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High-precision mass measurements with MLLTrap


The low energy facility Desir (Désintegration, Excitation et Stockage des Ions Radioactifs), proposed in the framework of the Spiral-2 facility of Ganil, will explore the ground-state properties of very exotic nuclides such as their decay mode, half-life, radius, deformation and mass. State-of-the-art equipment is currently prepared for Desir in particular a double Penning-trap mass spectrometer for high-precision mass measurements : MLLTrap, commissioned off-line at the Maier-Leibnitz Laboratory (MLL) in Munich, Germany. This setup is planned to be moved to the Desir facility as soon as low-energy beams will be available at Desir. A few letters of intent have been submitted for MLLTrap at Desir, as for example mass measurements in the region of 100Sn and of superheavy nuclides. IPN has proposed an on-line commissioning of MLLTrap using the exotic nuclides produced at the Alto facility. The knowledge obtained by setting up MLLTrap at Orsay is fundamental to carry out this new scientific topic in France. The Alto front end is configured to receive different driver beams : electrons for photo-fission of a uranium-carbide target and heavy-ion beams for fusion evaporation in suitable, medium-mass targets. The “cold” nature of photofission produces exotic neutron-rich species that are of superior purity, allowing accurate mass measurements uncompromised by the contamination present at other facilities. High-precision mass measurements in the region of the magic numbers 50 and 82 are of high interest for nuclear astrophysics (r and rp process) and can already be performed at Alto. In addition, the novel detector-trap developed at MLL for in-trap decay spectroscopy will allow for background free spectra via direct in-situ spectroscopy of stored ions.


The superposition of a strong homogeneous magnetic field B with a weak electrostatic quadrupolar field in a Penning-trap mass spectrometers (PTMS) allows trapping the ions in three dimensions. The mass m of the stored ion of interest, with a charge q, is obtained by measuring its cyclotron frequency νc=qB/(2πm). The impact on a specific field of physics (nuclear fine structure, astrophysics, neutrino physics among others) will rely on the relative uncertainty of the mass, which ranges from 10-11 to 10-7 for PTMS. Combining the 7-Tesla PTMS MLLTrap for high-precision mass measurements with the unique production of the Alto facility at Orsay offers an extraordinary opportunity for new low-energy nuclear physics. In particular mass measurements around78Ni and 132Sn are expected to be performed. An RFQ cooler-buncher is essential for a Penning trap. Such a device needs to be developed at Alto for MLLTrap. The design will be based on the RFQ cooler-buncher Colette that was built at CSNSM. It was installed at Isolde at CERN for an earlier experiment and is currently used at Mainz.

Both the instrument and the facility will see their performance enhanced, with the addition of new features (in-trap spectroscopy and fusion-evaporation reactions) and synergy with other new programs under development (laser spectroscopy).

Lay-out of MLLTrap for mass measurements (left) and decay spectroscopy (right)



Institut de Physique Nucléaire Orsay - 15 rue Georges CLEMENCEAU - 91406 ORSAY (FRANCE)
UMR 8608 - CNRS/IN2P3

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