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Radiation energy transfer

  1. Oct 19, 2004 #1
    I'm a bit confused about the way radiation is said to 'transfer' its energy to matter. I must be looking at it the wrong way, so I'd appreciate it if someone could explain what I am missing. I don't see how energy is transferred to atoms through ionization.

    The way I see it is charged particles like alphas and betas interact with atoms by either exciting the atom or causing an electron to be completely stripped from its shell. For excitation, I think I get it, the energy is lost in vibrations of the atom. But when ionization occurs, isn't the energy now transferred only to the stripped electron as kinetic energy? Or is it just some of the energy, with the rest causing the positively charged part of the ion pair to be energized somehow?

    This is also the way I look at gamma energy, imparting its energy to an electron via photoelectric interactions which will then go on to cause further ionizations. Don't we say that the energy of the photoelectron is equal to the energy of the gamma ray? Same thing with compton scatter, only the kinetic energy of the electron is now less than it would be in a photoelectric event, and there is a lower energy gamma now floating around.

    Now how do these electrons which are stripped deposit their energy? I just see them going on to cause more ionizations in an endless cycle of ionization.Any explanations would be greatly appreciated. Thanks.
  2. jcsd
  3. Oct 20, 2004 #2
    In a material, a fast electron leaves a trail of excitations and ionizations behind. All of these have lower energy than the original fast electron. They will interact with other electrons and with the lattice by electron-phonon coupling and produce heat. (Or in a scintillator, decaying excitations can produce some light.)
  4. Oct 20, 2004 #3


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    The radiation energy transfer occurs by means of absorbtion or emission of photons. You can find reams of info on this by going for texts/article on radiative transfer in atomic and nuclear physics, particularly older ones.
    Reilly Atkinson
  5. Oct 22, 2004 #4
    Hi coregis,

    Excitation of an atom works like this. Basically electrons "orbiting" some atomic nucleus have certain energy values because of the several interactions they undergo with each other and with the atomic nucleus. things like spin, angular momentum and spin-orbit-coupling are well described by QM and give us very accurate info on these possible energy levels. Suppose you have a photon (the particle corresponding to a light wave) that "collides" with the atom. The energy of the incident photon (which is kinetic energy) is absorbed by the atom, yielding an excited state.
    This means that because of the incident photon, an electron is knocked out of it's place in it's orbit around the nucleus and leaving an empty space behind.

    De-excitation occurs when this empty space (at energy level A) is filled up by some other electron coming from another energylevel B. As a consequence of this EM-radiation will be emitted with energy that corresponds exactly to the energy-difference of the two levels : B-A.

    Ofcourse there are several ways to excite an atom by several incident particles like photons or electrons,...

    Beware of the alpha and beta decays because they have nothing to do with excited states (ofcourse the gamma-decay DOES which i explained above). In alpha decay an unstable heavy nucleus decays into more stable alpha-nuclei. This process is described by QM-tunneling. The beta-decay is described by QFT because of the emitted electron that is created out of nothing (read : the vacuum). In the early days people thought that this electron had to come from the atomic nucleus itself...ahhh the old days...

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