DYNAMICS OF THE SUPERHEAVY SYSTEM IN THE 86Kr + 208Pb REACTION
V.A. Rubchenya1, 2*, A.A. Alexandrov3, S.V. Khlebnikov2, V.G. Lyapin2, 4, V.A. Maslov3,
Yu.E. Penionzhkevich3, G. Prete5, Yu.V. Pyatkov3, Yu.G. Sobolev3, G.P. Tiourin1, 2, W.H. Trzaska1,
4
, D.N. Vakhtin2, J. Aysto1, 4
1
Department of Physics, University of Jyvaskyla, Finland
KhlopinR adium Institute, St.Petersburg, Russia
3
Flerov Laboratory of Nuclear Reactions, JINR, Dubna, Russia
4
Helsinki Institute of Physics, Finland
5
INFN Laboratori Nazionali di Legnaro, Italy
2
The main problem in planning the
experiments aimed at the synthesis of superheavy nuclides is the prediction of the
formation of a compact compound superheavy nucleus, which decays to the ground
state by neutron- and γ-emission with a very
strong fission competition. This, in turn,
depends on the interplay of reaction channels
and ultimately on the dynamics of the fusionfission process. The reaction of 449 MeV 86Kr
with 208Pb was used in an attempt to produce
new super-heavy nuclei with Z = 118 [1].
Fragment mass and kinetic energy
distributions for this reaction at near Bass
barrier energies have been measured in Dubna
and the fission probability of the compact
compound system was estimated [2].
In the present work [3] results of the
investigation of the dynamics of the excited
super-heavy system with Z = 118 in the same
reaction at EKr = 600 MeV, which is well
above the interaction barrier, using n- and αparticle
probes
are
presented.
The
experiments were carried out at JYFL,
Finland.
Fig. 1. TKE distribution for all fragmentation
events (dashed) and in coincidence with neutrons
(dotted) and α-particles (solid).
The mass and kinetic energy of binary
fragments were determined by the TOF-
method.
TKE-distributions
for
all
fragmentation events (dashed curve), in
coincidence with n (dotted curve) and α (solid
curve) are shown in Fig. 1. One can see that
the TKE-curves in coincidence with n and α
tend to shift towards lower energies.
Fig. 2. The fragment mass distribution in
coincidence with n (lower part) and α-particles
(upper part).
Fragment mass distributions measured in
coincidence with neutrons (lower part) and α
(upper part) are shown in Fig. 2. Fragment
yields in the symmetric region increase
substantially when fragments were tagged by
α-particles and can be decomposed into a
symmetric, which may correspond to the
compound nucleus fission, and two
asymmetric
components,
which
may
correspond to deep inelastic collisions (dotted
curve) and fast or quasi-fission (dashed
curve). Thus, we can conclude that the
average total fragment kinetic energy (about
270 MeV) is considerably higher than the
value predicted for fission of the compound
nucleus (about 235 MeV) and that the
fragment mass and Ekin distributions tagged
by n or α correspond to more damped
collisions, where mass symmetric components
are enhanced.
Double differential distributions of αparticles were measured in coincidence with
fragments and have been analyzed within the
multiple-source model, which included 4
sources: two fragments, compound nucleus
and neck region between the fragments. The
comparison of the experimental spectra and
the calculated ones with α-multiplicities
MCN=0.015 from compound and MNF=0.05
from neck sources, and temperature T=2 MeV
is shown in Fig. 3.
Fig. 4. Pre- and post-scission neutron
multiplicities as functions of the fragment mass.
The values of post-scission neutron multiplicities,
predicted for compound nucleus fission, are
presented by open circles.
scission multiplicity with the experimental
limit of Mpre < 0.9 an estimation of the
lifetime of the composite system for
symmetric fragmentation Tpre ≤ 10-21 s was
obtained.
Thus, evidence of neck fragmentation in
heavy ion collisions at low energy was
obtained. A new experiment to investigate the
neck fragmentation process in greater detail is
planned in the nearest future.
REFERENCES
Fig. 3. The comparison of the measured α-particle
spectra (histogram) with calculated spectra (open
circles) at four different angles.
To describe the α-spectra one has to
assume high multiplicity for the neck
fragmentation, which exceeds by about a
factor of 4 the yields for 294118, predicted by
the model of ternary fission MNF = 0.013 [4].
Experimental pre- (solid squares) and postscission (solid circles) neutron multiplicities,
together with the theoretical values of postscission neutron multiplicities (open circles)
calculated for compound nucleus fission,
using model [5], are shown in Fig. 4. Here
gates on TKE were applied in order to
suppress elastic events, viz.│TKE - <TKE>│
< 70 MeV. One can conclude that the
averaged pre-scission neutron multiplicity is
close to zero, but the error is large Mpre =
0.0±0.9. By comparing the calculated pre-
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