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

difference comes from the opposite evolution of
the fragment multiplicity—see below.
Figure 2: Same as figure 1, for the light charged
particle (Z≤2) multiplicities. All statistical
errors amount to 0.04.
As appears in the table, the composite system with
N/Z=1.385 has been formed by three different entrance
channels. It is interesting to note that, within
the error bars, the measured multiplicities are the
same. This confirms that the selected quasi-fusion
sources have lost the entrance channel memory
and reached a high degree of equilibration [1].
fragments.
As already noted for total and lcp multiplicities, the
fragment multiplicity is independent of the entrance
channel mass asymmetry for a given composite
system.
Some indications of the evolution of the multiplicities
with the neutron-richness of the system were
already noted in [3,4] . The independence of the
entrance channel for a given N/Z is however an
original new result.
Dynamical simulations
The reaction dynamics can be followed by solving
the BNV transport equation that describes the evolution
of the one-body distribution function according
to the nuclear mean-field and two-body collisions
. The compressibility modulus is K=200 MeV,
And the free nucleon-nucleon cross-section with its
energy, angular and isospin dependence is used.
Two different prescriptions for the symmetry energy
term [5] are chosen: an asy-stiff one (linear
increase of the potential symmetry term with density)
or an asy-soft term roughly varying as the
square root of the density.
The first results, obtained for 124 Xe+ 112 Sn and
136 Xe+ 124 Sn at 45 MeV/nucleon, show the same
relative increase of the fragment number at the
end of the BNV calculation (i.e. before deexcitation
and filter by the experimental apparatus)
as observed experimentally. The asy-stiffness of
the EOS does not influence the fragment multiplicity
for the neutron poor system, while slightly more
fragments are produced in the asy-stiff case for the
neutron-rich one.
References
[1] F. Gagnon-Moisan et al., Proceedings of the IWM2009.
[2] G. Tabacaru et al., Eur. Phys. J. A 18, 103, 2003.
[3] J. F. Dempsey et al., Phys. Rev. C 54, 1710, 1996.
[4] G. J. Kunde et al., Phys. Rev. Lett. 77, 2897, 1996.
[4] V. Baran et al., Nucl. Phys. A703, 603, 2002.
Figure 3: Same as figure 1, for the fragment
(Z≥5) multiplicities. Statistical errors are
0.02.
The multiplicities for fragments (Z≥5) are displayed
in figure 3. Conversely to lcp multiplicities, they
only slightly increase (by about 5%) with the incident
energy. Indeed the systems become more
fragmented at higher energies, and the charge of
the products becomes smaller [2]. Thus a larger
number of them have a charge smaller than 5,
which is the lower limit for our “fragment” definition.
The interesting point is the increase of the
fragment multiplicity for neutron-rich systems. M frag
increases by about 8% when N/Z sys increases from
1.27 to 1.5. This larger fragment number can be
attributed to the larger quantity of available neutrons,
which helps the formation of more stable
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