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    Question:
    Explain why a photon of energy 2MeV could produce an electron-positron pair but not an proton-antiproton pair.

    I understand that rest mass of electron is less than proton but problem is that i have no figures to back up the 2MeV bit.

    I tried doing rest mass of electron times two divided by 1.6x10^-13 and rest mass of proton times two divided by 1.6x10^-13.

    I get for...
    ...electron - 1.138875x10^-17 and ...proton - 2.09125x10^-14.

    They are the correct respective side of 2 but too many decimal places out.

    Any ideas and help please?

    Cheers
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    (Original post by Stupiemountain)
    Question:
    Explain why a photon of energy 2MeV could produce an electron-positron pair but not an proton-antiproton pair.

    I understand that rest mass of electron is less than proton but problem is that i have no figures to back up the 2MeV bit.

    I tried doing rest mass of electron times two divided by 1.6x10^-13 and rest mass of proton times two divided by 1.6x10^-13.

    I get for...
    ...electron - 1.138875x10^-17 and ...proton - 2.09125x10^-14.

    They are the correct respective side of 2 but too many decimal places out.

    Any ideas and help please?

    Cheers
    A little tip for you: it's probably worth sticking to units of MeV rather than having confusing standard form all over the place. And remember your significant figures please

    I think as you have already identified, if the rest mass energy of the particle pair is greater than the energy of the photon, then pair production cannot occur (due to conservation of energy).

    Know that m_p = 938\mathrm{MeV}/c^2. So for production of a p\bar p pair, by E=mc^2, this requires at least 2m_pc^2 of photon energy, which is 1876\mathrm{MeV} \gg 2\mathrm{MeV}. Thus this pair production cannot occur.

    Clearly 2m_ec^2 = 1.022\mathrm{MeV} \ll 2\mathrm{MeV}, thus e^{+} e^{-}pair production can occur.

    (There is another subtlety here to do with conservation of momentum - meaning that the energy of the photon must actually be greater than 1.022\mathrm{MeV} - but this can be ignored for now.)
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