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Subjecting a physical system to extreme conditions is one of the means often used to obtain a better understanding and deeper insight into its organization and structure. In the case of the atomic nucleus, one such approach is to investigate isotopes that have very different neutron-to-proton (N/Z) ratios than in stable nuclei. Light, neutron-rich isotopes exhibit the most asymmetric N/Zratios and those lying beyond the limits of binding, which undergo spontaneous neutron emission and exist only as very short-lived resonances (about 10−21s), provide the most stringent tests of modern nuclear-structure theories. Here we report on the first observation of 28O and 27O through their decay into 24O and four and three neutrons, respectively. The 28O nucleus is of particular interest as, with the Z= 8 and N= 20 magic numbers1,2, it is expected in the standard shell-model picture of nuclear structure to be one of a relatively small number of so-called ‘doubly magic’ nuclei. Both 27O and 28O were found to exist as narrow, low-lying resonances and their decay energies are compared here to the results of sophisticated theoretical modelling, including a large-scale shell-model calculation and a newly developed statistical approach. In both cases, the underlying nuclear interactions were derived from effective field theories of quantum chromodynamics. Finally, it is shown that the cross-section for the production of 28O from a 29F beam is consistent with it not exhibiting a closed N= 20 shell structure.

A long-standing question in nuclear physics is whether chargeless nuclear systems can exist. To our knowledge, only neutron stars represent near-pure neutron systems, where neutrons are squeezed together by the gravitational force to very high densities. The experimental search for isolated multi-neutron systems has been an ongoing quest for several decades1, with a particular focus on the four-neutron system called the tetraneutron, resulting in only a few indications of its existence so far2–4, leaving the tetraneutron an elusive nuclear system for six decades. Here we report on the observation of a resonance-like structure near threshold in the four-neutron system that is consistent with a quasi-bound tetraneutron state existing for a very short time. The measured energy and width of this state provide a key benchmark for our understanding of the nuclear force. The use of an experimental approach based on a knockout reaction at large momentum transfer with a radioactive high-energy 8He beam was key.

Particle knockout scattering experiments1,2are fundamental for mapping the structure of atomic nuclei2–6, but their interpretation is often complicated by initial- and final-state interactions of the incoming and scattered particles1,2,7–9. Such interactions lead to reduction in the scattered particle flux and distort their kinematics. Here we overcome this limitation by measuring the quasi-free scattering of 48 GeV c–112C ions from hydrogen. The distribution of single protons is studied by detecting two protons at large angles in coincidence with an intact 11B nucleus. The 11B detection suppresses the otherwise large distortions of reconstructed single-proton distributions induced by initial- and final-state interactions. By further detecting residual 10B and 10Be nuclei, we also identified short-range correlated nucleon–nucleon pairs9–13and provide direct experimental evidence for separation of the pair wavefunction from that of the residual many-body nuclear system9,14. All measured reactions are well described by theoretical calculations that include no distortions from the initial- and final-state interactions. Our results showcase the ability to study the short-distance structure of short-lived radioactive nuclei at the forthcoming Facility for Antiproton and Ion Research (FAIR)15and Facility for Rare Isotope Beams (FRIB)16facilities, which is relevant for understanding the structure and properties of nuclei far from stability and the formation of visible matter in the Universe.

We propose to install a storage ring at an ISOL-type radioactive beam facility for the first time. Specifically, we intend to setup the heavy-ion, low-energy ring TSR at the HIE-ISOLDE facility in CERN, Geneva. Such a facility will provide a capability for experiments with stored secondary beams that is unique in the world. The envisaged physics programme is rich and varied, spanning from investigations of nuclear ground-state properties and reaction studies of astrophysical relevance, to investigations with highly-charged ions and pure isomeric beams. The TSR might also be employed for removal of isobaric contaminants from stored ion beams and for systematic studies within the neutrino beam programme. In addition to experiments performed using beams recirculating within the ring, cooled beams can also be extracted and exploited by external spectrometers for high-precision measurements. The existing TSR, which is presently in operation at the Max-Planck Institute for Nuclear Physics in Heidelberg, is well-suited and can be employed for this purpose. The physics cases as well as technical details of the existing ring facility and of the beam and infrastructure requirements at HIE-ISOLDE are discussed in the present technical design report.

Experiments with high-energy radioactive beams for nuclear astrophysics are discussed. Emphasis is paid to measurements of masses of exotic nuclei relevant for nucleosynthesis processes and the determination of dipole strength functions for neutron-rich nuclei relevant for both the r-process nucleosynthesis and neutron-star matter. Finally, an outlook is given on the experimental program planned at the radioactive-beam facility NuSTAR at the accelerator facility for anti-proton and ion research FAIR.

One-nucleon removal reactions at relativistic energies have been used as a spectroscopic tool for obtaining information about the ground state of neutron-rich oxygen isotopes. Using the FRS at GSI, the longitudinal momentum distributions of the emerging fragments after breakup were measured along with the one-nucleon removal cross-sections. The relative contributions of the remaining fragments in their ground and excited states have been determined for a few cases from measurements of γ rays in coincidence with the longitudinal momentum distribution. The controversial case of 23O has been directly addressed. Comparison of our exclusive momentum distributions for the one-neutron removal channel to theoretical momentum distributions calculated with an Eikonal model for the knockout process rendered a spin and parity of Iπ = 1/2+ for the 23O ground state. This result resolves the existent experimental discrepancy and supports sub-shell closure at N = 14 with a fairly pure 2s1/2 neutron in 23O.

Inelastic scattering of loosely bound nuclei by Coulomb interaction at intermediate energies (400–600 MeV/nucleon) has been utilized as a spectroscopic tool for exotic nuclei. The observed electromagnetic dipole (E1) strength above the one neutron threshold of neutron-rich C, Be, B and O isotopes can be explained by a non-resonant transition of a neutron into the continuum. The shape of these strength distributions reflects properties of the wavefunction of the released neutron in the nucleus and hence ground-state properties of these isotopes. Neutron capture cross-sections such as for the 14C(n,γ) 15C reaction which are of astrophysical relevance can be deduced indirectly.

During 1995–97, raptor assemblages were studied in ten riparian habitats in the arid south-west of the Northern Territory, Australia. The assemblages at different creeks differed in species composition, diversity, richness and pair density. They also changed from year to year, in parallel with changes in rainfall and food abundance. Differences in food abundance probably accounted for much of the spatial and temporal variation in the assemblages, although between-creek differences in habitat and the availability of nesting resources were also implicated. Most species appeared to respond to the environmental changes individualistically, but differences in pair spacing within and between trophic groups were suggestive of competitive interactions.

During 1995–97, reproductive requirements and success were quantified for raptor assemblages of up to 10 species on (usually dry) creeks in the south-west of the Northern Territory. Most breeders chose to nest in River Red Gums (E. camaldulensis) that were taller, of greater girth and more foliated than other trees generally available on the local drainages, but few interspecific differences in nest tree or nest site requirements were identified. Breeding densities and success fluctuated greatly during the study. Total assemblage productivity in 1996, a drought year, was 6–10 times lower than in 1995 and 8–15 times lower than in 1997. Declines in territory occupancy, breeding density, breeding success and the number of young fledged per active nest were characteristic responses to the drought for most species.

Coulomb breakup of neutron-rich nuclei around mass A∼ 20 has been studied experimentally using secondary beams (∼ 500–600 MeV/u) of unstable nuclei produced at GSI. The spectroscopic factor deduced for the neutron occupying s1/2level in 15C ground state is consistent with the earlier reported value. The data analysis for Coulomb breakup of 17C shows that most of the cross section yields the 16C core in its excited state. For 17–22O, the low-lying E1 strength amounts up to about 12% of the energy weighted dipole sum rule strength depending on neutron excess. The cluster sum rule limit with 16O as a core is almost exhausted for 17,18O, while for more neutron rich isotopes the strength with respect to that limit decreases.

Dissociation of a 240 MeV/u beam of 6He, incident on carbon and lead targets, has been studied in kinematically complete experiments to investigate low-lying excitation modes in the halo nucleus 6He. It is shown that alignment effects characterize the inelastic scattering and allow an unambiguous assignment of the spin of a narrow resonance observed in the excitation energy spectrum. The differential cross sections for the 6He inelastic scattering on carbon and lead targets were deduced from the measured momenta of the two neutrons and the α-particle. An analysis of these distributions shows that quadrupole and, possibly, monopole excitations characterize the hadronic interaction, while the dipole mode is dominating in Coulomb dissociation. Neither theoretically predicted new resonance states in 6He nor nuclear excitation of a dipole mode were found. Direct evidence has been obtained for strong suppression of Coulomb post-acceleration in direct Coulomb breakup in a lead target.

Abstract: This work demonstrates the feasibility of in-beam γ-spectroscopy employing radioactive ion beam species at relativistic energies of E = 210 ∼ 280 A · MeV. Neutron-rich nuclei below ⁴⁸Ca with neutron number between the two magic numbers 20 and 28 have been investigated. Using a Pb target, single step inelastic excitation originating mainly from Coulomb interaction takes place populating preferentially low-lying low-spin states and enabling the extraction of B(E2) values. On a C target, nucleon removal reactions are dominating and medium-spin states are populated thus offering spectroscopic information in neighboring nuclei. Cross sections of inelastic excitation are determined and compared to coupled-channels calculations. Two new states of the very neutron-rich nucleus ⁴⁴Ar at 1.78(8) MeV and 2.61(16) MeV are deduced from γ-transitions.

A captive Black-breasted Buzzard Hamirostra melanostemon cracked Domestic Chicken Gallus gallus eggs by using its bill to throw 15-65 g rounded stones at them in order to feed on the contents. The throwing method, feeding and post-feeding behaviour are described. Throwing accuracy improved during a series of trials conducted over 26 days. The egg-breaking activity is discussed in terms of tool usage and in terms of learned versus non-learned behaviour.

Sixty-seven breeding attempts of the Brown Goshawk Accipiter fasciatus were investigated on 26 temtories. Incubation generally commenced in October, although as early as mid-September and as late as early December. Incubation took 30- 35 days, most young hatching in November. The nestling period was usually 28-35 days; males generally fledged earlier than females. No breeding occurred outside the springlsummer period. Nests were placed high in mature Eucalyptus trees in woodland within 200 m of pasture. Nest trees were commonly on sheltered slopes of southerly or easterly aspect. Approximately 30% of nests were beside the trunk of the nest tree, the remainder being in branch forks 1-12 m from the trunk. Active nests were never closer than 1.6 km apart. Breeding density and success declined from 31.3 active nests1100 km⊃;, 2.31 young fledgedlactive nest in 1981 to 20.3 active nests1100 km⊃;, 1.54 young fledgedlactive nest in 1983. From 50-77% of territories held active nests in any one year during 1981-83. Particular nests were used for several years in succession on some territories, however there was no correlation between nest re-use and prior breeding success. Limited evidence was found for territory fidelity from year-to-year.

Abstract: Extracellular single-unit recordings were made from cerebellar dentate neurones in two conscious monkeys trained to perform wrist flexion and extension movement tasks that produced a range of static joint positions and dynamic velocities. The experiment was designed to establish whether there is a relationship between the discharge of dentate neurones and movement kinematics. The discharge patterns of 58 “wrist-related” neurones were correlated with joint position, duration of unidirectional movement (referred to as duration of velocity) and amplitude of velocity (peak velocity). Significant (P<0.05) correlations were found between the level of tonic discharge and static joint position in 21 of 58 (36%) neurones. Correlations between phasic discharge and at least one of the velocity variables were found in 17 of 43 (40%) neurones [7 of 43 (16%) showed a correlation between the duration of phasic excitation associated with movement and duration of velocity, 5 of 43 (12%) between the peak rate of phasic excitation and peak velocity and 10 of 43 (23%) between the number of discharges in the period of phasic excitation and peak velocity]. We conclude (for reasons outlined in the Discussion) that there is not a strong relationship between neuronal discharge and kinematic parameters of wrist movement in the dentate nucleus.

As part of a comprehensive study of uranium fragmentation at relativistic energies at the GSI projectile fragment separator, FRS, inclusive neutron-removal cross sections have been measured for severalxn channels at projectile energies of 600 and 950A MeV using targets of Al, Cu and Pb. The variation of the experimental cross sections with target nuclear charge is used to disentangle nuclear and electromagnetic contributions. The electromagnetic cross sections agree surprisingly well with a simple harmonic oscillator calculation of giant dipole resonances based on measured photonuclear cross sections and do not require an extra enhancement of the two-phonon giant dipole excitation as concluded from similar measurements with197Au.

Excitation of the two-phonon isovector giant dipole resonance was observed in 208Pb projectiles incident on different target nuclei (C, Sn, Ho, Pb, U) at high energy (640 A·MeV). Evidence is found for a two-step electromagnetic excitation mechanism. An enhancement of 1.33(16) for the excitation cross section is observed relative to the harmonic approximation. Neutron- and γ-decay probabilities are derived as well.

The folding- and azimuthal-angle and velocity distributions for the238U fission fragments have been measured in reactions with 100, 500, and 1000 A·MeV208Pb. These distributions were used to decompose the fission cross section into its electromagnetic and nuclear components. The fraction of electromagnetic fission was found to be 0.16±0.07, 0.48±0.08, and 0.60±0.04, respectively. The electromagnetic fission cross section as a function of the208Pb nucleus energy is compared with theoretical predictions. The measured fission cross section from nuclear reactions (˜1.5 b) is approximately constant between 100 and 1000 A·MeV.

The diet of the brown goshawk, Accipiter fasciatus, was studied during 1980-83 near Macclesfield, VIc. Birds made up 63% and mammals 26% of 1769 prey items. Rabbits, Oryctolagus cuniculus, accounted for 95% of mammalian items. Mammals were estimated to contribute more to dietary biomass than birds (54 v.46%). Other animals recorded in the diet were reptiles (Elapidae and Scincidae), insects (mostly Coleoptera and Orthoptera) and crustaceans (Engaeus spp.). Diet composition varied significantly from year to year, from season to season and, during breeding, between territories in close proximity to one another. There were significant differences in the diets of males and females. Males took relatively more birds (52 v.24%), more insects (41 v.21%) and fewer mammals (2 v. 48%). Differences in food between age classes were also significant. Compared to second-year and older birds, first-year birds took relatively fewer mammals (28 v.43%), fewer birds (23 v.49%) and more insects (43 v.0%).

Charge-loss and fission cross-sections of238U at 750 A·MeV were measured on Al, Cu and Pb targets. The charge-loss rate was obtained by the attenuation method. Fission was selected by detecting the pair of highly ionizing fragments. Since the neutron-loss cross sections were measured in a parallel experiment for the same projectiles, all cross sections contributing to238U collisions on nuclei are available now as function of the target mass number and can be compared with current models.

Charge distributions of fragments from low energy nuclear fission are investigated in reactions of highly fissile238U projectiles at relativistic energies (750 A·MeV) with a heavy (Pb) and a light (Be) target. The fully stripped fission fragments are separated by the Fragment Separator (FRS). Their high kinetic energies in the laboratory system allow the identification of all atomic numbers by using Multiple-Sampling Ionization Chambers (MUSIC). The elemental distributions of fragments observed at larger magnetic rigidities than the238U projectiles show asymmetric break-up and odd-even effects. They indicate a low energy fission process, induced mainly by dissociation in the electro-magnetic field for the U/Pb-system, or by peripheral nuclear interactions for the U/Be-system.