Results from USRBDX

Dear fluka users

I am sending neutrons from the Am-Be source towards the target of neutron moderating material to evaluate to moderating power of the material. I have sliced the target into several regions through which I score the neutron fluence exiting each region.

I expect to see a spectrum of moderated neutrons not being dominated by high enegy neutrons in lethargy plot. However, I get the opposite yet from the _sum.lis file I see that high energy neutron are indeed moderated


Could there be something I misunderstand and am doing wrong in my input and during plotting? Any assistance please

I attach my input and screenshot of the plot

ParaffinWax90.flair (9.1 KB)

Dear @Motlatsi_Vincent,

could you share your source AmBe spectrum as well?

David (838 Bytes)
Please find the attached spectrum.

One other issue is that I get a higher number of particle exiting region 1 tp region 2 than the requested primary particles. What could be the cause of this?

Dear @Motlatsi_Vincent,

the issue is that you don’t score the thermal neutrons. FLUKA is capable to simulate neutrons down to 1e-5 eV (1e-14 GeV), If you extend you scorings, you will see the contribution of thermal neutrons as well.

The explanation for the higher than 1.0 total response of the detector between REG1 and REG2 is twofold:

  1. You are scoring fluence. So if a neutron crosses the plane with an angle, it’s contribution is greater than 1. It is possible to score current (plain number of particles crossing the surface) in FLUKA as well.

  2. Some neutrons undergo multiple elastic interaction around the boundary, moving the particle between the two regions multiple times. This causes the neutron to be counted twice, (three times, …). This effect can be checked, by setting REG2 … REG10 to vacuum.


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