Pair production with 1 MeV positron beam?

Dear All
I am not sure whats going on and I hope somebody can help me out. I have a isotropic positron beam incident on thin tungsten foil. I have mgdraw code set up such that whenever there is a pair production in the foils, its recorded. The main idea was to run the simulation with photons and record both the pair production events and positron annihilation events in the foils. But I thought of trying with positrons as primary particle just to test things out. Not always, but every now and then I see that there is entry in the files for pair production event. Energetically its not possible. Can someone please take a look? It seems I cant attach the mgdraw out files. but i am attaching everything else.
saurabh mukherjee
POSH_mod_eff002.err (22.4 KB) POSH_mod_eff002.out (94.8 KB) POSH_mod_eff.inp (3.6 KB) POSH_mod_eff.flair (4.8 KB) mgdraw_pos_yield_copy.f (4.4 KB)


Two remarks:

  1. Pair production is possible: the available energy is not only the positron kinetic energy E (the primary kinetic energy minus whatever was lost to electronic stopping), but E+2m_ec^2, where m_e is the electron mass (keep in mind the positron annihilates with a poor electron, hence twice m_ec^2).

  2. Positron annihilation in flight leads to the emission of two photons, whose energies E1 and E2 can be asymmetric (but still verifying E1+E2=E+2m_ec^2). It can happen e.g. that E1 is slightly larger than 2m_ec^2: this photon may then go on to generate an electron/positron pair in a material. In mgdraw.f you’ll find the ICODE for positron annihilation at rest vs in flight to refine your analysis.



Dear Cesc
thank you for the answer. I thought I did some mistake with the mgdraw code. Like you said, with ICODE= 214 and KPART =7, I did saw quite a few gammas with energy above pair production threshold.
Thank you
saurabh mukherjee