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Relativistic energy and momentum questions.

  1. Sep 24, 2007 #1
    problems statement:
    1. a nucleus of mass m initially at rest absorbs a gamma ray (photon) and is excited to a higher energy state such that its mass now is 1.01m, find the energy of the incoming photon needed to carry out this excitation.

    2. A moving radioactive nucleus of known mass M emits a gamma ray in the forward direction and drops to its stable nonradiactive state of known mass m.
    Find the energy E_A of the incoming nucleus such that the resulting mass m nucleus is at rest. The unknown energy E_c of the outgoing gamma ray should not appear in the answer.
    attempt at solution
    1.well, for the first question i think this is fairly simple:
    from conservation of 4-momentum we have before 4-momentum is:(mc,0) after
    (E/c+E_ph/c,P) so we have : (mc)^2=(E/c+E_ph/c)^2-P^2=(E/c)^2-P^2+2EE_ph/c^2+(E_ph/c)^2=(1.01mc)^2+2EE_ph/c^2+(E_ph/c)^2 where (E/c)^2-P^2=(1.01mc)^2, here im kind of stuck with E which is not given, any hints?

    2.for the second the answer in the book is E_A=((M^2+m^2)/2m)c^2
    but i dont get it, here's my attempt to solve it:
    the before 4 momentum is (E_A/c,P) after: (E_c/c,0)+(mc,0)=(E_c/c+mc,0)
    which by the square of the momentums we get that:
    (E_A/c)^2-P^2=(E_c/c+mc)^2=(Mc)^2 but im not given P so im kind of stuck here again, i thought perhaps calculate it in the rest frame of M which means that the before is:
    (E_A/c,0) the after is (E_c/c,0)+(E/c,-P) but still don't get far with it, any help is appreciated, thanks in advance.
  2. jcsd
  3. Sep 24, 2007 #2


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    For question one, what is the 'rest energy' of the nucleus?
  4. Sep 24, 2007 #3
    well, if it wans't clear in my post, obviously it's mc^2, and i wrote in 4 momentum notation (mc,0) for the before the absorption of the photon.
  5. Sep 24, 2007 #4


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    Perhaps I'm missing something here, but couldn't you write;

    [tex]p^2 = (mc)^2 - (1.01mc)^2[/tex]
  6. Sep 24, 2007 #5
    well first, it should be minus that ofocurse cause this way we get a negatrive value where everything there is positive.

    and im not sure, what's wrong with what i wrote, first we have (mc,0) after that we have the absorption: (E/c+E_ph/c,p) now (E/c)^2-p^2=(1.01mc)^2 and
    E_ph=mc-E/c=mc-sqrt((p)^2+(1.01mc)^2) but how do you find p?
  7. Sep 24, 2007 #6
    i think that p=E_ph/c, am i wrong?
  8. Sep 24, 2007 #7
    ok, i solved question number 2.
  9. Sep 25, 2007 #8
    any news on question number 1?
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