Date: Tue, 28 Jan 2014 13:08:49 +0100
Dear Chris
I answer below both the technical and physical questions:
a) Technical answer:
the residual nuclei probabilities for low energy neutrons printed
in the output files for all isotopes heavier than alphas are the
cumulative probabilities normalized to the non-elastic cross section
(sigma_non = sigma_total - sigma_elastic)
Eg, if you have P_1, P_2, ..., P_n-1, P_n, the corresponding production
cross sections are:
sigma_1 = sigma_non * P_1
sigma_2 = sigma_non * (P_2-P_1)
...
sigma_n = sigma_non * (P_n-P_n-1)
Normally P_n should be 1 unless fission (fission products are NOT
printed in the output) is a possibility or the target isotope is
very light and fragments into alphas or lighter (see later)
For deuterons (d), tritons (t), helions (h), alphas (a), the same
applies but in this case probabilities are not cumulative, that is:
sigma_d = sigma_non * P_d
...
sigma_a = sigma_non * P_a
Indeed the cumulative/non cumulative issue can be somewhat confusing
and perhaps I'll change to fully non cumulative for the next release
In your case, the fact that you have 1 for both C-13 and C-14 simply
reflects that within the number of digits printed the C-14
probability is negligible wrt the C-13 one (it is not zero but too
small given the number of digits printed)
b) Physical issue:
the status of evaluated data for C-nat, 12-C and 13-C neutron cross
sections is complex. In a nutshell reliable evaluated data exist only
for C-nat and these are the data in the Fluka neutron library: of
course in principle they do not allow to say anything about residuals.
The residuals computed for Fluka make the approximation C-nat=C-12
with an ad-hoc correction for C-13(n,gamma)C-14 in the thermal region
where data exists. This ad-hoc addition was triggered a few years ago
by a request of people from PSI and you can probably find the relevant
exchanges in the Fluka mailing list archives: it fulfills the original
goal of accounting for most of C-14 production, which takes place
in the thermal region.
The plot you sent about C-13(n,gamma)C-14 is NOT a standard evaluation
based on experimental data which are very scarce (see later), but has
been rather computed by the SIG-ECN code. Those numbers are possibly
reasonable but they do not fit well with the available C-nat
transport cross section evaluated data. In particular the resonance
at ~150 keV does not show up in the C-nat evaluated data (see plot
attached).
Experimental data for (n,gamma) on C-nat, C-12 and C-13 are very few
out of the thermal region as you can easily check yourself in EXFOR.
The resonance at ~150 keV has been more or less measured (see PRC3 1737)
and its area is 112 b eV, which makes it negligible in almost all
practical situations where the thermal part will dominate as usual
(you can check yourself with your spectrum), unless very few
thermal neutrons are present. The C-12, C-nat (n,gamma) cross section
experimental data are also very scarce and this can explain the
inconsistency between the evaluated C-nat (n,gamma) and the known
C-13 (n,gamma) resonance at ~150 keV. In any case these cross sections
are very small and this could explain why they had never been measured
better/more in details.
Since no evaluated transport data set, computed or based on
experimental data, exists for both C-12 and C-13 (with the exception of
TENDL) there is no obvious way for us to have in the Fluka library
something which keeps the accepted and well known C-nat transport cross
sections together with the supposed C-13 (n,gamma) one as computed with
SIG-ECN.
TENDL (which is computed) has transport data sets for both C-12 and
C-13. The C-13 (n,gamma) data are the SIG-ECN ones, so in principle
one could use TENDL evaluations for C-12 and C-13 to build a Carbon data
set for Fluka instead of the well established C-nat everybody uses.
Unfortunately the TENDL (computed) evaluation for C-12 shows gross
disagreements wrt both experimental data and evaluated C-nat data
in the 10-20 MeV range up to a factor 1.5 on critical cross sections
like (n,non) and therefore I am not at all inclined to use it to
(slightly) improve on the already tiny C-14 production at the expense
of jeopardizing the bulk of neutron transport in carbon.
+----------------------------------------------------------------------+
| Alfredo Ferrari || Tel.: +41.22.76.76119 |
| CERN-EN/STI || Fax.: +41.22.76.69474 |
| 1211 Geneva 23 || e-mail: Alfredo.Ferrari_at_cern.ch |
| Switzerland || |
+----------------------------------------------------------------------+
On Mon, 20 Jan 2014, Chris Theis wrote:
>
> Dear all,
>
>
>
> a recent comparison regarding the production of residual nuclei by low
> energetic neutrons on natural Carbon revealed curiously low values of C-14
> by FLUKA. In order to understand this better we tried to print the neutron
> cross-section information (LOW-NEUT WHAT(4) = 4.0) but this did not prove to
> be helpful as the output is undocumented, which makes the correct
> interpretation impossible, and a posting to the discussion list did not
> yield any useful info either.
>
>
>
> We˘re afraid that the obvious assumption that the printed values, which are
> labeled "prob", denote production probability is somewhat questionable
> because the values per group sometimes sum up to >1.0. Also the fact that
> for example the production of C-13 and C-14 in group 58 both show a prob.
> of 1.0 remains somewhat mysterious to us as we assume that they both
> originate from (n,g) reactions on C-12 and C-13 respectively. Any comments
> on the correctness of these assumptions and an explanation of the print-out
> are highly appreciated.
>
>
>
> Analysis of X-section data which is publicly available in the IAEA's neutron
> gamma capture atlas or JEFF hint at the possibility that the (n,g) capture
> reaction of C-13(n,g)C-14 used by FLUKA could be considerably different and
> misses the significant peak around 0.1 MeV. Therefore, we wanted to ask if
> one of the developers could kindly confirm that the X-section used by FLUKA
> is similar to the IAEA data (see attachment) and that it includes the peak
> at 0.1 MeV.
>
>
>
> Thanks a lot for your help
>
> Chris & Helmut
>
>
>
> ---------------------------------------------------------------------------
> --------------
> Chris Theis
> CERN/DGS-RP - European Organization for Nuclear Research
> 1211 Geneva 23, Switzerland
> Phone: +41 22 767 8069 Office: 892-2A-015
> e-mail: Christian.Theis_at_cern.ch www: http://www.cern.ch/theis
> ---------------------------------------------------------------------------
> --------------
>
>
>
>
>
(image/gif attachment: cnat-ng-endf.gif)