Re: [fluka-discuss]: less than 20 MeV neutrons

From: Alberto Fasso' <fasso_at_slac.stanford.edu>
Date: Wed, 30 Jul 2014 05:19:00 -0700 (PDT)

Dear Takashi,

Francesco's answer is correct, but a little cryptic on an important point.
It says that the multigroup approach "does not envisage the explicit
production of charged secondaries but of photons and neutrons only (apart
from specific exceptions)".
One of these specific exceptions is rather important for you, especially since
you noticed that it was missing below 20 MeV: the recoil protons. While 4He and
heavier nuclei are not produced explicitely below 20 MeV (but their
contribution to dose is accounted for as kerma as stated by Francesco), recoil
protons are always produced explicitely and transported in detail. If they were
missing, it was due to the DEFAULTS you used, as pointed out by Mario in a
previous answer.

Alberto

On Wed, 30 Jul 2014, Francesco Cerutti wrote:

>
> Dear Takashi,
>
> no surprise. 20 MeV is exactly the threshold below which neutrons are
> treated through multigroup approach, which does not envisage the explicit
> production of charged secondaries but of photons and neutrons only (apart
> from specific exceptions), that's why you cease to see them (and cannot
> see them), according to what is explicitly said in the manual (10.3.3). At
> the same time, their contribution to energy deposition is taken into
> account (in the kerma approximation).
>
> Best wishes
>
> Francesco
>
> **************************************************
> Francesco Cerutti
> CERN-EN/STI
> CH-1211 Geneva 23
> Switzerland
> tel. ++41 22 7678962
> fax ++41 22 7668854
>
> On Tue, 29 Jul 2014, Maruyama, Takashi wrote:
>
>> I am trying to find energy deposition in a 320 micron-thick Silicon layer
>> from incident neutrons. The neutron energy is monochromatic and set by the
>> BEAM card. When I use the energy density per region in .out file, everything
>> looks good; the energy deposition varies smoothly at 20 MeV. I wanted to find
>> what particles are contributing to the energy depositions. So I use mgdraw to
>> dump these particles. Above 20 MeV neutron incident, I see many recoil
>> protons, 4He and some e-'s contributing to the energy deposition. However, as
>> soon as I cross 20 MeV neutron energy, these recoil protons and 4He
>> disappear, and only e-'s are present. There must be a switch or threshold for
>> recoil particle tracking, but I cannot figure out what. Please help.
>>
>> Takashi Maruyama
>> SLAC
Received on Wed Jul 30 2014 - 16:08:02 CEST

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