Modeling complex features in the fission of light actinides with GEF

Slide 1
Title and authors

Slide 2
Already in the 1970th, abnormal behaviour of fission probablities and FF angular distributions
were observed in the fission of thorium isotopes. These observations were attributed to the 
existance of a triple-humped fission barrier.
In the mean time, it is well established that fission of light actinides proceeds by the
passage from the almost spherical ground state over three barriers. The figure shows the result
of a recent theoretical caculation by Bernard et al. for 226Th.
To the best of our knowledge, there exist no investigations about the influence of the
third barrier on the FF yields.
This is the subject of the present talk

Slide 3 
The impact of the third barrier is expected to correlate with the height of the third barrier
in comparison with the two inner barriers. For the inner barriers, the barrier heights were
deduced from measured fission probabilities. Unfortunately, some (although limited) information
on the height of the third barrier is only available for the lighter actinides around Th isotopes.
Therefore, we estimated the height of the third barrier for heavier nuclei by an 
"educated guess".
We observe that the maxima of the three barriers in 230Th follow a linear dependence. 
For heavier nuclei, the second barrier decreases primarily because the shape of the 
macroscopic barrier changes in a systematic way. 
Thus, we used a linear extrapolaton to estimate the height of the third barrier for the heavier
nuclei. After fixing these three point relative to the ground state, there is a smooth line
drawn in the figure, just to guide the eye. The result looks rather reasonable.
We see that we expect that the influence of the third barrier (if there is any) appears in the
ligher nuclei, while it should fade away in the heavier systems.

Slide 4
This slide shows the FF Z distributions for thermal-neutron-induced fission of systems very close 
to the nuclei selected in the previous slide.
We see (in black) the Z yields from the ENDF-B/VII evalution. We observe that the position and
the shape of the heavy peak are very similar for the heavier systems, but the lightest system
(229Thnth,f) deviates appreciably.
For a better comparison, the red lines repeat the Z distribution of 239Pu(nth,f).

Slide 5
Similar data at higher energies are scarce, whith the exception of 14-MeV-neutron-induced fission of
a few systems.
The available data for 239Pu, 235U and 232Th are shown in this slide.
We observe that the position and shape of the Z distribution from 232Th(n-14MeV,f) is much
closer to the heavier sytems. 
We think that the deviation can be caused by a contribution of low-eneryg fission due to
multi-chance fission (fission after the emission of one or two neutrons).  

Slide 6
We assume that the strong deviation of the Z distribution of the 229Th(nth,f) from that of the other
systems is caused by the suppression of trajectories with less mass asymmtry. The mass-dependent
prompt-neutron multipicities suggest that these are the more compact configurations at scission.
It seems also that the deviation occurs only at low excitation energies, where the third barrier
may only be traversed by tunneling.
I follows that the memory on the distribution in mass asymmetry estblished after the second 
barrier is erased in this case (low E* -> tunneling through the third barrier).
At higher energies (no tunneling), the memory on the distribution in mass asymmetric established
after the second barrier is presesrved. 
A quantitative model has been developed and implemented in GEF.

Slide 7
Here we see the results of two sets of calculation from GEF in blue, compared with the data from
ENDF-B/VII (in black).
Left column: Full code with the suppression effect included.
Right column: GEF result, disregarding the suppression effect.
We see the strong effect for 229Th(nth,f) that was shown already before.
The two uranium isotopes show already a non-negligible effect.
The lowest inset shows the results for 238U(spont. fission). Due to the lower excitation energy,
the suppression effect, in particular for the S1 fission channel, is drastic.

Slide 8
One would like to know at which excitation energy the suppresssion effect disappears. 
We did not find any good data on fission-fragment distributions as a function of excitation
energy. However, the change of the FF yields, in particular the variation of the yield of the
S1 fission channel (almost spherical heavy fragment near 132Sn) has a large impact on the TKE.
This slide shows measured TKE values (in black) for 238U(n,f) as a function of incident-neutron energy.
(These are the only data that we found that show the impact of the suppression effect on the TKE.
The data of similar kind for other systems do not reach down enough in excitation energy.)
The data are compared with two GEF calculations, with and without the suppression effect.
One can see that the suppression effect disappears already at E* = 8 MeV, that means a 
neutron energy of 4 MeV. 

Slide 9
Now, we can use GEF for performing systematic calculations of fission yields, including the
suppression effect by the third barrier. As we saw in the previous slide, the suppression
effect has an impact on the FF distributions in the range of neutron energies of a few MeV.
Thus this suppression effect by the third barrier should be considered in the simulation of
fission reactors, in particular when the operate with fast neutrons.
It has also an impact on the calculation of the anti-neutrino yields and energy spectra
as well as other fission data.

Slide 10
Conclusion:
Mentions, where the new extended GEF code is available, and that the yields
are included in the Janis software.

Supplement
Eventually for the discussion.