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Accueil du site > Installation ALTO > Dispositifs expérimentaux > MLL-TRAP

MLL-TRAP

Spectroscopy and level lifetimes for MLLTrap

Contact : E. Minaya Ramirez minaya@ipno.in2p3.fr

Motivation

The energy and lifetime of a nuclear state are a manifestation of the underlying nuclear structure. Measuring these quantities is therefore an essential first step to a better understanding of the nuclear forces at play. In the case of deformed actinide and transactinide nuclei, measuring the lifetime (and therefore the reduced electric quadrupole transition rate B(E2)) of members of a rotational band can also give a handle on the quadrupole moment of the nucleus, which can be used as an input to models and/or to constrain models. Knowledge of the deformation is also crucial for modelling the flow of gamma rays emitted from the nucleus and understanding the competition between fission and gamma decay for example in the survival of evaporation residues produced in fusion­ evaporation reactions. The 2+ energies of deformed nuclei are sensitive to shell effects. Indeed, the decrease of pairing correlations at a closed deformed shell gap leads to an increase of the moment of inertia of the nucleus and therefore a decrease of the 2+ energy. Studying the systematic behavior of 2+ energies can therefore allow to map out the evolution of pairing and in combination with lifetime measurements the evolution of shell structure as a function of neutron and proton numbers.

As far as lifetimes of excited rotational states are concerned, they have been measured in actinide nuclei via fast timing methods following alpha decay, recoil distance measurements, Coulomb excitation (scattering experiments or Mossbauer spectroscopy). The data however are scarce as only a handful of cases for which target/source material is readily available have been studied. For heavier systems or systems for which the nucleus of interest has to be synthesized using a nuclear reaction, no lifetimes are available and one has to rely on phenomenological prescriptions, which link the transition energies to lifetimes. For the lighter actinides, the 2+ energies have been measured through combined alpha/beta and conversion electron spectroscopy of sources or from extrapolation of the ground-state rotational band measured through in-beam gamma-ray spectroscopy. For Z≥100 nuclei, 2+ energies have been measured only in 254,256Fm through the beta decays of 254m,256mEs, in 252Fm through the fine structure of the alpha decay of 256No and extracted from the extrapolation of the ground state rotational bands in 256Rf, 252,254No, 246,248,250Fm. Again, as for lifetimes, there is a desperate need for more measurements in the actinide and trans-actinide region. The goal of this project is to continue the development started in Munich on alpha and conversion-electron spectroscopy using the spectroscopy tower of the MLL Penning trap. The feasibility of in-trap conversion-electron spectroscopy has been demonstrated but so far no measurements related to alpha decay have been performed inside a Penning trap. The other unique feature we would like to develop is the possibility of measuring lifetimes of low-lying rotational states populated by alpha decay.

Methodology

Heavy deformed nuclei, which alpha decay populate the ground-state of the daughter nucleus but also the 2+ excited state with an intensity of the order of 15-30% of the total alpha-decay branch. Once populated, the 2+ state decays to the ground state mostly via internal-conversion-electron emission since the transition energy is very low (<100 keV). The idea is to measure precisely the 2+ energy and also the distance travelled by the recoiling daughter nucleus before the electromagnetic emission occurs because this distance directly depends on the lifetime of the 2+ state.

Schematic illustration of the lay-out of the detectors in the trap (top) and fringe field of the superconducting solenoid (bottom).

High-precision mass measurements with MLLTrap

Motivation

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.

Methodology

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)


 

IPN

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

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