Thermal neutron not detected

Dear experts,

I am contacting you regarding the detection of low energy neutrons.

Through source.f routine, I inject a proton spectrum interacting with thin layer of Beryllium to generate
a first neutron spectrum. These neutrons are then propagating through few mm of water, to try to thermalize them. With USRBDX card, I am able to see well the neutron spectrum generated after the Beryllium foil (see picture attached). However, the neutron spectrum in output of the water box don’t seems to show thermalized neutrons, but the inverse, with lower flux, which is not what I am expecting to see.

I suppose that something is wrong in my physics card /scoring card, could you help me to figure this out ?
Neutrons.inp (3.1 KB)

I attached as well the inp. file.

Looking forward to your help.


Dear @marine.huault

your target is very small (few mm) inside a void space. You don’t have enough material to thermalise the neutrons. If you want to see the thermal neutrons out of this small geometry you need to run a lot of statistics
However my guess is that your target will not be in located in outerspace, there will be material around, table, support, walls etc, which will be the main source of neutron thermalization. If you omit them then the results will be wrong.

Few other remarks on your input

  • You don’t need a SOURCE card if you don’t have a specialized source Fortran routine
  • There is no need to redefine the water. There is a predefined WATER material
  • You can safely remove the LOW-MAT cards. The new fluka 4.4 has by default activated the neutron point-wise treatment which is what you need for this simulation
  • If you want to specify Be-9 as material you need to create a dedicated MATERIAL and fill the A: field with 9 (along with the Z and ρ). Otherwise as you have in your input the BERYLLIUM is assumed natural for all particles except for low energy neutrons using Be-9 group wise.
  • The lowest energy of neutrons in FLUKA is 1e-15 GeV, while you cut your plots at 1e-12 GeV