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NUSTAR-SHE-05 GSI SCIENTIFIC REPORT 2009
174
GSITemplate2007
The capture process and first steps to cold fusion
S. Heinz, V. Comas, S. Hofmann, D. Ackermann, J. Heredia, F.P. Heßberger, J. Khuyagbaatar,
B. Kindler, B. Lommel, and R. Mann
GSI, Darmstadt, Germany
The synthesis of heavy elements in cold fusion reactions
using lead and bismuth targets reveals maximum crosssections
for central collisions at energies below the barrier
predicted by the Bass model. The two-centre shell model
[1] gives an approach to explain fusion at such low energies.
It assumes that the nucleons move in a potential
which is formed by combining the potentials of the individual
nuclei. In central collisions at near-barrier energies
the relative velocities and angular momenta are small and
the valence nucleons can move between the reaction partners
as soon as the outer orbits come into contact. We
investigated the applicability of the two-centre shell
model by following the capture process and first steps to
the compound nucleus in the superheavy system 64 Ni +
207 Pb (Zproj + Ztarget = 110). This was done by studying
transfer products with proton numbers beyond the target
proton number, which have been created under the same
kinematical conditions which result in maximum yields of
fusion products, namely, in central collisions and at nearbarrier
energies.
The experiment was performed at the velocity filter SHIP
[2] which allows the detection of reaction products at angles
of (0 ± 2)°. We measured at beam energies of 4.80,
5.00, 5.20, 5.40, 5.53 and 5.92×A MeV. We detected several
transfer products with proton numbers 84 ≤ Z ≤ 88
via their α-decays. Figure 1c shows the excitation functions
representatively for the isotopes 211 Po, 212 At, 213 Rn,
213 Fr and 214 Ra (Z = 84 to 88). The excitation functions
for isotopes with Z = 84, 85 and 86 are rather narrow
(FWHM = 15 MeV) with maxima located below the Bass
barrier (arrows). They are comparable to those from cold
fusion reactions leading to superheavy elements after oneneutron
evaporation (figs. 1a and b). At low energies fusion
or transfer is hindered by the Coulomb barrier while
towards higher energies other evaporation or fission
channels open and lead to the decrease of the yield. The
location of the maxima corresponds to a distance of ≥ 14
fm between the nuclear centres, which just allows for the
contact of the surfaces. According to the two-centre shell
model this is sufficient to start the exchange of valence
nucleons between the cores. For transfer products with Z
> 86 the maxima of the excitation functions are located at
higher energies and their half-widths are larger which
points to higher excited primary products in comparison
to the previous case. They are comparable to fusion excitation
functions for evaporation channels with more than
one nucleon. If and to what extent also the occupation of
specific neutron and proton shells influences the capture
process will be subject to future experiments.
All observed transfer products originate from deep inelastic
reactions. This is reflected by the total kinetic energies
Figure 1. Excitation functions for 1n evaporation channels
in the cold fusion reactions 58 Fe + 208 Pb and 64 Ni +
208 Pb (a, b) and for different isotopes produced in transfer
reactions in 64 Ni + 207 Pb collisions (c). The energy scale
on the abscissa gives the excitation energy of the compound
nuclei 266 Hs and 271 Ds.
in the exit channel, which are independent of the bombarding
energy and have values slightly below the Viola
energy.
The massive transfer of nucleons, which we observed at
very low beam energies, has also an applicatory aspect.
The low beam energies lead to transfer products at low
excitation energies. This enhances their survival probability
against particle evaporation and fission. Therefore,
such transfer reactions could be used to create new isotopes
of heavy nuclei, especially on the neutron-rich side
of the nuclear chart.
References
[1] D. Scharnweber, W. Greiner, and U. Mosel, Nucl.
Phys. A 164, 257 (1971).
[2] S. Hofmann and G. Münzenberg, Rev. Mod. Phys. 72,
733 (2000).