Thomson scattering: ©©It is the scattering of electromagnetic radiation by a free non-relativistic charged particle. The electric and magnetic components of the incident wave accelerate the particle. As it accelerates, it, in turn, emits radiation and thus, the wave is scattered. Thomson scattering is an important phenomenon in plasma physics and was first explained by the physicist J. J. Thomson 1856-1940 N o b el Prize 1906 The main cause of the acceleration of the particle will be due to the electric field component of the incident wave. The particle will move in the direction of the oscillating electric field, resulting in electromagnetic dipole radiation. The moving particle radiates most strongly in a direction perpendicular to its motion and that radiation will be polarized along the direction of its motion. Therefore, depending on where an observer is located, the light scattered from a small volume element may appear to be more or less polarized.©© Daniele Dallacasa ± Radiative Processes and MHD Section -4 : Thomson Scattering, Compton scattering & Inverse Compton Thomson scattering photon-electron interaction -1- hv ≪ mec2 ± for a non relativistic particle, the incoming (low-energy) photons can be expressed as a continuous e-m wave and the magnetic field (Ho=Eo) can be ªneglectedº (i.e. studied as a consequence of the E field behaviour). 2 ± the average incoming Poynting flux is <|S|> = cEo /8π ± the charge feels a force due to a linearly polarized incoming wave: ± the energy of the scattered radiation is the same as the incoming wave Thomson scattering (2) ± the force is due to a linearly polarizesd wave: F = e E o sino t 2 ¨ e E o d = e r d = e r¨ = sin t m o ± the dipole moment is e 2 E d = − o sin t = a t 2 o m o e 2 E ± which describes a dipole with amplitude d = − o o 2 m o ± taking into account the Larmor©s formula, the time averaged emitted W is ¨2 4 2 d W 2 d e E d 〈 〉 = 〈 〉 = 〈 o sin2 〉 = 〈S 〉 d 3 c 3 8m2 c 3 d 4 ± where d e 2 2 2 = sin =r o sin d m2 c 4 is the differential cross section for the scattering Particle acceleration (due to incoming radiation) the emitted power by the scattering particle is: from this equation we define the electron Thomson cross section by integrating over all the angles: 2 Back to the ©©geometric problem©© the angular distribution of the radiation is given by and this means that each individual scatter produces polarized radiation Cross section independent from frequency (breaks down at high frequencies when the classical physics need quantum mechanics) Compton scattering ©©In physics, Compton scattering or the Compton effect, is the decrease in energy (increase in wavelength) of an X-ray or gamma ray photon, when it interacts with matter. Inverse Compton scattering also exists, where the photon gains energy (decreasing in wavelength) upon interaction with matter. The amount the wavelength increases by is called the Compton shift. Althoughnuclear compton scattering exists, what is meant by Compton scattering usually is the interaction involving only the electrons of an atom. A.H. Compton 1892-1962 Compton effect was observed by Arthur Holly Compton in 1923, for which he earned the 1927 Nobel Prize in Physics. The effect is important because it demonstrates that light cannot be explained purely as a wave phenomenon. Thomson scattering, the classical theory of charged particles scattered by an electromagnetic wave, cannot explain any shift in wavelength. Light must behave as if it consists of particles in order to explain the Compton scattering.©© Compton scattering photon-electron interaction -1- Compton scattering photon-electron interaction -2- energy conservation: 2 2 2 2 4 = h i me c h f p e c me c momentum conservation: p i = p f pe p=E /c =h /c photon momentum 2 2 2 p e = pi −p f pe = pi −p f ⋅p i−p f = p i pf −2 pi p f cos 2 2 2 2 2 2 p e = h i h f − 2h i f cos multiply by c 2 then take the square of energy conservation 2 2 2 4 2 2 2 2 2 4 2 2 2 p e c m e c = h i h f me c 2h i me c −2hf me c −2h i f 2 2 compare the two by highlighting p e c we get 2 2 2 2 −2h i f cos = 2h i m e c −2h f me c −2h i f 1−cos i −f 1 1 1 2h 2 1−cos = 2h m c 2 − = = − i f e i f 2 h h f i me c i f 1 1 f i h − = − = 1−cos 2 f i c c me c Compton wavelength for electrons h h − = 1−cos = 1−cos = = 0.02426 A f i o o m c me c e Compton scattering a Java applet showing the geometry of the scattering can be found at http://www.student.nada.kth.se/~f93-jhu/phys_sim/compton/Compton.htm Compton scattering a Java applet showing the geometry of the scattering can be found at http://www.student.nada.kth.se/~f93-jhu/phys_sim/compton/Compton.htm h = i h e 2 1hi 1−cos /me c h e−i = ⋅1−cos = o 1−cos = 0.02426⋅1−cos [A] me c Final energy of the scattered photon is a function of: ± initial energy of the photon ± scattering angle The photon always looses energy, unless θ = 0, and the scattering is closely elastic when ≫ (i.e. hv≪ m c2 ) l lc e When the photons involved in the collision have large energies, the scattering become less efficient and quantum electrodynamics effects reduce the cross section: the Thomson cross section becomes the Klein-Nishina cross section Cross ± section: Thomson and Klein ± Nishina In case hv ~ m c2 , the probability of interaction decreases, also as a function of the e scattering angle; additional constraints come from relativistic quantum mechanics; h if we use ≡ x 2 me c 3 1x 2x 1x 1 13x = T −ln12 x ln12x − 4 [ x 3 12x 2 x 12x 2 ] 3 1 1 known as the Klein ± Nishina cross section; if x ≫1 = ln2 x T 8 x 2 it originates from a differential cross section based on the scattering angle d 1 = r 2 [P h , − P 2h ,sin2 P 3 h ,] d 2 e where the probability is defined as 1 P h , = h − 1 2 1 cos me c and the Klein ± Nishina cross section becomes the Thomson cross section at low photon energies Cross ± section Klein ± Nishina The Klein ± Nishina cross section is relevant at high photon energies (Hard X-Rays and beyond) x = Inverse Compton scattering photon-electron interaction -3- Moving, energetic (relativistic), electrons ªcool downº by increasing the energy of seed photons. To make computations simpler, it is possible to consider the centre of momentum frame where the photon energy is much less than m c2 and then can be used. e sT In the electron rest frame we have Thomson scattering 1. Lab system ⇨ electron rest frame: h e = h L 1 − cos 2. Thomson scattering in electron rest frame h e © = h e 3. Electron rest frame ⇨ Lab system h L © = h e © 1 cos1 © i.e. ⇨ 2 h L © ≈ h L Very efficient energy transfer from electron to photon! Inverse Compton scattering The scattering angle is important for the energy of the outgoing electron: 1 - maximum energy gained by the electron if q =p and q1© = 0 ⇨ in the electron rest frame the photon is blue ± shifted (face on collision) 2 ± minimum energy gained by the electron if q = 0 and q1© = p ⇨ in the electron rest frame the photon is red ± shifted (end on collision) the maximum energy that can be gained is (averaged over all angles): 4 © ≈ 2 3 4 or, in terms of frequency © ≈ 2 3 In case it is possible to measure both the initial and final photon energy/frequency, then g 2 can be deduced 3 © ≈ 4 Inverse Compton scattering emitted energy It is a Lorentz invariant, i.e. the same in the laboratory and electron rest frame E © = h © = h 1 cos = E 1 cos Let©s consider a region where there is a plasma of relativistic electrons and a radiation field: in the electron framework, the photon energy is 2 d d © 〈E © 〉 = = c rad = c 2〈1 cos2〉 〈E 2 〉 dt dt © T 8 T rad since the emitted energy is a Lorentz invariant. It is then possible to write 1 〈E 2 〉 = U 〈1 cos2〉 = 1 2 rad ph 3 and, finally, the energy coming out the scattering region is d 1 out = c U 2 1 2 dt T ph 3 The energy associated with the photons, prior of the scattering is d in = −c U dt T ph and then the net effect of the Inverse Compton scattering is d out d in d 1 − = = c U 2 1 2 − 1 dt dt dt T ph 3 ic [ ] d 4 2−1 = 2 2 then = c 2 2 U dt 3 T ph ic d 4 2 2 and if we recall the synchrotron emission = c T U H dt syn 3 d dt U we get syn = H d U ph dt ic The ªLocal Radiation Fieldº Evaluation of Urad: as taken in a number of selected locations X-rays radio Spatial variation of the total radiation field throughout the Galaxy.