Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Particle physics in relativity. Please help, Urgent.

  1. Oct 11, 2005 #1
    Here's the problem:

    " A particle is in its first excited state with energy Ea, before it decays to its ground state with energy Eg by emitting a photon with an energy hbar*omega

    (omega = 2Pi*frequency)

    Why will the photon energy be smaller than DE= Ea - E?

    Show that:
    hbar*omega = DE (1 - DE/2Ea)"

    Now the first part seems easy enough. The photon has a momentum, so the particle must get a momentum as well, equal and opposite, so a part of the energy will be used to give the particle that momentum.

    The second part is a little harder though... I have no idea.

    So far I've managed:
    hbar*omega = DE ( 1 - EKinetic/DE)
    hbar*omega = DE (1 - (gamma-1)Eg/DE)
    hbar*omega = DE ( 1- (DE + ((2gamma*Eg*Ea -2EgEa)/DE))/2Ea)
    But I'm not so sure that is correct.

    Any help would be most welcome

    (Ps. I'm only first year, so please keep it reasonably comprehensible please)
  2. jcsd
  3. Oct 11, 2005 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    Correct. And the key to the second part is to quantify the energy used to give the particle that recoil momentum.

    The momentum of the photon is: [itex]p_{photon} = h/\lambda = \hbar \omega/c[/itex]

    Since this is equal to the recoil momentum of the particle, the intial recoil speed of the particle is [itex]v = p_{photon}/m = \hbar\omega/cm[/itex].
    Since [itex].5mv^2 + \hbar\omega = DE[/itex], we have:

    [tex]\frac{(\hbar\omega)^2}{2c^2m} + \hbar\omega - DE = 0[/tex]

    That is a quadratic equation in terms of [itex]\hbar\omega[/itex].
    See if the solution to that gives you the result they are looking for.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook