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Introduction

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An introduction to the SPIRAL project by A. Villari

 
    The study of the properties of the atomic nucleus far from the valley of stability is an old and passionate subject of research. The pioneering efforts of ISOLDE at CERN, followed by other equally important centres based on the so-called Isotopic Separation On-Line (ISOL) technique, have contributed much to our present knowledge of the atomic nucleus far from stability.

    An important breakthrough in the study of the fundamental properties of these nuclei came about when Symons and collaborators [1] demonstrated the usefulness of heavy-ion beams for the production of neutron-rich nuclei. Quickly following on from this development the first results of experiments using fast radioactive ion beams (RIB) appeared. For example, the first measurements of the interaction cross sections for light nuclei, made at the BEVALAC, USA by Tanihata and collaborators [2], provided the first evidence for the existence of an unexpected halo for the nucleus 11Li. From the first experiments in 1979, projectile fragmentation has also been used at GANIL (see LISE and SPEG), GSI-Germany, MSU-USA and RIKEN-Japan to produce and study reactions induced by radioactive beams. Separators have been built at these centres in order to maximize the RIB production and purity. All these facilities have in common the fact that RIBs are produced by projectile fragmentation at energies between 70 and 2000 A.MeV. It turns out from the principle of production and separation using a spectrograph that the optimum efficiency of the process is reached when the RIB has a velocity similar to that of the primary beam. The production process, however, does imply losses in intensity and/or quality of the secondary beam, which become increasingly important as the beam is slowed down. From this point of view, the study of secondary beam reactions at low energies using intense RIBs is very difficult.
 
 
SPIRAL

The SPIRAL project at GANIL was funded in December 1993 by the IN2P3/CNRS, DSM/CEA and the Regional Council of Lower Normandy. Under the project leader, M. Marcel Lieuvin, a large collaboration between laboratories in France and Europe has been established in order to construct and develop SPIRAL. The project is based on the ISOL technique for production of Radioactive Ion Beams (RIB).
  The SPIRAL project at GANIL was funded in December 1993 by the IN2P3/CNRS, DSM/CEA and the Regional Council of Lower Normandy. Under the project leader, M. Marcel Lieuvin, a large collaboration between laboratories in France and Europe has been established in order to construct and develop SPIRAL. The project is based on the ISOL technique for production of Radioactive Ion Beams (RIB): The primary light or heavy-ion beam accelerated by the GANIL cyclotrons bombards a production target located inside a well shielded cave. The radioactive atoms produced by nuclear reactions will be released from the target, which will be at high temperature (~2300 K), and pass through a transfer tube into a permanent magnet ECRIS (NANOGAN-III). The radioactive atoms are then ionised up to charge states corresponding to q/m = 0.09 to 0.40. After extraction from the ECRIS, the low-energy RIB (acceleration voltage from 7kV to 34kV) are selected by a relatively low resolution separator of Delta-m/m = 4x10-3. They can be either deliver to the low-energy beam line LIRAT or injected into the K=265 (B*rho=2.344Tm) compact cyclotron CIME. The corresponding energy range varies from 1.7A to 25A MeV. After acceleration, the RIB is selected in magnetic rigidity by the modified alpha-shaped spectrometer and directed to the existing experimental areas. The separation between isobars is performed for the most part by the cyclotron (which is a good spectrometer for fast ions) with a resolution of Delta-m/m~ 5 x 10-4. It is also possible to improve the resolution of the cyclotron with an appropriate detuning of the isochronism during the acceleration of the ions. This procedure can improve the resolution in mass to Delta-m/m~ 1 x 10-4. Additional separation can be achieved by placing a stripper foil or a degrader in the dispersive focal plane of the spectrometer . In the latter case, however, significant losses in RIB intensity will occur. SPIRAL is producing radioactive beams for experiments since September 2001. Since, a number of successful experiments and technical achievements have been performed as detailed in the SPIRAL activity report published in 2008.

In parallel, an important R&D effort is devoted to the production of new RIBs by means of dedicated Target Ion source Systems (TIS) at the SIRa test bench.  

    A similar facility exists at Louvain-la-Neuve, Belgium, which uses a 30MeV (200mA) proton beam to produce RIBs and accelerate them with a K=110 cyclotron. Following on the same direction other low energy RIB facilities exist at ORNL - USA and ISAC at TRIUMF.
 

[1] T.J.M. Symons et al., Phys. Rev. Lett. 42, 40 (1979)  and G.D. Westfall et al., Phys. Rev. Lett. 43, 1859 (1979)
[2] I. Tanihata et al., Phys. Lett. B160, 380 (1985) and Phys. Rev. Lett. 55, 2676 (1985)

 

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