Re: [fluka-discuss]: Observed Anomalies for low energy (10 MeV) neutron beam.

From: Andrea Tsinganis <>
Date: Tue, 10 May 2016 09:52:13 +0200

Dear Mina,

Concerning your questions:

1) Observing g-rays with energies higher than the incoming neutron is not
particularly surprising; for example, the capture of a thermal neutron
(practically "zero" energy) can lead to the emission of g-rays of several
MeV. The energy comes from the nuclear reaction, rather than the neutron.

Of course, this is valid within reason: it is difficult to imagine a g-ray
cascade from a nuclear de-excitation that will give g-rays of e.g. 100MeV,
even if your neutron carries that energy. The nucleus will in that case
lose most of the excitation energy through particle emission, plus g-rays
again in the MeV range. So you are less likely to observe Eg>En as En
increases (already at your 30MeV case).

2) Please look at the first several paragraphs of chapter 10 of the manual
(Low energy neutrons in FLUKA), in particular section 10.1.1 "Possible
artefacts". For neutrons <20MeV, the angular distribution after scattering
is discretised into 3 polar angles (the azimuthal angle is sampled
uniformly, so you will see "circles" if you plot your USRBIN in a cut
perpendicular to the z-axis).

This explains the unphysical feature you observe and also why it is more
prominent in the case of a few centimetres of lead (where the probability
of neutron interaction is relatively low and most neutrons interact only
once or not al all, rather than in polyethylene, where more interactions
take place and the effect is smeared out (although still visible).

None of this is relevant for your test cases >20MeV, where indeed you do
not observe this effect.

Best regards,

On Tue, May 10, 2016 at 1:18 AM, Mina Nozar <> wrote:

> Hello everyone,
> I am doing a study to optimize neutron collimation for a setup. I
> initially wanted to know the neutron attenuation lengths for Lead, Iron,
> Concrete, and High density Polyethylene for incident neutron beam energies
> of 10, 30, 100, and 500 MeV.
> There are two features in the results of the 10 MeV neutron beam that I do
> not understand.
> 1) When I look at photon fluence in the shielding material and in a
> detector placed one meter from the shielding, I see photons at energies
> above the beam energy of 10 MeV. How can this be?
> 2) When I look at the side-view distribution of the neutron fluence, I see
> some angular dependencies (peaks and valleys). Where are these angular
> dependencies coming from?
> I only see these features for the 10 MeV neutron beam runs. Attached,
> please find plots for 1) and 2) above.
> Thanks in advance,
> Mina

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Received on Tue May 10 2016 - 11:24:07 CEST

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