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

Fluo X: Deexcitation time measurements
using the Atomic Clock method
IPNO Participation: D. Jacquet, M.F. Rivet, L. Tassan Got
Collaboration : GANIL Caen, CEA/IRFU/SPhN Saclay, CEA/DIF/DPTA/SPN Bruyères le Châtel, LPC
Caen, INFN (Italy), NIPNE Bucharest (Romania)
Après le succès des expériences de mesure de temps de fission utilisant la méthode de blocage dans une
cible monocristalline qui a permis de mettre en évidence la très grande stabilité des noyaux de 120 et 124
protons, un nouveau programme expérimental a été entrepris auprès du Ganil: il s’agit d’ étudier les temps
de vie de noyaux exotiques ( super-lourds ou de masse moyenne mais déficients en neutrons ) grâce à une
méthode dite de l’Horloge Atomique basée sur la décroissance des lacunes crées dans les couches atomiques
profondes au cours des collisions entre ions lourds. Un tel programme permet naturellement d’étendre
les études de stabilité des super-lourds en s’affranchissant des contraintes liées à la qualité cristallographique
de la cible requise par la méthode de blocage. La méthode permet également en s’intéressant
aux temps d’émission de particules d’évaporation, de suivre l’évolution des paramètres de densité de niveau
en fonction de l’asymétrie lorsqu’on s’éloigne de la vallée de stabilité.
The FLUO-X experiments
To extend further the exploration of the superheavy
element stability , probed through the measurement
of their fission time, that we initiated with
the crystal blocking technique for the Z=120 and
Z=124 nuclei [1], we started last year a new program
of decay time measurements based on the
Atomic Clock method. The principle of this method
relies on the decay of the atomic inner shell vacancies
which are created in the reaction products
during the collision. K-shell holes are produced in
the reaction partners electron clouds during the
approach phase due to time-dependent Coulomb
field. If the projectile velocity is small compared to
the K-shell electron velocity, the electronic levels
adjust quasi adiabatically to reflect the increased
binding of the combined projectile and target nuclear
charge, ending with K-shell holes in the
Figure 1: View of the 3 Ge detectors
surrounding the Vamos target and
operating under vacuum.
united atom (UA) whose decay might be observed.
Such a method can be applied to probe the stability
of superheavy elements through fission time
measurements as well as to evaporation time
measurements of medium-mass exotic nuclei in
order to study the evolution of decisive parameters
controlling the decay processes. We thus proposed
two experiments to the Ganil PAC in 2008
which have been accepted and scheduled during
fall 2009. The first one is entitled "Level density
parameter investigated through evaporation time
measurements". Considering the essential role of
level densities in any statistical code, and more
generally in the thermodynamical behavior of
asymmetric nuclear matter, the aim is to derive
information on their evolution with asymmetry
when moving away from the stability valley to the
neutron deficient region. Due to the direct dependence
of the particle emission rate ( or emission
time) on the level density parameters involved ,
one expects different emission time spectra for the
successive nuclei evaporating particles during the
deexcitation cascade of a compound nucleus when
assuming different level density parameters.
These differences may end with changes by more
than one order of magnitude for the most probable
times, as well as for the average times.
Fluorescence yields of element characteristic X-
rays, resulting from the filling of collision-induced
inner shells vacancies can be used to measure
particle emission time thanks to a simple relationship
linking the number of characteristic X-ray
N X , the (atomic) lifetime of the K-vacancy and the
(nuclear) lifetime of the emitting nucleus. This nuclear
lifetime can be extracted provided the number
of emitting nuclei and of K-vacancies in the
considered emitter are also measured. Because of
the very tiny impact parameter range associated to
nuclear processes as compared to atomic length
scales, the latter quantity can be deduced from X-
ray production in coincidence with elastic scattering
events. Regarding the atomic lifetimes, they
are experimentally well known from X-ray line
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