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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).


 

IPN

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

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