Resonant laser ionization and spectroscopy are widely used techniques at radioactive ion beam facilities to produce pure beams of exotic nuclei and measure the shape, size, spin and electromagnetic multipole moments of these nuclei. However, in such measurements it is difficult to combine a high efficiency with a high spectral resolution. Here we demonstrate the on-line application of atomic laser ionization spectroscopy in a supersonic gas jet, a technique suited for high-precision studies of the ground- and isomeric-state properties of nuclei located at the extremes of stability. The technique is characterized in a measurement on actinium isotopes around the N=126 neutron shell closure. A significant improvement in the spectral resolution by more than one order of magnitude is achieved in these experiments without loss in efficiency.

International audienceThe results of preparatory experiments and the preliminary designs of a new in-gas laser ionization and spectroscopy setup, to be coupled to the Super Separator Spectrometer S3 of SPIRAL2-GANIL, are reported. Special attention is given to the development and tests to carry out a full implementation of the in-gas jet laser spectroscopy technique. Application of this novel technique to radioactive species will allow highsensitivity and enhanced-resolution laser spectroscopy studies of ground- and excited-state properties of exotic nuclei

a b s t r a c tThe in-gas laser ionization and spectroscopy (IGLIS) technique is used to produce and to investigate short-lived radioactive isotopes at on-line ion beam facilities. In this technique, the nuclear reaction products recoiling out of a thin target are thermalized and neutralized in a high-pressure noble gas, resonantly ionized by the laser beams in a two-step process, and then extracted from the ion source to be finally accelerated and mass separated. Resonant ionization of radioactive species in the supersonic gas jet ensures very high spectral resolution because of essential reduction of broadening mechanisms. To obtain the maximum efficiency and the best spectral resolution, properties of the supersonic jet and the laser beams must be optimized. To perform these studies a new off-line IGLIS laboratory, including a new high-repetition-rate laser system and a dedicated off-line mass separator, has been commissioned. In this article, the specifications of the different components necessary to achieve optimum conditions in laser-spectroscopy studies of radioactive beams using IGLIS are discussed and the results of simulations are presented.

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The method of laser spectroscopy in supersonic gas jets was proposed for high-resolution and highefficiency in-gas laser ionization and spectroscopy studies of short-lived nuclei. The flow properties of such supersonic gas jets have been characterized under off-line conditions. Planar laser-induced fluorescence spectroscopy of seeded copper atoms has been applied to nonintrusively measure velocity, temperature, and relative density profiles of gas jets formed by different de Laval nozzles. For validation, planar laserinduced fluorescence spectroscopy was applied on supersonic free jets with well-known flow parameters. The performance of the in-gas-jet laser spectroscopy method is determined by the achievable spectral resolution, which requires an optimization and a precise manufacturing of the nozzle inner contour as well as a pressure matching of the background medium at the nozzle exit. Our studies now enable a thorough understanding and quantification of these requirements and a determination of the final performance of the in-gas-jet method. Additionally, a comparison between the experimental results and the numerical calculations was performed for the temperature, velocity, and Mach number profiles of underexpanded and quasiuniform jets formed by a de Laval nozzle.