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

From: Francesco Cerutti <Francesco.Cerutti_at_cern.ch>
Date: Wed, 30 Jul 2014 14:55:16 +0200

For neutron energy below 20 MeV (differently from higher energy), FLUKA
can explicitly produce protons only if you have hydrogen ("recoil
protons") and/or nitrogen (from (n,p)). Therefore, in case of a
monochromatic neutron beam < 20 MeV on silicon, you will never see them,
no matter which defaults/thresholds you adopt

Francesco

**************************************************
Francesco Cerutti
CERN-EN/STI
CH-1211 Geneva 23
Switzerland
tel. ++41 22 7678962
fax ++41 22 7668854

On Wed, 30 Jul 2014, Alberto Fasso' wrote:

> 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 - 16:20:33 CEST

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