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

width measurements.
This relationship simply illustrates the fact that an
atomic vacancy which has been created in the
entrance channel phase of the collision in the reaction
partners — and transferred to the fused nucleus
after electronic orbital arrangements — disappears
either because it decays through X ray
emission, characteristic of the formed element, or
because a nuclear decay process has occurred,
leading to a new element whose characteristic inner
shell X-ray is different. These competing processes
between nuclear and atomic decays will
eventually continue along the decay chain. The
experiment performed at Ganil in November 2009
focused on the compared decay of 103 In and 109 In
formed in the reactions 76 Kr+ 27 Al and 82 Kr+ 27 Al at
340 MeV.
The second experiment entitled "Super-Heavy Element
Fission Time Measurements Using Inner-
Shell Ionization” is the first step of the SHE stability
measurement program which extends our crystal
blocking experiments. The lifetimes of K- and L-
shell vacancies of such atoms, in the 10 -18 s and
10 -17 s range, respectively, should allow to measure
fission times much longer than the nuclear formation
time. In particular by choosing fissioning
nuclei with atomic numbers near or at predicted
closed shells for protons and/or neutrons, one
should be able to probe the stability of super-heavy
elements as the high fission barriers expected in
the vicinity of shell closures should lead to fission
times long enough for K and L X-ray emission from
the compound nucleus. Moreover, X-ray observation
from super-heavy compound nuclei gives access
at the same time to their charge identification
and to a quantitative estimate of their fission time,
bringing thus valuable and complementary pieces
of information to the crystal-blocking experiments
performed at GANIL. In addition, this method allows
the study of systems for which single crystal
targets are not available.
In a first step, this alternative method for fission
time measurement, independent of any nuclear
model has been applied in December 2009 to the
Z=120 element formed in U induced reactions on
Ni targets. The confirmation of the existence of
long lifetime ( > 10 -18 s) components for such heavy
elements by two independent experimental methods
is of crucial interest as within the present theoretical
approaches it seems quite difficult to reconcile
the very tiny measured residue cross sections
and the rather high cross sections associated to
compound nuclei surviving against fission longer
than 10 -18 s. In addition , by studying the 238 U +
58 Ni and 238 U + 64 Ni systems, the evolution of fission
times with neutron numbers in the proximity of
the neutron shell closure can also be studied, providing
thus very sensitive tests of nuclear shell
models.
The experimental planning
Successful tests have been realized in September
2008, in order to check the background counting
rates registered in the X-ray Germanium detectors
at low energy and the feasibility of the lifetime
measurement experiments based on the Atomic
Clock technique. 2009 has been mainly dedicated
to the realization and tests of the different parts of
the experimental set-up: the sectorized ionization
chambers constituting the first stage of the
charged fragment telescopes built in Ganil have
been tested in June. Then we tested the sensitivity
of diamond monocristal detectors to radiation damage
with 7.5 A MeV 129Xe beams, in order to
check whether this type of detectors could be used
to detect the elastically scattered projectiles in our
experiments, to deduce the inner shell ionization
probabilities. Finally it appeared that the pulseheight
delivered by these diamond detectors was
quite rapidly affected by the radiation damage,
which led us to use alternatively the elastically
scattered target nuclei in standard detectors located
at large angles.
Furthermore, all the mechanics needed to install
the FluoX detection set up around the Vamos target
have been designed and built this year: a set of
supports for the Germanium detectors operated
under vacuum, allowing precise displacements has
been designed in IPNO whereas the reaction
chamber hosting the fragment detectors was
drawn in Ganil. A few days of beam time have
been used in September to check the whole setup,
including the Vamos spectrometer and finally
the two scheduled experiments have been successfully
performed in the last months of 2009.
The analysis of these two experiments just started
in January 2010.
References
[1] Morjean M. et al «Fission time measurements: a new probe
into super-heavy element stability», Physical Review Letters,
101, 072701, 2008
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