Why neutron was produced when 5 MeV electron bombard on iron target?

Dear experts,

I use a 5 MeV electron bombarding on iron target, with photonuc card on for material IRON. In my mind, the threshold of (gamma, n) is above 7 MeV, so there will be no neutron. But I found the neutron dose is not zero in the USRBIN card. So what’s the problem?
Attached is the input and the neutron dose map from USRBIN .

By the way, I have two other questions, needed to be checked if I am correct:

1、 the electronuclear interaction is not needed for shielding calculation, even for 250 MeV electron.
2、 For electron beam, the dose recorded by ALL-PART including the dose from NEUTRON+PHOTON+ELECTRON, but only NEUTRON+PHOTON are useful for shielding calculation.

Thank you for any comments,

Best wishes

e5.inp (4.9 KB)

Dear @biyj,

I have a few comments:

  • As you can see in the picture below, your beam is not hitting target and probably the neutrons are produced in the concrete walls

  • While in general electronuclear interactions are not the most relevant ones in shielding calculations, particularly for a 5 MeV beam, they are implemented in Fluka and you can turn them on for free.

  • You ask about dose but you’re actually using dose equivalent in your input. Either way, I think you’re making an unnecessary differentiation here. Use ALL-PART, it is simpler to implement. Also, look in the manual and/or search on the forum to find out more about the estimation of the ambient dose equivalent. Which is made folding particle fluence with conversion factors.

Looking at your input I also notice a few things worth commenting about.

  • You are using Portland concrete. In general concrete can be pretty different. You have to be sure to be using one with a composition similar to the one you have in reality.

  • The second AUXSCORE card associate NEUTRON also to the “pho” scoring, I’m not sure this is what you wanted rather than a typo.

  • The fourth AUXSCORE card is unnecessary because the AMB74 conversion factors are those used by default for a DOSE-EQ estimate.

Dear @amario

Thank you for your reply. It solved my questions.

I do refer to “dose equivalent” as “dose” for short. I need a comment if I am right: For shielding calculation, what we need is (photon fluence * conversion factor+ neutron fluence * conversion factor), electrons fluence * conversion factor is not required for shielding calculation since electrons are “secondary particles” in the process of photon energy deposition.

Many thanks

From a software point of view, also the electrons are needed, because if an electron produced in a USRBIN cell A crosses the USRBIN cell B, it will deposit energy also in this one and you would have to take it into account.
Also, don’t forget that in real life the energy is always deposited by electron interactions.
Last, since Fluka takes automatically care of this when scoring the generalized particle DOSE-EQ, I would say that you are overthinking this specific point.