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

pattern is not trivially due to a change in the size of
the quasi-projectile. After a shape analysis in the
center of mass frame, only events with a
total forward detected charge larger than 80% of
the Au charge were considered.Two different procedures
aiming at selecting events with negligible
neck contribution were adopted. In the first one [6]
(I) by eliminating events where the entrance channel
dynamics induces a forward emission,in the
quasi-projectile frame, of the heaviest fragment
Z 1 .For isotropically decaying quasi-projectiles, this
Figure 3: Size of the heaviest fragment Z 1 versus
total excitation energy. That picture is constructed
using the fit parameters extracted from the equivalent-canonical
distribution. The distance between
the two maxima, liquid and gas peaks, projected
on the excitation energy axis corresponds to the
latent heat of the transition.
projectile selections. The results (for one quasiprojectile
selection) are illustrated in figure 3.
Figure 2: Upper part, measured distribution of the
charge of the largest fragment normalized to the
charge of the quasi-projectile detected in Au+Au
collisions at three different bombarding energies.
Lower part, weighted distributions obtained considering
the same statistics for each excitation energy
bin.
procedure does not bias the event sample but only
reduces the statistics. In a second strategy (II) the
reduction of the neck contribution is obtained by
keeping only ``compact'' events by imposing (i) an
upper limit on the relative velocity among fragments,
and (ii) a constant quasi-projectile size within
10% (see [7] for details). In both cases fission
events were removed. The results obtained with
the two different selection methods are shown in
figure 2.To take into account the small variations of
the source size, the charge of the heaviest fragment
Z 1 has been normalized to the source size.
After the weighting procedure (lower part of
figure 2), a bimodal behavior of the largest fragment
charge clearly emerges for both selection
methods [8].
Those weighted experimental distributions can be
fitted with an analytic function (see [8] for more
details). From the obtained parameter values one
can estimate the latent heat of the transition of
the hot heavy nuclei studied (Z~70) as
ΔE=8.1 ± 0.4 (stat) +1.2 -0.9 (syst) MeV per nucleon.
Statistical error was derived from experimental
statistics and systematic errors from the
comparison between the two different quasi-
References
[1] P. Chomaz et al., Phys. Rev. E 64, 046114, 2001.
[2] B. Borderie and M. F. Rivet, Prog. Part. Nucl. Phys. 61, 551,
2008.
[3] D. H. E. Gross, World Scientific, Vol. 66 of World Scientific
Lecture Notes in Physics, 2002.
[4] F. Gulminelli, Nucl. Phys. A 791, 165 , 2007.
[5] M. Di Toro et al., Eur. Phys. J A 30, 65, 2006.
[6] M. Pichon et al. (INDRA Collaboration), Nucl. Phys. A 779,
267, 2006.
[7] E. Bonnet et al. (INDRA and ALADIN Collaborations), Nucl.
Phys. A, 1, 2009.
[8] E. Bonnet et al. (INDRA and ALADIN Collaborations), Phys.
Rev. Lett. 103, 072701, 2009.
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