Exotic nuclei

Structure of exotic nuclei via direct reactions

The structure group of the DPhN conducts experimental studies to determine the structure and reactions of radioactive nuclei far from the valley of stability. These nuclei of short life, unstable, radioactive, are called exotic when they develop unusual structures (large extension of matter, halo or neutron skin, shell structure with new magic numbers). The properties (nuclear size, nucleon shell occupation) can become very different from what is observed for the stable isotope (s). Nuclei at the boundary of nuclear cohesion, bound but unstable, such as exotic nuclei, give new observables which are compared to the calculated ones, to test the assumptions of the nuclear models and to make them evolve.
In order to determine the new properties of very weakly-bound neutron-rich nuclei and to determine the possible modifications of their shell structure, the group measures direct reactions which involve the exotic nucleus of interest, either as the product or as the projectile (if the beam is available). These data are extracted from the elements on the structure of the nucleus, its spectroscopy (energies of excitation, spectroscopic factors) and its interaction with a target.

The programs of the group are carried out mainly in two directions:

- with the radioactive beams produced at low energy (between 10 and 60 MeV/n) at the Grand Accélérateur National des Ions Lourds, GANIL (Caen);

-with the rare radioactive ion beams produced at the RIBF facility of the RIKEN accelerator (Japan) at energies between ~150 and 300 MeV/n.

The GANIL radioactive beams are produced either by the SPIRAL (Source de Production d'Ions Radioactifs et d'Accélération en Ligne) facility or by fragmentation of stable nuclei on the production target of the LISE area.

Experimental technics with the GANIL beams:

spectroscopy of light charged particles with MUST2 (MUrs à STrips, 2^egeneration) The MUST2 Si-strip detectors have been designed and built by the groups of nuclear structure of GANIL, IPN and IRFU/SPhN with the technical teams of these laboratories.
tracking of the incident beam trajectories with the beam detectors CATS (Chambre à Trajectoires de Saclay, developed by IRFU/SEDI) - BTD (Beam Tracking Detectors).
detection of ejectiles produced by the reactions at forward angles, in a spectrometer, (for instance VAMOS).

MUST2 experimental program of the group, see page MUST2 réactions and the MUST2 site.

Array of 8 MUST2 telescopes in the VAMOS area at GANIL for the E655S SPIRAL experiment. [Photo Irfu, V.Lapoux]

New targets for the reaction studies with exotic beams

For the direct reactions induced by the beams produced at very low intensity, projects have been developed to build new targets. Specific pure cryogenic H₂ or D₂ targets are used instead of the standard CH2 or CD2 films. The goals are to increase the performances for the cross sections measurements with a better luminosity and the reduction of the physical background due to the reactions on the ¹²C content of the usual targets. In collaboration with the DACM, DEDIP and DIS departments, new programs were developed to optimize the reaction targets used at GANIL (CHyMENE project), at RIKEN (MINOS, in use) and for the next programs with FAIR at GSI (project COCOTIER). Depending on the experimental programs and the type of beams (energy, intensity) and foreseen reactions, the R&D may involve the development of a specific detection device surrounding the target, as was the case for MINOS.

Spectroscopy program at RIKEN at high energy (200 to 300 MeV/n)

To study the shell structure in the vicinity of the expected new doubly-magic nuclei, far from the valley of stability, knock-out reactions are studied the spectroscopy of these isotopes, using beams produced at very low intensity, around 1particle/s even less. The experimental setup includes a new device developed by IRFU in the years 2010-2014, it has been implanted at RIKEN: MINOS (MAgic Numbers Off Stability) was specially developed for the experiments performed with radioactive beams at very low intensity. The device was conceived to improve efficiency for the gamma spectrometry. MINOS is a thick H₂ cryogenic target surrounded by a cylindrical Time Projection chamber TPC.

The set-up and technics for MINOS at RIBF combine:

the spectroscopy of excited states produced by knockout, either by detecting the photons in the DALI2 gamma spectrometer, or by invariant mass method in the neutron detector NEBULA and a calorimeter used for the heavy ejectile,
the identification of the heavy ejectile in a spectrometer (SAMURAI or ZeroDegree).
the reaction vertex tracking in the Time Projection Chamber; the reconstruction of the vertex position is crucial to improve the energy resolution in the measurement by gamma spectroscopy or invariant mass method. The use of a thick target (10 to 15 cm) increases the luminosity by a factor ~10 compared to standard targets. With the MINOS target+TPC it is possible to study the most exotic nuclei, only available at RIBF. The beams under study have very low intensities, a few particles per second (pps) or even less than 1 pps.

View of the MINOS TPC in the Samurai experimental area of the RIBF facility (RIKEN Accelerator). The beam direction is from left to right. [Photo Irfu, V.Lapoux]

Programs (2014-2017) and projects (2018-2022++)

- Direct reactions and spectroscopy at GANIL with the SPIRAL beams and MUST2, results and highlights cf MUST2-reactions. Campaigns for the spectroscopy of neutron-deficient nuclei will be performed in the LISE area using MUST2, the objectives are to study the unbound states and in particular, to search for 0+ states via (p,t) transfer reactions.

- Studies of dineutron correlations in the Borromean nuclei of ¹¹Li and of ¹⁴Be (Samurai-18 experiment at RIBF with MINOS).

- Spectroscopy of 2+ states of rare exotic nuclei produced by the RIBF machine (RIKEN), the measurements were carried out during a series of campaigns done with MINOS, see MINOS-campaigns. New RIBF experiments with MINOS are foreseen in the years 2018-2020 (spectroscopy of neutron-deficient nuclei around ⁴⁶Fe, around ¹⁰⁰Sn; in the neutron-rich region around ³⁹Mg).

The group is studying three experimental programs with the exotic beams produced by FAIR, triggering the target developments foreseen at IRFU (cf page "nuclear structure studies at FAIR" for details):

- Spectroscopy of weakly-bound exotic nuclei,

- Study of short-range nuclear correlations (COCOTIER),

- Study of the production of exotic hypernuclei.

Contact Minos and FAIR targets: A. Corsi ; Contact CHymene: A.Gillibert

Contact page "Exotic nuclei" and MUST2: V. Lapoux

#3410 - Last update : 06/12 2018

Nuclear Physics Division

Institute of Research into the Fundamental Laws of the Universe