RE: U-238 fission yield

Hi Lucia,

- difference between FLUKA and MCNPX:
for U-238, and neutron energies below 20MeV, FLUKA uses evaluated cross
sections based on ENDF/B-VI, Jeff and Jendl. Fission neutrons are
created using a group-dependent fission probability and are emitted
isotropically with an energy sampled from a fission spectrum which is
specific for the isotope. For energies above 20MeV fission is fully
treated by models. For MCNPX it is similar, however for energies between
20-150MeV the xSections are based on computed xSection sets, thus the
comparison shown by you shows a reasonable agreement, certainly within
the uncertainties of the respective assumptions.

- missing 'dip'
the usual 'dip' in the mass spectra can be nicely seen at typical
fission spectra energies of ~2.8 MeV (or fully separated in case of
spontaneous fission), however at 40 MeV it's already 'filled' (then
becoming 'Gaussian' for higher E) - thus the distribution is what one
expects.

- how to score fission reactions only:
in your case (40MeV) this will not make a big difference, however if
needed you can use the scoring user-routine (usrrnc.f, don't forget
USERWEIG WHAT(5) > 0.0) and check the common <RESNUC> for LRNFSS. The
latter will be true if the parent reaction was fission.

hope this helps, cheers

Markus

> -----Original Message-----
> From: owner-fluka-discuss_at_mi.infn.it [mailto:owner-fluka-
> discuss_at_mi.infn.it] On Behalf Of Lucia Sarchiapone
> Sent: Montag, 19. J=E4nner 2009 11:15
> To: fluka-discuss_at_fluka.org
> Subject: U-238 fission yield
>=20
> Hi,
>=20
> I want to evaluate the fission yield from 40 MeV proton on U-238 (more
> precisely, a UC compound).
> As you can see from the attached picture, I get reasonable results,
> even
> compared to MCNPX data, but I cannot explain where does the central
> peak
> come from, I mean the one corresponding to A=3D115.
> I thought this fission yield shape was typical for higher proton
> energies, while for 40 MeV protons I would expect a 'valley' in that
> region (110 < A < 120).
> The other question I have, is it possible to score just fission
> products
> excluding nuclei emerging from other reactions?
> Thanks a lot
>=20
> Lucia
Received on Mon Jan 19 2009 - 15:07:47 CET