Dear Andrea,
Thank you very much for the clear explanations. They make sense and I
understand.
Wish you the best,
Mina
On 16-05-10 12:52 AM, Andrea Tsinganis wrote:
> 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,
> Andrea
>
>
> On Tue, May 10, 2016 at 1:18 AM, Mina Nozar <nozarm_at_triumf.ca
> <mailto:nozarm_at_triumf.ca>> 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 Wed May 11 2016 - 02:43:59 CEST