Re: [fluka-discuss]: Neutrons on carbon still

From: Francesco Cerutti <Francesco.Cerutti_at_cern.ch>
Date: Wed, 5 Aug 2015 18:58:06 +0200

Hi Francesca,

you believe to shoot a 18 MeV (kinetic energy) neutron, whereas you are
shooting a 18 MeV/c (momentum) neutron, which - as written in the
quite useful Beam properties section of the output - has a kinetic energy
of 170 keV. So we are in a region (group #144) where one just expects
capture (apart from elastic), indeed yielding 13C (on 12C) and 14C (on the
small fraction of 13C).

Note also the following:

- your interpretation of the residual nuclei probabilities is wrong, see
http://www.fluka.org/web_archive/earchive/new-fluka-discuss/6203.html

- you are in the world apart of low energy neutrons (also at 18 MeV
kinetic energy), based on library data and not on models. This means that
EVAPORAT and COALESCE are totally pointless. And also LAM-BIAS does not
apply (pointless too), as you can realize looking at the (missing) effect
on number (vs weight) of performed low-energy neutron interactions and CPU
time.

- additionally, as a continuation of the above point, remember that -
apart from very few exceptions - charged particles (including residual
nuclei) by low energy neutrons are not generated explicitly and so cannot
be transported (IONTRANS pointless). You can ask FLUKA to evaluate the
production rates of low energy neutron reaction residues, but these do not
exist as particles in the simulation. The adopted group-wise treatment
prevents from simulating exclusive reaction channels.

Ciao

Francesco

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

On Wed, 5 Aug 2015, francesca.belloni_at_libero.it wrote:

> Dear all,
>
>        I am still trying to get alphas out of n+diamond_material interaction, but with no
> success.
> So I switched to another code and for a 5 m thick diamond, I get alphas (and other heavy
> ions).
> Then I went back to FLUKA and reduced the input at the minimum: only a diamond 500 cm
> thick and neutrons striking it.
> I asked RESNUCLEI in the diamond region, but I only get C13 and C14, as product of n+natC.
> Could anyone give a look, please?
>
> If I look at the output file everything seems to have been activated
> In my *.inp LOW-NEUT, WHAT(4)=4 asks for a printout of  the information about the residual
> nuclei. This is dumped on the *.out file.
>
> If I look at my *.out file I see that the neutrons I am shooting belong to the Neutron
> (low energy)
> Xsec group number 5:
>
>
> Neutron group n.:    5  upper limit 1.8221E-02 GeV,  average energy 1.7995E-02 GeV,
>                                   velocity 5.7846E+09 cm/s,  momentum 1.8477E-01 GeV/c
>
>  
> Later on, on the same *.out file, there is written that the FLUKA medium number 26
> corresponds to the Xsec medium number 1.
>
>
>    Fluka medium     Name     Xsec medium  atomic density          Id. 1        Id.
> 2        Id. 3
>      number                   number     ( at/(cm barn) )
>
>          1        BLCKHOLE         0         0.0000E+00               0           
> 0            0
>
>          2        VACUUM        1000         0.0000E+00               0           
> 0            0
>
>         26        DIAMOND          1         1.7649E-01               6          
> -2          296
>
>
> Going down again in the *.out file there is written:
>
>  Residual nuclei informations for medium    1
>
>   Group:    1   A     Z    Prob
>
>               12     6    7.54E-01
>               11     5    9.09E-01
>               13     6    9.09E-01
>               12     5    9.36E-01
>                9     4    1.00E+00
>               14     6    1.00E+00
>                1     1    7.44E-02
>                2     1    1.07E-01
>                4     2    6.44E-02
> [..]
>
>  Group:    5   A     Z    Prob
>               12     6    7.87E-01
>               11     5    9.11E-01
>               13     6    9.12E-01
>               12     5    9.48E-01
>                9     4    1.00E+00
>               14     6    1.00E+00
>                1     1    4.18E-02
>                2     1    1.18E-01
>                4     2    5.25E-02
>
>
> Therefore it means that in FLUKA the neutrons I am shooting have a certain probability
> (different from 0.0)  to interact with the diamond and give alphas,  Be-9,  B-11 and B-12.
>
> I thought it could have been a problem of statistics (I shoot 9E+7 neutrons on a material
> where lam-bias=-0.001) . But looking at those probabilities, it seems it's more likely to
> get B-12 than C-13  (IF i am understanding the table correctly...which is not sure).
>
> But I get C-13 and C-14 only.
>
>
> What am I missing,please?
>
> Thanks,
>
> regards,
>
> Francesca
>
>
>

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Received on Wed Aug 05 2015 - 20:21:15 CEST

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