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I have encountered many differential crossections: [tex]\frac{d\sigma}{dE_+d\Omega _+ d

\Omega _-}[/tex]

(Pair production of electrons and positrons)

Where E+ is energy of positron. However, in all of these crossections, the energy of the electron; E- is included in the formula, e.g eq 2.1.1 (http://www.irs.inms.nrc.ca/EGSnrc/pirs701/node22.html)

So let's say I know the incident photon energy, k, and want to evaluate the probability to get a positron with energies between E+(1) and E+(2), should I replace the E- in the formula with (assuming that recoil energy of the nucleus is neglectable): E- = k - E+ , then integrating over dE+ ?

E- is not an independent variable, but I am wondering why all sources I have encountered so far do this? -> Putting E- and p- into the equations when they are dependent on E+ and k... is it just for making the formulas more nice and symmetric?

This might be a very trivial question, but input from someone else would save my day :-)

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# Question on differential crossection, pair production

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