I can't help you with EGSnrc since I only speak GEANT4-ish. But from what process would you expect secondary electrons, assuming x-rays in the 100 keV range?

Hi Patrik, Thank you for your reply.

In my case, the x-rays are both the diagnostic (kV) and therapeutic (MV). The problem that I am trying to figure out is as follows.

A beam of x-rays passes through a medium. At a given point inside the medium, (i) what the spectrum of primary photons (after they get attenuated) looks like, (ii) what the spectrum of only secondary photons (due to any or multiple processes of Photoelectric effect, Compton scattering and pair production) looks like, of course neglecting very low energy secondary photons. and (iii) what the combined spectrum (of both primary and secondary photons) looks like. I am not talking about the secondary electrons.

Also, hopefully I can begin with monoenergetic x-rays in MonteCarlo and move towards a realistic beam of x-rays.

Doing MC you don't need to make approximations, spectra are almost as easy to use as monochromatic beams. To solve this questions, I would use TOPAS (topasmc.org) based on Geant4. You describe your problem in simple statements and Geant4 does the particle transport. You can add filters for particle generation (primary/secondary) to define what your scorer sees.

There should be no secondary photons from photo and compton, so just attentuation according to energy. But sometimes doing MC gives you unexpected results and the world is not as simple as it seemed :-)

  Patrik

(I'm not related to Topas development, I'm just a very happy user)

Hi Rajeshs,

from my interpretation of your question it would have a very simple way to get to an answer. As already said, with Monte Carlo computations you have a lot of flexibility, so you can start with mone-energetic photons. Then, you ask the code to give you the number spectrum, digitized with photon energy bins that you want, and one narrow bin with its low-energy limit just below the photon energy of the primaries. At every region in the phantom the primaries are then in that narrow energy bin; everything outside this bin is a secondary.

As already said, the lower-energy diagnostic x-rays absorb quickly; any secondaries they produce have lower to much lower energies, and should stay physically closer to where they were generated than the primary diagnostic x-rays. The therapeutic x-rays may be multi-MeV, and they will scatter all over the place. If your person were to be made from kryptonite or a more conventional high atomic number material such as tungsten or lead, you would also make energetic K-lines and even annihilation photons, but I suspect that this is only a minor effect in low-Z phantoms that mimic real people.

Now, if you were interested in the energy spectrum of the photons (rather than the number spectrum) or in the dose produced by each part of the spectrum, things would be different.

This is a technical question and it may not be an easy task if you are not experienced with Mortran and/or C++ codes. On the other hand, the solution is not too much complex. All you need to do is to assign a proper value to the weighting factor of the particle which is being transported. I mean, WT(NP)=0 for primary particles and  WT(NP)=1.0 for secondary particles. Then, multiply deposited energy (EDEP) by the weighting factor while calculating dose value in a region.   

Hello!

You can take a look at the cavity user code of EGSnrc. In this user code, there is the option to simulate primary only and scattered photons. Also, you can obtain the fluence  at a given scoring plane. Hence, you will obtain the spectrum. I hope this can help you! Best regards!

http://nrc-cnrc.github.io/EGSnrc/doc/pirs898/cavity.html