Re: Re: [fluka-discuss]: Biasing Air Activation

From: Felix Ernst Horst <>
Date: Wed, 22 Apr 2015 11:06:23 +0200


Dear Vittorio,

Sorry, my Mailing Client sent the answer on Alberto's suggestion
directly to him and later when i saw that I tried to forward it to the
fluka-discuss adress but that failed.

For O-16 and N-14 the photoneutron production cross section is indeed
very low - around two orders of magnitude smaller than the one of e.g.

I will also try the simulation with a simple geometry and very high
cutoff as you suggested.

A question besides FLUKA: Do you have a hint where to look for
well-resolved and evaluated cross section data for the purpose of direct
determination of the activation yield by folding it with the photon
spectrum? The data i found in the JENDL library for O-16 and N-13 looks
very smooth and seems not to include the fine structure as e.g. shown in
plots in "P. Oblozinsky, Handbook on photonuclear data for applications:
Cross-sections and spectra, IAEA-TECDOC-1178 (2000).". For my photon
spectrum with a cutoff energy of 18 MeV the choice of which cross
section i use might effect the result by factor of two or more i guess.

I attached a comparison of two such data sets as .png.

Thank you


Here another copy of my last answer to Alberto:

Dear Alberto,

Thank you for the fast response.

I attached my input file and the output of detector 41, which scores
resulting nuclei over the whole air volume around the electron linac
that i modelled (~ 180 m^3). Certainly the activation is mostly produced
in the photon field whose dimension is given by the collimator setting
but i like to have an estimate for the activation averaged over the
whole air volume.
Actually you see some N-13 with high uncertainty, but no O-15.
The output of detector 42, scoring the local activation in a small air
volume at the isocenter (0/0/0) was still empty after that run.

I activated photonuclear reactions at all energies and in all materials,
so the neutrons which cause the capture activation in air might come
from the tungsten collimators and shielding which are the main neutron

The cutoff energy of my bremsstrahlung spectrum (18 MeV) is not much far
from the production threshold of O-15 (15.7 MeV) so that only few of the
generated photons may cause that reaction. Could that be a reason for
the bad statistics?

The procedure of tracklength-scoring the spectral photon fluence and
folding it with the cross section sounds much more easier than my
method, thank you for that hint.



Am 2015-04-21 23:23, schrieb Vittorio Boccone:

> Dear Felix, I have some problem to read correctly your last e-mail, hope it's not the mailing-list software which is messing the things up.
> I guess it's mostly a problem of statics. In order to get a feeling of your generation cross section you can try to run a pilot simulation of a long tube surrounded by a black hole, where you cut everything below the phot.nucl. threshold.
> What Alberto suggest is actually a very practical way to get it directly.
> Best,
> Vittorio
> 2015-04-21 14:54 GMT+02:00 Felix Ernst Horst <>:

Felix Horst, B.Sc.
Institut für Medizinische Physik und Strahlenschutz (IMPS)
Technische Hochschule Mittelhessen
Wiesenstr. 14, 35390 Gießen
Raum D10.12a
Telefon: 0641 309 2642

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Received on Wed Apr 22 2015 - 12:39:30 CEST

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