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super heavy elements

For highly fissile nuclei, the relative weights of long lifetime components (LLCs-typically longer than 10-18 s) in the fission time distribution are strongly correlated to the fission barrier heights. Consequently fission time measurements can be used to probe the stability of nuclei in the super heavy element (SHE) region and localize possible islands of stability far beyond the point reachable today by synthesis experiments.

Fission time measurements undertaken at Ganil using the blocking technique in single crystals have evidenced LLCs with significant probabilities for Z=120 and Z=124 nuclei formed in 238U +Ni and 238U+Ge reactions [Phys. Rev. Lett. 2008 101 072701]. To get an indisputable confirmation and to extend further our exploration, we have recently used a completely independent method, based on fluorescence yield measurements. This technique derives average fission times of compound nuclei from the yield of characteristic X-rays arising from the decay of atomic inner shell vacancies created during the collision, provided the fission times are in the same range than the vacancy lifetimes.

We used in a first 238U+64Ni experiment a dedicated set-up providing us with the fission fragment detection angle, energy and atomic number. Three planar germanium detectors were used for X-ray detection, located at the same polar angle with respect to the beam direction, but at different azimuthal angles allowing to explore various relative angles with respect to the detected fragment directions.

X-rays characteristic of the element Z = 120 have been sought in coincidence with fragments detected between the grazing angle and 70°. In the whole Z range of fission-like fragments arising from complete capture reactions (either complete fusion followed by fission or quasi-fission reactions) a broad peak located around 200 keV - the expected value for K X-rays from Z=120 - has been observed after a proper unfolding procedure allowing to discard the random events in the X spectra. 

Due to the Weisskopf broadening of the lines resulting from the finite fission time of the nuclei, no peak can be observed in the X spectrum for emitter’s lifetimes shorter than 10-19s. Therefore, quasi-fission processes cannot contribute to the observed peak. The position, width and magnitude of this peak as recorded with the three germanium detectors clearly demonstrate that it is dominantly populated by XK emission characteristic of the element Z=120. A quantitative analysis led to a minimum mean fission time of 2.5 × 10−18 s. This work [Phys. Rev. Lett. 108, 122701 (2012)], cross-checking our previous blocking fission time results, provides us with evidence for transiently formed unbinilium atoms characterized by their electronic inner shell structure. 

Recent Hartree-Fock-Bogolubov calculations at finite temperature [Journal of Physics : Conf. Series 282, 012009 (2011)] taking into account the quite different evolutions with temperature of the pairing energies involved at the saddle deformation and at the ground state configuration are possibly able to account for the sizable proportions of LLCs measured in the fission time distributions. X-ray fluorescence technique gives thus now access to studies of the stability of nuclei that cannot be reached nowadays by synthesis experiments.

Figure 1 : Experimental Xrays spectrum in coincidence with fragments with 35≤Z≤90 showing the K Xrays characteristics of Z=120.



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