L. V. Grigorenko

Radioactive decays at limits of nuclear stability

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.

Measurement of the {beta}{sup +} and Orbital Electron-Capture Decay Rates in Fully Ionized, Hydrogenlike, and Heliumlike {sup 140}Pr Ions

We report on the first measurement of the beta+ and orbital electron-capture decay rates of 140Pr nuclei with the simplest electron configurations: bare nuclei, hydrogenlike, and heliumlike ions. The measured electron-capture decay constant of hydrogenlike 140Pr58+ ions is about 50% larger than that of heliumlike 140Pr57+ ions. Moreover, 140Pr ions with one bound electron decay faster than neutral 140Pr0+ atoms with 59 electrons. To explain this peculiar observation one has to take into account the conservation of the total angular momentum, since only particular spin orientations of the nucleus and of the captured electron can contribute to the allowed decay.

New results on the halo structure of 8B

The longitudinal momentum distribution of Be fragments after fragmentation of B and the one-proton removal cross section (σ-1p) in a carbon target were measured at 1440 MeV/u with the fragment separator FRS used as an energy-loss spectrometer. The results show a narrow momentum distribution with a FWHM value of 91 ± 5 MeV/c and a large cross-section, σ-1p = 98 ± 6 mb. Both these results support the interpretation of a spatially extended proton orbit forming a halo in the B ground state. The momentum distribution and the one-proton removal cross section are shown to be reproduced with a three-body wave function for 8B where the target is treated as a black disc in the breakup process.

Prospective candidates for the two-proton decay studies I: structure and Coulomb energies of 17Ne and 19Mg

We discuss perspectives of studies of two-proton emission from the unbound ground state of 19Mg. The structure of 19Mg and the emission process are studied in a three-body model. Accurate estimates of the decay energy are provided in order to estimate the lifetime. We conclude that observation of the fragments from the in-flight decay of the nucleus, created by high-energy fragmentation, is an adequate experimental technique for the broad range of possible lifetimes of 19Mg. The 17Ne nucleus is studied in a three-body model to calibrate the model for 19Mg. Basing on the Coulomb displacement energy analysis the weight of the s2 component in 17Ne is predicted to be 40–50%.

The 8He and 10He spectra studied in the (t, p) reaction

The low-lying spectra of 8He and 10He nuclei were studied in the 3H(6He, p)8He and 3H(8He, p)10He transfer reactions. The 0+ ground state (g.s.) of 8He and excited states, 2+ at 3.6–3.9 MeV and (1+) at 5.3–5.5 MeV, were populated with cross sections of 200, 100–250, and 90–125 μb/sr, respectively. Some evidence for a 8He state at about 7.5 MeV was obtained. We discuss a possible nature of the nearthreshold anomaly above 2.14 MeV in 8He and relate it to the population of a 1− continuum (soft dipole excitation) with a peak value at about 3 MeV. The lowest energy group of events in the 10He spectrum was observed at ∼ 3 MeV with a cross section of ∼ 140 μb/sr. We argue that this result is consistent with the previously reported observation of 10He providing the new 10He g.s. position at about 3 MeV

Two-proton radioactivity studies with 45Fe and 48Ni

In an experiment at the SISSI/LISE3 facility of GANIL, we have studied the decay of the two proton-rich nuclei 45Fe and 48Ni. We identified 30 implantations of 45Fe and observed for the second time four implantation events of 48Ni. In 17 cases, 45Fe decays by two-proton emission with a decay energy of 1.154(16) MeV and a half-life of T_1/2 = 1.6+0.5_-0.3 ms. The observation of 48Ni and of its decay allows us to deduce a half-life of T_1/2 = 2.1+2.1_-0.7 ms. One out of four decay events is completely compatible with two-proton radioactivity and may therefore indicate that 48Ni has a two-proton radioactivity branch. We discuss all information now available on two-proton radioactivity for 45Fe and 48Ni and compare it to theoretical models.

Probing the 11Li halo structure through β-decay into the 11Be∗(18 MeV) state

An experimental study of beta-delayed charged particles (H, He, Be) from Li-11 shows a beta-feeding to an excited state at 18.15(15) MeV in Be-11 with a large Gamow-Teller strength, B-GT greater than or equal to 1.6. Branching ratios and reduced widths of 2-, 3- and 5- particle decay channels of this state are extracted, A strong suppression of the neutron branch from the 18 MeV state to the Be-10(g.s.) is observed, We suggest that the measured partial width may be sensitive to the (p(1/2))(2) component in the Li-11 halo, A rough estimate indicates that the (p(1/2))(2) component is not the dominant one in the halo wave-function.

Democratic Decay of 6Be Exposed by Correlations

The interaction of an E/A=70-MeV (7)Be beam with a Be target was used to populate levels in (6)Be following neutron knockout reactions. The three-body decay of the ground and first excited states into the α+p+p exit channel were detected in the High Resolution Array. Precise three-body correlations extracted from the experimental data allowed us to obtain insight into the mechanism of the three-body democratic decay. The correlation data are in good agreement with a three-cluster-model calculation and thus validate this theoretical approach over a broad energy range.

Proton-proton correlations observed in two-proton decay of 19Mg and 16Ne

Proton-proton correlations were observed for the two-proton decays of the ground states of 19 Mg and 16 Ne. The trajectories of the respective decay products, 17 Ne + p + p and 14 O + p + p, were measured by using a tracking technique with microstrip detectors. These data were used to reconstruct the angular correlations of fragments projected on planes transverse to the precursor momenta. The measured three-particle correlations reflect a genuine three-body decay mechanism and allowed us to obtain spectroscopic information on the precursors with valence protons in the sd shell.

Two-proton radioactivity and three-body decay. III. Integral formulas for decay widths in a simplified semianalytical approach

Three-body decays of resonant states are studied using integral formulas for decay widths. The theoretical approach with a simplified Hamiltonian allows semianalytical treatment of the problem. The model is applied to decays of the first excited 3/2- state of 17Ne and the 3/2- ground state of 45Fe. The convergence of the three-body hyperspherical model calculations to the exact result for widths and energy distributions are studied. The theoretical results for 17Ne and 45Fe are updated, and uncertainties of the derived values are discussed in detail. Correlations for the decay of the 17Ne 3/2- state are also studied.