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

one of them is made with a neptunium of different
origin, with a higher content of 241 Am. Any problem
with the 235 U reference target can be discarded
because it leads to the expected cross sections for
234 U, as shown above, and also 233 U. The backing
thicknesses have been measured by the slowing
down of the α particles, and the O and H contents
Fig. 2. Fission cross section of 234 U above the threshold, obtained
from the measured ratio to 235 U and the 235 U cross section.
lar for all targets, the ratio of efficiencies between
targets is close to 1. This can be checked for the 4
targets of 237 Np and also for the 2 targets of 234 U.
As illustrated in figure 1 the efficiency is reproducible
within 3%. The angular distribution of fission
fragments affects the efficiency due to the truncation
of the detectable angle range. This effect has
been included as a correction on the ratio of efficiencies.
Overall the systematic uncertainty is
about 4%.
Figure 2 shows the fission cross section of 234 U
above 100 keV. The reference cross section for
235 U is taken from ENDF-B7 up to 30 MeV and
from the high energy extension of JENDL-3.3
above. The steep rises at 0.6, 7 and 15 MeV correspond
to the openings of 1st, 2nd and 3rd chance
Fig. 4. Fission cross section of 237 Np below the fission threshold
have been assessed by RBS of 3 MeV protons. In
addition the angular distribution varies with energy,
but is similar for 237 Np and 235 U. All these effects
have been included in the efficiency correction and
Fig. 5. Fission cross section of 209 Bi
Fig. 3. Fission cross section of 237 Np above the threshold, obtained
from the measured ratio to 235 U and the 235 U cross section.
fission. Our data are in good agreement with previous
measurements, but above 1 MeV we are lower
than the data from Lisowski [2] which extend up to
400 MeV.
Similarly figure 3 shows the fission cross section of
237 Np above 500 keV. Although the shape is very
similar to the one derived from the most recent
measurements (Shcherbakov [3], and also
Lisowski not plotted in the figure), our data are systematically
higher by 6 % above 1 MeV. It’s worth
mentioning that these higher values are obtained
equally with each of the 4 237 Np targets although
we consider that our data are safe
Figure 4 shows the cross section between 1 and
300 keV confirming the recent measurement from
Tovesson which invalidates the ENDF-B6 database
which is now correctly updated in ENDF-B7.
Figure 5 shows the cross section obtained for 209 Bi.
It’s worth stressing that our data are in agreement
with previous measurements which lie essentially
below 200 MeV. The systematics based on these
measurements departs from our values above 200
MeV where data were lacking At 1 GeV our data
join the Prokofiev systematics [4] defined for proton-induced
fission.
References
[1] L. Ferrant, University Thesis, Orsay, 7 Sept 2005
[2] P. Lisowski et al., Conf. on Nucl. Data for Sci. and
Technol. Juelich p. p. 732 (1991).
[3] O. Shcherbakov et al., J. Nucl. Sci. and Tech., Supplement
Vol. 2 p. 230 (2002).
[4] A. V. Prokofiev. NIM A, 463:557, 2001.
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