High energy electrons (I'm not sure exactly what they classify as high energy) mostly lose energy through bremsstrahlung, while high energy photons mostly lose their energy by electron-positron pair production. The radiation length is a characteristic "length" for these processes in a material, although it is usually measure in g / cm^-2 . You get this by multiplying a length by the density of the material. The radiation length is defined for electrons as the length over which the electron loses all but 1/e of its energy, while for photons it is defined as 7/9 of the mean free path.
On a similar note, low energy electrons lose energy through various types of scattering as well as ionization. Since the ionization loss rises logarithmically in energy and the bremsstrahlung increases linearly, there is a critical energy where the energy lost to ionization and the energy lost to bremsstrahlung are equal. For most materials, this critical energy is in the tens of MeV, so it seems that radiation length can be a useful parameter for most of the energies we see at JLab.
High energy electrons means "minimum ionizing particles", basically electrons with 'over a certain total energy' deposit the same amount of energy per length (dE/dx). For more details see the Bethe-Bloch formula. Therefore most systems have a hodoscope of something light where the electrons are minimum ionizing (we use bc408 plastic scintillator) and a calorimeter of something heavy (we us CsI crystals) to stop the particle and measure the full energy.
ReplyDeleteelectron bremsstrahlung and electron-positron pair production are actually the same feynman diagram, just twisted! for a hillarious look at this examine the Undergraduate thesis of Loreto Peter Alonzi III from W&M
ReplyDeleteseriously they just handed out honors for anything
also, of course I could say this about everything, take a look at what Leo has to say
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