Nuclear excitations in plasmas: the case of 84mRb

by David Denis-Petit (Centre d' Etudes Nucleaires de Bordeaux Gradignan)

Thursday 30 Oct 2014, 14:30 → 15:30 Europe/Zurich

26-1-022 (CERN)

The development of high intensity lasers has opened up new opportunities for nuclear physics studies. Lasers are promising tools to study nuclear properties in extreme plasma conditions, which cannot be reached with conventional particle accelerators. In a plasma, the interactions between a nucleus and its electronic cloud can be strongly modified and unusual excitation processes cannot be neglected anymore such as nuclear excitation by electronic capture (NEEC) and nuclear excitation by electronic transition (NEET). The NEEC (unobserved) and NEET (observed in 197Au, 189Os and 193Ir in accelerator based experiments) are exotic processes, which can be dominant in particular plasma conditions of temperature and density. We have undertaken a joint experimental and theoretical program to investigate the 84mRb excitation rate in laser produced plasma. The long-lived isomeric state of the 84Rb (energy of 463.6 keV, T1/2=20.26min) can be excited towards a higher lying short-lived state. According to the literature, the nuclear transition energy involved in this excitation is 3.05 (20) keV. It is possible to find atomic transitions, which match this nuclear transition for charge states between 29+ and 33+ allowing the NEET process to take place. Searching for the NEET process in plasma requires an accelerator to produce the 84Rb isomeric state and a high energy - high intensity laser to create the plasma. This kind of facility exists at GSI (Darmstadt, Germany) where the PHELIX high-energy laser is combined with the UNILAC ion accelerator. To prepare the NEET experiment, nuclear and atomic physics experiments were conducted. We have made experiments at ELSA (CEA/DAM, Bruyères-le-Châtel) and at TANDEM/ALTO (Orsay) accelerators to measure the nuclear transition energy with high accuracy. Indeed, the accuracy of this transition energy found in the tables was insufficient (200 eV) in comparison with the atomic one (few eV) . We have also measured the X-ray spectra emitted by a Rb plasma at PHELIX with different laser energies and pulse configurations. These spectra were compared with theoretical ones to determine plasma conditions (charge states, temperature…) reached during the experiment. Thanks to these plasma properties nuclear excitation rates were evaluated to determine if the excitation of the 84mRb could be highlighted in an experiment with PHELIX.