Re: [fluka-discuss]: Photofission cross section of Uranium

From: Francesco Cerutti <>
Date: Sat, 7 Mar 2015 23:07:32 +0100

Dear Felix,

unfortunately you hit a famous issue. From the release notes of FLUKA

- There is a known issue with photofission: the photonuclear interaction
   model is not up-to-date with respect to the present fission model.
   As a result, photofission can be heavily underestimated at low photon
   energies for fissile materials.

This is exactly your case. Underestimation becomes as large as you
reported down to around 10 MeV photon energy, where your bremsstrahlung
spectrum lies.

On the other hand, as long as fission is far enough from being one of the
dominant disintegration channels, you can still trust the cross sections
of the latter ones.



Francesco Cerutti
CH-1211 Geneva 23
tel. ++41 22 7678962
fax ++41 22 7668854

On Thu, 5 Mar 2015, Felix Ernst Horst wrote:

> Dear FLUKA experts,
> We carried out an experiment where we irradiated uranium samples (natural
> isotope composition) with bremsstrahlung photons from a clinical 18 MeV
> electron accelerator. We have then determined the fission yield by
> quantitative gamma spectrometry (actually we measured I-131 and calculated
> from its activity to the integral fission yield).
> We have modelled that accelerator in FLUKA. Its output is expressed in
> monitor units and due to an additional simulation of the monitor ionization
> chamber calibration situation with a water phantom we are able to convert the
> FLUKA results normalized per primary electron into results normalized per
> monitor unit.
> When i fold the photon spectrum obtained by USRTRACK with the photofission
> cross section of U-238 (from "A. V. Varlamov et al., Atlas of Giant Dipole
> Resonances (1999)") i come very close to the experimentally determined
> fission yield value (fissions per gramm uranium per monitor unit) while the
> fission yield i scored with RESNUCLE is about a sixth of the measured value.
> I checked my normalization to the volume and density and could not find any
> errors there.
> Is such a large underestimation by FLUKA possible or could there be another
> explanation?
> And can i trust the cross sections for other photonuclear reactions such as
> neutron disintigration from lead or tungsten?
> Thank you and regards!
> Felix
> --
> 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 Sun Mar 08 2015 - 00:23:53 CET

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