exotic nuclei structure and reaction noyaux exotiques ... - IPN - IN2P3

Fission cross sections of actinides at nTOF,
from resonances to spallation
IPNO Participation: L. Tassan-Got, L. Audouin, C. Paradela, C.-O. Bacri, C. Stéphan, B. Berthier, D.
Tarrio, D. Trubert, C. Le Naour
Mesures de sections efficaces de fission à nTOF, des résonances à la spallation
Le faisceau de neutrons de l’installation nTOF du CERN offre des possibilités uniques : gamme en énergie
étendue (0.7eV à 1GeV), très bonne résolution en énergie des neutrons, haut flux instantané particulièrement
utile pour les cibles radioactives. Nous avons mesuré les sections efficaces de fission par détection
en coincidence des fragments de fission, ce qui permet d’éliminer la contribution des autres types
de réaction à haute énergie, et réduit le bruit de fond de radioactivité. Les isotopes pour lesquelles les
mesures ont été effectuées sont 232 Th, 233 U et 234 U jouant un rôle important dans le cycle du thorium,
237 Np comme cible possible de l’incinération, ainsi que nat Pb et 209 Bi qui interviennent dans les cibles de
spallation. Toutes les mesures ont été effectuées relativement à 235 U et 238 U considérés comme références.
Pour le 237 Np nous trouvons une section efficace de fission supérieure à celle des bases de données.
Nuclear reaction data for minor actinides play a
key role in the development of a new generation of
nuclear reactors. The aim for future reactors is to
significantly reduce nuclear waste radiotoxicity
through partitioning and transmutation techniques,
thereby decreasing the repository needs for highlevel
radioactive waste.
Recently, to fulfil the data requirements for these
new reactors, there has been a demand for extensive
neutron-induced nuclear cross section measurements
for actinides. Inside this line we measured
the fission cross section of 234 U and 237 Np. The
incineration of 237 Np, one of the more abundant
isotopes in the spent fuel from current reactors, is
an important issue in transmutation. 234 U plays an
important role for the development of the thorium
cycle, where it acts as an analogue to 240 Pu in the
Pu/U cycle of the present-day fast reactors. Most
previous measurements of these isotopes were
performed a few decades ago in limited neutron
energy ranges.
nat Pb and 209 Bi play a key role in accelerator driven
systems (ADS) since the spallation target will most
likely consist of lead or lead-bismuth. In addition
209 Bi(n, f) has been recommended as a crosssection
standard for neutron energies above 50
MeV but new measurements are needed.
The very broad energy range (thermal to 1 GeV) of
neutrons delivered at the nTOF facility at CERN
allowed accurate measurements of the fission
cross sections of 233 U, 234 U, 237 Np, nat Pb and 209 Bi.
They all have been performed in reference to 235 U
and 238 U permanently present in the stack of targets.
.
The detection system was built in order to detect
the 2 fragments in coincidence and hence discriminate
fission events from other types of reaction :
radioactivity, recoils of reactions appearing at energies
above 10 MeV, and spallation reactions at a
few tens of MeV. The experimental setup is made
of a stack of 10 parallel plate avalanche counters
(PPAC) interleaved with 9 targets including the 2
reference isotopes. The ambiguities arising from
the possible crossing of 2 detectors by the same
fission fragments are solved by the coincidence
time between detectors.
Fig. 1. Fission yields ratios between different samples of the
same isotope, normalized by the actinide content. The ratio
between the two 234 U samples is shown by the dotted line with
symbols, while the ratios between the 237 Np samples and their
average are indicated by solid colored lines.
The detailed characteristics of the targets are described
in [1]. They are made by electrodeposition
on a 2 µm thick aluminium foil, over a disk of 8 cm
in diameter. The content of the deposited layer has
been obtained by α counting with an accuracy better
than 1 %, and the spatial distribution has also
been scanned by α counting and by the RBS technique.
Due to pile-up problems, localisation measurement
could not be used for a systematic monitoring of
the detector acceptance. Therefore the cross sections
measurement rely only on the coincidences
of the fast anode signals. The detection efficiency
is mainly determined by the limitation of the solid
angle resulting from the stopping of the fission
fragments in the backing and the PPAC electrodes
when the fission angle is higher than 50°. The
thicknesses of the stopping layers being very simi-
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