ARIS2011 établit une cartographie des noyaux

Au cours des 10 à 20 dernières années, les possibilités d’expérimentation pour l’étude des noyaux exotiques se sont beaucoup développées, grâce à des progrès techniques impressionnants permettant la production d’espèces nucléaires rares, aussi bien au repos que dans des faisceaux énergétiques. ARIS2011, la première d’une série de conférences sur le thème des avancées de la science des isotopes radioactifs, a été l’occasion de passer en revue ce domaine et d’examiner les résultats les plus récents. Ce nouveau rendez-vous pour les physiciens résulte de la fusion de deux conférences internationales, consacrées respectivement aux noyaux exotiques et masses atomiques et aux faisceaux nucléaires radioactifs, qui existaient depuis les années 1960.

The roots of the first conference on Advances in Radioactive Isotope Science, ARIS 2011, go back to CERN in 1964, when the then director-general Victor Weisskopf called for proposals for on-line experiments to study radioactive nuclei at the 600 MeV synchrocyclotron. Why this should be done – and how – became the subject of a conference held in Lysekil, Sweden, in 1966 and a year later experiments began at ISOLDE, CERN’s Isotope Separator On Line (CERN Courier December 2004 p16). Following this successful start, in 1970 CERN organized a first meeting on nuclei far from stability in Leysin, Switzerland.

Since then there have been regular conferences within the field, with more specialized meetings arising hand in hand with increasingly sophisticated technical developments (see box). Three years ago the community felt that the time was ripe to streamline the conferences by merging all of the physics into a single meeting held every three years. The result was that at the end of May this year some 300 physicists met in the beautiful medieval town of Leuven in Belgium to attend ARIS 2011. The success of the meeting, with its excellent scientific programme, indicates that this was the perfect decision.

Over the past two decades the experimental possibilities for studying exotic nuclear systems have increased dramatically thanks to impressive technical developments for the production of rare nuclear species, both at rest and as energetic beams. New sophisticated detection methods and data-acquisition techniques with on- and off-line analysis methods have also been developed. The two basic techniques now used at laboratories worldwide are the isotope separator on-line (ISOL) and in-flight production methods, with several variations.

Conference highlights

The conference heard the latest news about plans to make major improvements to existing facilities or to build new facilities, offering new research opportunities. The review of the first results from the new major in-flight facility, the Radioactive Isotope Beam Factory at the RIKEN research institute in Japan, was particularly exciting. The production of 45 new neutron-rich isotopes together with results from the Zero-Degree Spectrometer and the radioactive-ion beam separator, BigRIPS, gave a glimpse of the facility’s quality. Future installations, such as the Facility for Antiproton and Ion Research (FAIR) at GSI, SPIRAL2 at the GANIL laboratory, the High Intensity and Energy ISOLDE at CERN, the Facility for Rare Isotope Beams at Michigan State University (MSU) and the Advanced Rare Isotope Laboratory at TRIUMF were also discussed, together with the advanced plans to build EURISOL, a major new European facility complementary to FAIR.

The nuclear mass is arguably the most basic information to be gained for an isotope. Its measurement has involved various techniques, but a paradigm shift came with the development of mass spectrometers based on Penning traps and such devices are now coupled to the majority of radioactive-beam facilities. This has led to mass-determinations of unprecedented precision for isotopes in all regions of the nuclear chart, making it possible in effect to walk round the mass "landscape" and scrutinize its details (figure 3).

Recent results from the ISOLTRAP mass spectrometer at CERN, which has been in operation for more than 20 years, have a precision in the order of 10–8 for the masses of isotopes with half-lives down to milliseconds. The first determination of masses of neutron-rich francium isotopes, where the mass of 228Fr (T1/2 = 39 s) is a notable example, were presented at ARIS 2011. The JYFLTRAP group, using the IGISOL facility at the physics department of the University of Jyväskylä (JYFL), presented masses for about 40 neutron-rich isotopes in the medium-mass region. The SHIPTRAP spectrometer at GSI has made measurements of masses towards the region of super-heavy elements; 256Lr, produced at a rate of only two atoms a minute, is the heaviest element studied so far. The TRISTAN spectrometer at TRIUMF has boosted precision by "breeding" isotopes to higher charge-states – for example, in a new measurement of the mass of the super-allowed β-emitter 74Rb, which is relevant to the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. Results from isochronous mass-spectroscopy with the CSRe storage ring at the National Laboratory of Heavy Ion Research in Lanzhou were also presented. Ion-traps are now routinely used as a key instrument for cooling and bunching the radioactive beams. This gives an improvement of several orders of magnitude in the peak-to-background ratio in laser spectroscopy experiments, or can be used prior to post-acceleration.

The determination of nuclear matter and charge radii has been important to the progress of radioactive-beam physics. The observation of shape co-existence in mercury isotopes at ISOLDE was the starting point for impressive developments, with lasers playing the key role. The most recent results from the same area of the nuclear chart are measurements using the Resonance Ionization Laser Ion Source at ISOLDE of isotope shifts and charge radii for the isotope chain 191–218Po. Another demonstration of the state of the art was shown in the determination of the charge radius of 12Be using collinear laser spectroscopy based on a frequency comb together with a photon-ion coincidence technique. Electron scattering from radioactive beams will be the next step for investigating nuclear shapes; the ELISe project at GSI and SCRIT at RIKEN are examples of such plans.

The determination of matter radii by transmission techniques, pioneered at Berkeley in the mid-1980s, led to the discovery of halo states in nuclei. These are well known today but the main data are limited to the lightest region of the nuclear chart. A step towards heavier cases was presented at ARIS in new data from RIKEN, where results from RIPS and BigRIPS indicate a halo state for 22C and 31Ne and maybe also for 37Mg.

The use of laser spectroscopy in measuring charge radii, nuclear spins, magnetic moments and electric quadrupole moments has been extremely successful over the years. New results from the IGISOL facility – mapping the sudden onset of deformation at N = 60 – and from the ISOLDE cooler/buncher ISCOOL – for copper and gallium isotopes – were highlighted at the conference. The two-neutron halo nucleus 11Li continues to attract interest both theoretically and experimentally, where a better determination of the ratio of the electric quadrupole moments between mass 11 and 9 was needed. Now a measurement at TRIUMF based on a β-detected nuclear-quadrupole resonance technique has yielded a value of Q(11Li)/Q(9Li) = 1.077(1). Here, the cross-fertilization between beam and detector developments has led to laser-resonant ionization becoming an essential ingredient in the production cycle of pure radioactive, sometimes isomeric, beams.

Nuclear-structure studies of exotic nuclei were the topic of many contributions at ARIS. There is progress on the theoretical side with large-scale shell model calculations in the vicinity of 78Ni leading to a unified description of neutron-rich nuclei between N = 40 and N = 50. The evolution of collectivity for the N = 40 isotopes has provided many interesting experimental results. From the strongly deformed N = Z nucleus 80Zr, collectivity is rapidly decreasing to 68Ni with a high-lying 2+ state at 2.03 MeV, suggesting a doubly magic character. Going to 64Cr, there is a new deformed region illustrated by a 2+ state at 470 keV, and research at the National Superconducting Cyclotron Laboratory at MSU has found an enhanced collectivity for 78Sr, with a quadrupole deformation parameter of β2=0.44.

Many of the talks at ARIS addressed the "island of inversion". A recent result from REX-ISOLDE identifies an excited 0+ state in 32Mg, illustrating shape coexistence at the borders of the island (CERN Courier September 2011 p39). Many new results – a rotational band in 38Mg observed by BigRIPS, isotope shifts for 21–32Mg measured at ISOLDE, β-decay for chromium isotopes from Berkeley and shape-coexistence in 34Si and 44S – add to the understanding of this interesting region of the nuclear chart. A new island of inversion, indicated by data for 80–84Ga from the ALTO facility in Orsay, was also discussed.

Continuing with nuclear structure, data from GANIL and its MUST 2 array on d(68Ni, p)69Ni give access to the d5/2 orbital, which is crucial for understanding shell structure and deformation in this mass region. The reaction d(34Si, p)35Si shows a density dependence of the spin-orbit splitting leading to a depletion of the nuclear matter density and resulting in a "bubble nucleus" – a topic also discussed in a theory talk.

The doubly magic nucleus 24O has attracted interest for a decade, from experimental and theoretical viewpoints. At ARIS, the coupled-cluster approach was presented as an ideal compromise between computational costs and numerical accuracy in theoretical models, while the absence of bound oxygen isotopes up to the classically expected doubly magic nucleus 28O presents a theoretical challenge.

Experimentally, there is an impressive series of data – over a wide range of elements – from the MINIBALL array at ISOLDE. One of the highlights here was the observation of shape coexistence in the lead region. A theory talk pointed out that the nuclear energy-density functional approach, both for mean-field and beyond-mean-field applications, is an efficient tool for calculations on medium-mass and heavy nuclei.

Early experiments with radioactive beams revealed exotic decay-modes such as β-delayed particle emission. Today these processes are well understood and used as workhorses to learn about the structure of exotic nuclei. The study of β-delayed three-proton emission from 43Cr and two-proton radioactivity from 48Ni using an Optical Time Projection Chamber at MSU was also presented at ARIS. Here it is clear that in future the study of the most exotic decay modes will use active targets, such as in the Maya detector developed at GANIL and the ACTAR-TPC project being planned by GANIL together with MSU. An interesting new result concerns the observation of β-delayed fission-precursors in the Hg-Tl region, where an unexpected asymmetric fragment-distribution has been observed for the β-delayed fission of 180Tl.

Unbound nuclei or resonance states are sometimes debated as "ghosts" without any physics significance. However, developments over the past 5–10 years have provided a huge amount of data, so that most of the previously empty spots on the nuclear chart for the light elements are now filled. The production of 10He and 12,13Li from proton-knockout reactions from 11Li and 14Be, respectively, is a particularly spectacular case. The knockout of a strongly bound proton from the almost unbound nucleus 14Be results in a 11Li nucleus that together with two neutrons shows features that can only be attributed to an unbound 13Li nucleus. Many of the resonance states might be populated in transfer reactions in inverse kinematics, in which the exotic nuclei are used as an energetic beam directed towards a target that was earlier used as the beam. The HELIOS spectrometer, which will use neutron-rich beams from the CARIBU injector at the Argonne Tandem Linear Accelerator System, is a model for what might develop at many facilities in the future.

The super-heavy-element community was represented in several talks at ARIS. Having produced all elements up to Z = 118, the next step is to tackle the Z = 120 barrier, an exciting goal that could become a reality with reactions such as 54Cr+248Cm. Nuclear spectroscopy is also climbing towards ever higher mass numbers and elements, as demonstrated by data from JYFL for 254No. One exciting talk concerned the chemical identification of isotopes of element 114 (287,288Uuq), which is found to belong to group 14 (in modern notation) in the periodic table – the group that contains lead, tin, germanium, silicon and carbon.

The acquisition of data pertinent to nuclear astrophysics has grown tremendously thanks to the access to nuclei in relevant regions of the nuclear chart. Results include the study at JYFL and at the Nuclear-physics Accelerator Institute (KVI), Groningen, of β-decay of 8B for the solar-neutrino problem and the work at ISOLDE, JYFL and KVI on β-decays of 12N and 12B, which are important for the production of 12C in astrophysical environments. Data from ISOLDE and JYFL on the neutron-deficient nuclei 31Ar and 23Al, relevant for explosive hydrogen burning, were also discussed, as were results from MSU relating to the hot carbon–nitrogen–oxygen cycle and the αp-, rp- and r-processes in nucleosynthesis. GANIL has results on the reaction d(60Fe,p)61Fe, which is relevant for type II supernovae, while the Radioactive Ion Beam Facility in Brazil in São Paulo has data on the p(8Li,α)5He reaction. Calculations for proton scattering on 7Be in a many-body approach, combining the resonating-group method with the ab initio no-core shell model, were also described at the conference.

Exotic nuclei can also provide information about fundamental symmetries and interactions. The painstaking collection of data over decades has provided an extremely sensitive test of the unitarity of the top row of the CKM matrix. Today there are precise data for 13 super-allowed β-emitters, which give a value of 0.99990(60) for this quantity. In this context, there are plans for measurements with the Magneto Optical Trap at Argonne of β-neutrino correlations for 6He and the electric dipole moment for 225Ra. The high-precision set-ups – WITCH at CERN, LPC Trap at GANIL and WIRED at the Weizmann Institute – were also discussed at the conference. The claim is that this kind of experiment – the high-precision frontier – will to some extent complement the high-energy frontier in understanding the deepest secrets of nature.

Finally, a review of the different techniques using radioactive isotopes in solid-state physics presented the current state of the art, together with some recent results. This work was pioneered at CERN and has over the years become an important ingredient at many facilities.

In summary, ARIS 2011 turned out to be a successful merger of the former ENAM and RNB conferences (see box). The talks, supported by an excellent poster show, covered the field perfectly. The talks are available on the conference website and the organizers had the excellent idea of putting the posters there too – this is "a first", to be followed in future.