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Probing the atom with light high velocity particles

  1. Jan 15, 2012 #1
    1. The problem statement, all variables and given/known data

    The first part asks to find the necessary momentum of a "relativistic projectile particle" in order to resolve a diameter of 10^-10m (i.e. the atom). The second part asks to find the corresponding energy of the projectile particle, in GeV.

    2. Relevant equations

    λ = h/p

    E2 = p2c2+m02c4

    These are the only relevant equations I can think of.

    3. The attempt at a solution

    The first part seems easy, I'm assuming I have that one right. It looks as easy as just λ=10^-10m, so p = h/10-10 ≈ 6.63 * 10-24kgms-1 to resolve the atom.

    The second part seems less obvious, the only equation I know of that can answer it is the relativistic energy equation, however the problem does not say what the projectile particle is or its mass. E could be approximated to be pc if the mass is very small, but this approximation would only be reasonable if the particle's invariant mass was significantly less than p/c, which in this case is ≈10-32kg. I don't know of many particles which are this light, so it seems unreasonable to make that approximation. Are there any equations I am forgetting which give the energy of the particle? I'm suspicious that I have forgotten something very basic. Of course, if the first part is wrong, it may mean that I can safely make the approximation.

    Thanks for any help.
  2. jcsd
  3. Jan 15, 2012 #2


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    Are you sure about the exponent on that p value? And, sure, they must mean ignore the mass. I can think of some particles that are light enough. Photons and neutrinos would be good. Though neutrinos would have other problems.
  4. Jan 16, 2012 #3
    This problem is just totally weird. The momentum needed to probe something as large as the atom (not even the nucleus, the whole atom) is not very high. So even if they did want me to ignore the rest mass, the energy E = pc is going to be nowhere near the scale of GeV. Which leads me to think that they want me to include the rest mass, which they have given me absolutely no information about. I'll post the full problem word for word in case I have misread or misinterpreted anything:

    a). Estimate the momentum in SI units needed for a relativistic projectile particle
    to be used in a scattering experiment to resolve the structure of a target particle of
    diameter 10^−10 m (eg. an atom).
    b). Find the corresponding energy of the projectile particle in giga-electron-volts
  5. Jan 16, 2012 #4


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    The necessary momentum involves the smallest energy of a particle with the given wavelength. It can be even a photon with zero rest mass. You have written the expression of E in terms of the momentum: E2 = p2c2+m02c4, so E≥pc. If the rest mass differs from zero the energy must be higher than that minimum value.

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