Ambient dose equivalent of neutron due to proton on Cu target

Dear experts,

In the present simulation 18 MeV proton beam on a thick Cu target (with a thin Havar foil layer before that) has been considered. The intention was to estimate source term at 1 m in terms of Ambient dose equivalent of neutron dose. I have placed few detectors at different angles and found that dose rate is high at the backward direction. The dose rate is also high at 90 degree angle as compared to 0 degree angle. I expected neutrons will be more forward focused and dose rate at 0 degree will be more than dose rate 90 degree.

Can you please guide if I my understanding is wrong or if I am missing something in the input?

I am attaching the dose equivalent estimated via this simulation.

Source_18MeVproton.inp (7.6 KB)



Dear @riya,

I’ve run your input adding a USRBIN mesh scoring to visualize where the neutrons are going. Zooming on your dump, I got this plot:

It makes sense to me: the neutrons are not emitted isotropically because some of them are absorbed by the dump itself and the majority is backscattered.

Dear @amario

Yes, I also observed the same. I scored neutron fluence using usrbin and usrbdx to check the spectra. The situation made me wonder if that is what happens in a beam dump. I don’t have much experience in this domain, so I was confused with other literatures on shielding calculations for beam dump room where it has been stated that dose rate at 0 degree is higher than dose rate at 90 degree.

  1. Is this phenomena then dependent on beam energy?

  2. I am wondering if this is related to interaction of secondary neutrons. Is there any way to stop reactions of secondary neutrons in FLUKA for further verification?

  3. Also the concept of ‘reaction to be forward focused’ valid for only thin target ?

This may not be related to FLUKA simulation, but if you can guide regarding this query, it will be helpful.



Try using a target that has dimensions equal to the projected range plus lateral and longitudinal straggling. Use SRIM to calculate these.

Dear @sunil ,

Thank you for the suggestion. I will do that.