RE: [fluka-discuss]: Neutron cross section

From: Chris Theis <Christian.Theis_at_cern.ch>
Date: Thu, 20 Feb 2014 16:20:36 +0000

Hi Jonathan,

> For activation, you talk about a separate table: what kind of table is it? How is it computed? How Fluka can solve
> Bateman equation with this table?

Activation data for low-energy neutrons is available for example from evaluated data libraries like ENDF or JEFF. You can find more info which data FLUKA is using for low-energy neutron calculations in section 10.4.1.2 in the manual.

As Alberto mentioned already, FLUKA has its own table which describes the production cross-section for residual nuclei. Combining this and the info on the particle fluence, which one gets directly from tracking the particles, one can calculate the respective isotope production rate. So far this is independent from the Bateman equations.

> Normally, Bateman equation needs some information to be solved: fluence, cross-section etc (or other variables
> link to these last one). I really want to know how the activation works with FLUKA.

The Bateman equations are only used later if you want to calculate the time dependent evolution of a nuclide production source term. The differential equations basically describe the temporal change of the number of nuclides considering a nuclide production rate and the possible decay of nuclides via decay chains.

If you apply a Laplace transform on the diff. equ. you can find an analytic, closed-form expression which can give you the activity of a specific isotope after a defined irradiation pattern. There are also other ways to do this but as far as I know FLUKA uses this analytic expression to calculate the final activities. As this step is somewhat separated it can also be done off-line as long as the information of the nuclide production rate is available from the Monte Carlo simulation.

Cheers
Chris


From: owner-fluka-discuss_at_mi.infn.it [mailto:owner-fluka-discuss_at_mi.infn.it] On Behalf Of Jonathan Derrien
Sent: 20 February 2014 16:32
To: Alberto Fasso; fluka-discuss_at_fluka.org
Cc: Vittorio Boccone
Subject: RE: [fluka-discuss]: Neutron cross section

Dear Alberto,

This is a part of one of the output file (.out):

1 Cross sections for media 1
                      (Residual nuclei informations available)
 GROUP SIGT SIGST PNUP PNABS GAMGEN NU*FIS EDEP PNEL PXN PFISS PNGAM PNP PNA
          barn barn GeV/col
    1 2.118E+00 2.806E+00 0.000 1.325 1.512 0.000 0.001 0.564 5.356E-01 0.0000 0.0002 0.0080 0.0066
    2 2.123E+00 2.832E+00 0.000 1.334 1.514 0.000 0.001 0.575 5.463E-01 0.0000 0.0003 0.0086 0.0072
    3 2.146E+00 2.875E+00 0.000 1.340 1.498 0.000 0.001 0.581 5.527E-01 0.0000 0.0003 0.0092 0.0077
                                                                                 .
                                                                                 .
                                                                                 .

As you can see, there are PNP and PNA. For activation, you talk about a separate table: what kind of table is it? How is it computed? How Fluka can solve Bateman equation with this table? Normally, Bateman equation needs some information to be solved: fluence, cross-section etc (or other variables link to these last one). I really want to know how the activation works with FLUKA.

Thank you,

Jonathan

--
[http://polytech.ulb.ac.be/signature/img6.php?fname=Jonathan+Derrien&ftitle=Teaching+Assistant+-+PhD+Student&fservice=Nuclear+Metrology+Service&fcp=165/84&fext=2097&fax=&hasmail=true&fmail=jonathan.derrien&fmailext=%40ulb.ac.be&web=http://mywebsite.ulb.ac.be&x=46&y=10&]
> Date: Thu, 20 Feb 2014 15:24:17 +0100
> From: fasso_at_mail.cern.ch<mailto:fasso_at_mail.cern.ch>
> To: fluka-discuss_at_fluka.org<mailto:fluka-discuss_at_fluka.org>
> CC: jderrien_at_ulb.ac.be<mailto:jderrien_at_ulb.ac.be>; dr.vittorio.boccone_at_ieee.org<mailto:dr.vittorio.boccone_at_ieee.org>
> Subject: RE: [fluka-discuss]: Neutron cross section
>
> Hi Jonathan,
>
> reading your first email, I wasn't sure where you had found PNP and PNA.
> Now that you say you refer to the cross sections printed by LOW-NEUT, I
> can assure you that neither is there with such names. The only ones that
> more or less look similar are the following (I am quoting from the Manual,
> Chapter 9):
>
> PNUP = upscatter probability (can be different from zero only if
> there are several thermal groups)
> PNABS = probability of Non-ABSorption (= scattering).
> It is = SIGST/SIGT, and can sometimes be > 1 because of
> (n,xn) reactions
> PNEL, PXN, PFISS, PNGAM = partial cross sections, expressed as
> probabilities (i.e. ratios to SIGT). In the order:
> non-elastic, (n,xn), fission, (n,gamma)
>
> Group cross sections are mainly for neutron transport, not for activation.
> The only exceptions are PNGAM and the 14-N(n,p)14-C cross section. The
> situation is different for the few cases where pointwise cross sections
> can be used (6-Li and 10-B(n,alpha)7-Li). In FLUKA, activation by
> low-energy neutrons is obtained by a separate table of residual
> nuclei, where the values are group dependent and uncorrelated with the
> actual sampled reaction.
>
> Alberto
>
> On Thu, 20 Feb 2014, Jonathan Derrien wrote:
>
> > Hello Vittorio,
> >
> > Thank you to trying to answer my question. Well, for neutron with energies
> > below 20 MeV, cross-section libraries are used. These cross-sections can be
> > seen by using the command LOW-NEUT.  Have you any idea about how compute the
> > activation cross-section thanks to the data given by LOW-NEUT?
> >
> > Best,
> >
> > Jonathan
> > --
> > [mywebsite.ulb.ac.be&x=46&y=10&]
> >
> > ____________________________________________________________________________
> > Date: Thu, 20 Feb 2014 14:26:53 +0100
> > Subject: Re: [fluka-discuss]: Neutron cross section
> > From: dr.vittorio.boccone_at_ieee.org<mailto:dr.vittorio.boccone_at_ieee.org>
> > To: jderrien_at_ulb.ac.be<mailto:jderrien_at_ulb.ac.be>
> > CC: fluka-discuss_at_fluka.org<mailto:fluka-discuss_at_fluka.org>
> >
> > Dear Jonathan,
> > What do you exactly mean with "data used by Fluka,  I'm not sure if I'm
> > mislead by the name but I had some memories that those could be the
> > probabilities of emission of a neutron and a proton (Pnp) and the one for
> > the emission of a neutron and an alpha (Pna).
> >
> > best,
> > V.
> >
> >
> > On Thu, Feb 13, 2014 at 10:55 AM, Jonathan Derrien <jderrien_at_ulb.ac.be<mailto:jderrien_at_ulb.ac.be>>
> > wrote:
> > Hello dear Fluka Users,
> > I'm using the data used by Fluka for the low neutron and  I found
> > among the cross section and different probabilities two probabilities:
> > PNP and PNA. I do not see what it is, so what is their meaning?
> > Moreover, does anyone know how from these data to determine the
> > activation cross section?
> > Sincerely,
> >
> > Jonathan Derrien
> >
> > --
> > [mywebsite.ulb.ac.be&x=46&y=10&]
> >
> >
> >
> >

Received on Thu Feb 20 2014 - 18:25:38 CET