelike properties of radiation 2.6 Photons and X-ray production X-ray tube • The deceleration of electrons, brought to rest in the target material, result in the emission of a continuous spectrum of electromagnetic radiation. • For a given electron energy, there exists a well-defined minimum wave-length , and its value depends on applied potential V, being the same for all target material.
Chapter 2 Photons-particlelike properties of radiation bremsstrahlung process: brems (deceleration) + strahlung (radiation) • measurement of gives the ratio • Comparisons: photoelectric effect: a photon is absorbed, its energy and momentum is transferred into electron and recoiling nucleus. bremsstrahlung: a photon is created, its energy and momentum coming from a colliding electron and nucleus.
Chapter 2 Photons-particlelike properties of radiation 2.7 Pair production and pair annihilation pair production mass is the same, but charge and magnetic moment are opposite Ex: Pair production cannot occur in empty space ?
Chapter 2 Photons-particlelike properties of radiation (1) momentum conservation: at rest (2) total relativistic energy conservation: • Anderson (1933): the first experimental evidence for the pair production process, and existence of positrons during an investigation of cosmic radiation. bubble-chamber photographer of electron-position pair formation
Chapter 2 Photons-particlelike properties of radiation • experiment: free electron cannot emit photon with energy • Why ? Dirac’s theory positron exists • Dirac’s relativistic quantum mechanics of electron: • Dirac’s assumption: a vacuum consists of a sea of electrons in negative energy levels which are normally filled at all points in space. (2) (1) (3) occupied states
Chapter 2 Photons-particlelike properties of radiation 2.8 Cross sections for photon absorption and scattering • photoelectric effect: absorption • pair production : absorption • Rayleigh scattering (no wavelength shift) • Comptonscattering (wavelength shift) • cross sections: describe the probability of absorption or scattering The scattering, photoelectric, pair production, and total cross section for lead No. of photoelectric absorption photon number No. of atom per unit area in a slab cross section, its unit is 1/area Photoelectric effect: Scattering: Pair production:
Chapter 2 Photons-particlelike properties of radiation Ex: Evaluate, in terms of the total cross section, the attenuation of a parallel beam X-rays in passing through a thick slab of matter. a beam of photons passing through a slab photon number after penetrating a slab at position x attenuation coefficient attenuation length • As increases, absorption or scattering of photons in the matter is more rapid, and its value depends on photon energy .
