Re: [fluka-discuss]: Deposited Energy for monochromatic Low energy neutrons

From: Andrea Tsinganis <Andrea.Tsinganis_at_cern.ch>
Date: Mon, 20 Mar 2017 12:13:38 +0200

Dear Cristian,

What you observe is the result of the treatment of neutrons <20 MeV in
FLUKA.

In particular, with a few exceptions listed in the manual (Chapter 10),
energy deposition by low-energy neutrons is handled in most materials with
the kerma (kinetic energy released in matter) approximation, without
explicit generation of secondary particles (recoil nuclei, reaction
products etc.). This means that, if you generate any number of neutrons of
the same energy (i.e. belonging to a specific energy group out of the 260
available) the energy deposition value you will receive e.g. with EVENTBIN
will always be the same. In fact, if your region contains more than one
element, you will receive a few discrete values, each corresponding to one
nuclear species.

There is unfortunately no easy workaround to this that I am aware of to
generate a realistic energy deposition distributions (except for some
materials like Hydrogen which can be treated in a point-wise manner). In
some cases, it may be possible to make a two step calculation, recording
the rate of these interactions and generating the corresponding recoil
nuclei or other particles using energy and angular distributions from
libraries. Of course this is quite time-consuming and may be completely
impractical in certain cases e.g. if you need to study many energies or
expect many reactions to contribute significantly.

A short time dedicated to reading Chapter 10 of the manual will provide
additional insight into this and other possible artifacts when dealing with
low-energy neutrons in low density targets.

Best regards,
Andrea


On Mon, Mar 20, 2017 at 11:30 AM, Cristian Antochi <
VasileCristian.Antochi_at_roma1.infn.it> wrote:

> Hello,
> I am trying to simulate in FLUKA the interactions of a primary neutron
> beam below 20 MeV in a low density gas target of Helium.
> I tried to send a monochromatic beam on the target and I scored the Energy
> through USRBIN and EVENTBIN.
> The problem is that the deposited energy is always the same no matter what
> card I activate. For example, with a primary beam of 1MeV with 3^10^6
> primaries, I get 1674 interactions with an energy deposit of exactly
> 0.33033 MeV every single time. Why does that happen? The elastic
> interaction should give a distribution of deposited energy up to the
> cinematic cut-off at ~0.6MeV...
> Is there anything I should do to activate elastic scattering at low
> energies?
>
> I'm attaching the input file
>
> Thank you,
> Cristian Antochi


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Received on Mon Mar 20 2017 - 12:06:18 CET

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