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Different ways of slowing down an individual particle

  1. May 14, 2009 #1
    I was wondering how you can slow down a particle. I know you can have it run into something. I know if its charged, you can introduce it in an electric field. Is there any other way. I remember learning something about a specific type of field that slows down all mass passing though it, which increases the energy of the field. Can anyone help me out here?
  2. jcsd
  3. May 14, 2009 #2
    The most common way of slowing down charged particles (excluding electrons) is to use materials, such as beryllium, copper, or tungsten (among others) by making use of dE/dx, the energy loss based on collisions of the slowing particles with atomic electrons (not Coulomb scattering on nuclei). The major concern about using materials to stop the particles is multiple scattering, which increases the beam size (divergence). Very roughly, the energy loss is about 2 MeV of energy loss per gram/cm2 (gram per cm-squared) of the material. Positive and negative particles (positive and negative muons, pions) have a slightly (very small) different dE/dx stopping distance. If the charged particles are bunched in time, then RF cavities can decelerate them by properly phasing the RF power. If the beam current is sufficient, power can be extracted from the beam. Electrons are stopped by collisions with high-Z nuclei that produce bremsstrahlung and electromagnetic showers of large numbers of mainly positrons and electrons. Uncharged strongly interacting particles (e.g., neutrons) are best slowed by collisions with low-mass nuclei, like hydrogen, such as in water or hydrocarbons, or in graphite. Neutrinos are uncharged weakly interacting particles, and usually pass through the Earth without any interaction at all.

    Photons interact by either electromagnetic or nuclear interactions. Photoelectric effect, (up to k-shell binding energies of atoms), Compton scattering (up to a few MeV), and pair production (of positrons and electrons) will attenuate photons. So will nuclear interactions, like gamma,n interactions, such as O16(gamma,n)O15.

    Ultra-cold (1 micro electron volt) neutrons can be further slowed down by letting them drift vertically upward. Roughly half cannot reach an elevation of 10 meters before stopping and falling back down.

    Dark matter?? dark energy?? WIMPS?? neutralinos?? tachions?? monopoles??
    Last edited: May 15, 2009
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