No abstract

The last decades brought an impressive progress in synthesizing and studying properties of nuclides located very far from the beta stability line. Among the most fundamental properties of such exotic nuclides, usually established first, is the half-life, possible radioactive decay modes, and their relative probabilities. When approaching limits of nuclear stability, new decay modes set in. First, beta decays become accompanied by emission of nucleons from highly excited states of daughter nuclei. Second, when the nucleon separation energy becomes negative, nucleons start to be emitted from the ground state. Here, we present a review of the decay modes occurring close to the limits of stability. The experimental methods used to produce, identify and detect new species and their radiation are discussed. The current theoretical understanding of these decay processes is overviewed. The theoretical description of the most recently discovered and most complex radioactive process - the two-proton radioactivity - is discussed in more detail.Comment: Review, 68 pages, 39 figure

The three-body breakup 6 He→ 4 Heϩnϩn is studied experimentally, using a secondary 6 He ion beam of 240 MeV/nucleon incident on carbon and lead targets. Integrated cross sections for one-and two-neutron knockout and differential cross sections d/dE* and d/d for inelastic nuclear or electromagnetic excitations into the 6 He continuum are presented. The E1-strength distribution is deduced from electromagnetic cross sections and is found to exhaust (10Ϯ2)% of the energy-weighted Thomas-Reiche-Kuhn sum rule or (40Ϯ8)% of the cluster sum rule for excitation energies below 5 MeV. Both the energy-weighted and non-energy-weighted dipole cluster sum rules are almost exhausted integrating the strength up to 10 MeV, a fact from which the root-mean-square distance between the ␣ core and the two valence neutrons of r ␣Ϫ2n ϭ(3.36Ϯ0.39) fm is derived. The known I ϭ2 ϩ ͑1.80 MeV͒ resonance in 6 He is observed in nuclear inelastic scattering; model-dependent values of the quadrupole deformation parameter ␦ 2 ϭ(1.7Ϯ0.3) fm or B(E2,0 ϩ →2 ϩ )ϭ(3.2Ϯ0.6)e 2 fm 4 are derived. No clear signature could be obtained for predicted higherlying 2 ϩ resonances, but low-lying continuum strength of multipolarity other than dipole, likely of monopole and quadrupole multipolarity, is indicated by the data. Two-body correlations in the decaying 4 Heϩnϩn system are investigated. The astrophysical relevance of the data with regard to the two-neutron capture process 4 He(2n,␥) 6 He is briefly discussed.

In the centres of stars where the temperature is high enough, three alpha-particles (helium nuclei) are able to combine to form 12C because of a resonant reaction leading to a nuclear excited state. (Stars with masses greater than approximately 0.5 times that of the Sun will at some point in their lives have a central temperature high enough for this reaction to proceed.) Although the reaction rate is of critical significance for determining elemental abundances in the Universe, and for determining the size of the iron core of a star just before it goes supernova, it has hitherto been insufficiently determined. Here we report a measurement of the inverse process, where a 12C nucleus decays to three alpha-particles. We find a dominant resonance at an energy of approximately 11 MeV, but do not confirm the presence of a resonance at 9.1 MeV (ref. 3). We show that interference between two resonances has important effects on our measured spectrum. Using these data, we calculate the triple-alpha rate for temperatures from 10(7) K to 10(10) K and find significant deviations from the standard rates. Our rate below approximately 5 x 10(7) K is higher than the previous standard, implying that the critical amounts of carbon that catalysed hydrogen burning in the first stars are produced twice as fast as previously believed. At temperatures above 10(9) K, our rate is much less, which modifies predicted nucleosynthesis in supernovae.

The halo structure originated in nuclear physics but is now encountered more widely. It appears in loosely bound, clustered systems where the spatial extension of the system is significantly larger than that of the binding potentials. A review is given on our current understanding of these structures, with an emphasis on how the structures evolve as more cluster components are added, and on the experimental situation concerning halo states in light nuclei.

No abstract

No abstract

The ''island of inversion'' nucleus 32 Mg has been studied by a (t, p) two neutron transfer reaction in inverse kinematics at REX-ISOLDE. The shape coexistent excited 0 þ state in 32 Mg has been identified by the characteristic angular distribution of the protons of the ÁL ¼ 0 transfer. The excitation energy of 1058 keV is much lower than predicted by any theoretical model. The low-ray intensity observed for the decay of this 0 þ state indicates a lifetime of more than 10 ns. Deduced spectroscopic amplitudes are compared with occupation numbers from shell-model calculations. The evolution of shell structure in exotic nuclei as a function of the proton (Z) and neutron (N) number is currently at the center of many theoretical and experimental investigations [1,2]. It has been realized that the interaction of the last valence protons and neutrons, in particular, the monopole component of the residual interaction between those nucleons, can lead to significant shifts in the single-particle energies, leading to the disappearance of classic shell closures and the appearance of new shell gaps [3]. A prominent example is the collapse of the N ¼ 20 shell gap in the neutron-rich oxygen isotopes where instead a new magic shell gap appears for 24 O at N ¼ 16 [4,5]. Recent work showed that the disappearance of the N ¼ 20 shell can be attributed to the monopole effect of the tensor force [3,6,7]. The reduced strength of the attractive interaction between the proton d 5=2 and the neutron d 3=2 orbitals causes the d 3=2 orbital to rise in energy and come closer to the f 7=2 orbital. In regions without pronounced shell closures correlations between the valence nucleons may become as large as the spacing of the single-particle energies. This can thus lead to particle-hole excitations to higher-lying single-particle states enabling deformed configurations to be lowered in energy. This may result in low-lying collective excitations, the coexistence of different shapes at low energies or even the deformation of the ground state for nuclei with the conventional magic number N ¼ 20. Such an effect occurs in the ''island of inversion'', one of most studied regions of exotic nuclei in the nuclear chart. In this region of neutron-rich nuclei around the magic number N ¼ 20 strongly deformed ground states in Ne, Na, and Mg isotopes have been observed [8-11]. Because of the reduction of the N ¼ 20 shell gap, quadrupole correlations can enable low-lying deformed 2p-2h intruder states from the fp shell to compete with spherical normal neutron 0p-0h states of the sd shell. In this situation the promotion of a neutron pair across the N ¼ 20 gap can result in deformed intruder ground states. Consequentially, the competition of two configurations can lead to the coexistence of spherical and deformed 0 þ states in the neutron-rich 30;32 Mg nuclei [12]. Coulomb excitation experiments have shown that 30 Mg has a rather small BðE2Þ value for the 0 þ gs ! 2 þ 1 transition [13,14] placing this nucleus outside the island of inversion. The excited deform...