Nuclear Physics Division

From 2015, the LINAG accelerator in GANIL will be able to produce stable heavy ion beams of unprecedented currents. They will be a factor 10 to 100 more intense than present beams. These stable ion beams will enable to deepen our knowledge in many aspects of physics by various experiments:

• Synthesis and spectroscopy of super heavy elements
• Synthesis and spectroscopy of nuclei at or beyond the proton drip line
• Multinucleonic transfer and deep inelastic reactions
• Study of production mechanisms and reaction products distributions
• Study of ground state properties of rare atoms
• Ion-ion interaction for atomic physics

To use these high intensities above 10¹⁴ particles per second, which is beyond the limit of any detector, it is necessary to develop a new kind of device. It should separate the interesting nuclei which are only a tiny part of the transmitted ions after the target – most of them are beam ions that did not interact with the target. The aim is to obtain after separation counting rates of the order of 1kHz or less, compatible with a normal detection. The S³ spectrometer is designed to achieve this goal. S³ is a device that includes:

• A highly durable target, able to sustain high current beams
• A momentum achromat that rejects and blocks the primary beam ions
• A mass spectrometer that selects the different reaction products and measure their mass with high resolution (>1/300)
• Different detection setting adapted to the running experiment (decay spectroscopy station, low energy branch…)

Superconducting high aperture magnets, combining quadrupole, sextupole and octupole fields, permit a high transmission and a high mass resolution.

Irfu has taken in charge the “primary selection line” of S³, which in located at the first momentum dispersive plane. Its function is to ensure the rejection of the majority of the primary beam that passed trough the target without nuclear interaction. These ions, with different charge state can be blocked in different parts of the Primary rejection line. Each beam dump must be able to sustain the very high power of the incoming beam (up to a total of 40kW). When the beam is within the momentum acceptance of the spectrometer, specific, thin beam dumps (“fingers”) are designed to block the very high power density of the beam (up to 5kW/cm²) while letting most of the nuclei of interest pass through. When the beam is outside of the acceptance, the quadrupole magnets with embedded sextupole corrections have a horizontal aperture to bring the beam in a lateral dump. Due to the very high intensities, the resulting radio-activation of the material is also very important. All beam dumps can be fully enclosed in a lead shielding, ensuring a minimal radioactivity to the environment and the staff.

The Irfu is also involved in the development of a high resolution decay spectroscopy station. Located at the end of S³, this set-up will perform alpha, proton, electron and gamma spectroscopy of the nuclei that have been transmitted through S³. This device is specially designed to study the decays of superheavy elements that are produced by fusion-evaporation reactions, in order to study already known elements of identify new ones. This detection system uses low dead zone, large size silicon detector optimized for the detection of heavy ions. Its innovative electronics combine high resolution with high dynamics and a very low dead time, to be able to detect the fast decay chains following the heavy ion implantation.

S³ is under construction and its commissioning will start in 2015.

Visit the S³ webpage at GANIL.