During an experiment carried out at the accelerator of the Australian National University (Canberra, Australia), a French-Australian collaboration (GANIL Caen, IPN Orsay, IRFU/DPhN Saclay, ANU Canberra) first identified the fragments created in quasi-fission reactions with atomic numbers Z up to plutonium (Z=94) and mass A. For this study, near-fission reactions were induced during collisions between 48Ti projectile ions, accelerated to 276 MeV, and target atoms of 238U. The atomic numbers were deduced from the characteristic fluorescence X-ray emissions and the masses from the angular correlations and velocities of the emerging fragments. The data collected highlights shell effects which increase the production of nuclei around the magic number Z=82 (lead) in near-fission reactions. These results, which will make it possible to optimize experiments aimed at creating heavier elements by fusion, as well as the prospects opened up by this original experimental approach in the field of nuclear fission and fusion, have led to a publication in the journal Physical Review Letters (M. Morjean et al., Phys. Rev. Lett. 119, 222502).
Mass identification (A) and atomic number (Z) of the quasi-fission fragments.
This plot shows the distribution of events according to the mass ratio R and photon energy (X-rays or γ) Ephotons, where R=A2/(A1+A2), A1 and A2 are the masses of the quasi-fission fragments. The color scale associated with the number of collected events (the frequency of occurrence) is shown on the right. The masses A1 and A2 were determined from the times of flight of the fragments to the detectors. The particularly intense numbers of shots near R = 0.20 and 0.83 correspond to very short interactions in time during which the projectile and target kept their respective masses, Aprojectile = 48 and Atarget = 238. The photon energy allows us to assign an atomic number when it corresponds to an X-ray emission energy of fluorescence characteristic of an element (some characteristic energies are indicated by dotted lines in the figure). The characteristic energies corresponding to the lighter of the two quasi-fission fragments are below the detection threshold and only the Z of the heavier partner can be identified on this plot thanks to the accumulation of events near the characteristic energies.
Competition in quasi-fission and fission in the synthesis of super heavy metals
Near-fission, in which projectile and target nuclei exchange nucleons before separating in a zeptosecond time (10-21 s), is the dominant mechanism in interactions between heavy nuclei. This mechanism, identified in the 1970s, remains the least well controlled of the interaction mechanisms between heavy ions despite its role in inhibiting the synthesis of very heavy elements. It is in strong competition with the fusion mechanism used to synthesize new super-heavy elements (Z>104), elements that can only exist if microscopic effects of sufficiently intense quantum origin stabilize them. The heaviest element synthesized to date is Oganesson (Z=118), but recent theories of nuclear physics predict that a whole region of elements, probably extending well beyond Z=118, should be stabilized by these microscopic effects.
Problem of the synthesis of super-heavy elements
The last synthesized elements required increasingly expensive experiments, using whole months of very intense beams provided by different heavy ion accelerators in Russia or Japan. The synthesis of superheavy elements is only considered complete when (statistically) a few copies of the compound nuclei formed survive fission and can be identified in the appropriate detection system. To exceed the limit of Z=118, it will therefore be necessary to optimize experiments on the nature of the colliding nuclei and the incident energy in order to maximize both the effective fusion cross-section and the fission survival probability of the superheavy compound nuclei thus formed. Unfortunately, the very similar characteristics of some of the quasi-fission and post-fission fragments do not allow reliable measurement of the effective fusion and quasi-fission sections. In this context, realistic modelling of the quasi-fission process is essential in order to achieve reliable predictions of effective fusion sections.
Contribution of this new data on the quasi-fission mechanism
Thanks to data collected by the Franco-Australian experience, new information is now available on the quasi-fission mechanism. In this experiment, a maximum of the production of quasi-fission fragments was observed for the magic number of protons Z=82, whereas many previous experimental and theoretical studies suggested that the magic number of neutrons N=126 played a decisive role. For different numbers of nucleons exchanged (overall balance of the numbers of neutrons and protons exchanged between the projectile and the target), the respective contributions of the numbers of exchanged neutrons and protons were determined. Experience shows that the number of exchanged nucleons is strongly correlated with the angle at which the fragments separate. This separation angle, on the other hand, is correlated with interaction times between nuclei (typically in the range of 10-22 seconds) and thus gives access to the speed at which neutrons and protons are exchanged (on average) since the first moments of the reaction. Calculations based on the most efficient theoretical microscopic approach in this field [the Time Dependent Hartree-Fock (TDHF) approach] overestimate the exchange of protons and underestimate that of neutrons, but predict the average interaction times and the determining role of the magic number Z=82. The study of nucleon exchanges, for different systems of colliding nuclei, will allow a fine adjustment in this model of the parameters governing nucleon exchanges.
publication: M. Morjean et al., Phys. Rev. Lett. 119, 222502 (2017)
Contacts: Antoine Drouart (Irfu), Maurice Morjean (Ganil)