Final Report Summary - LIGHTHALOS (Crowns of neutrons: reactions and decays to penetrate the halos of Be-14 and He-8)
Predicting the staggering variety of configurations, properties, and behaviours, which spontaneously arises from the mutual interaction of a relatively small (from few to few hundred) number of nucleons, is one of the most fundamental – and yet still unmet – challenges in the study of nuclei. The wealth of experimental studies which led and constrained the major theoretical developments of the first century of nuclear science focussed, not surprisingly, on nuclei which can be found in nature, or could at least be produced with relative ease using reactions with accelerated beams of stable nuclei. These nuclei lie on, or close to, what is known as the “valley” of beta stability. In the last two decades, however, thanks to the development of radioactive ion beams (RIB), nuclear physics made unprecedented steps to “climb out” of the valley of stability, in order to explore regions of isotopes with either much larger or much smaller neutron-to-proton ratios.
These nuclei exhibit such “exotic”, unexpected properties to have caused real paradigm shifts in our understanding of the nuclear many-body system, and bear important consequences on related fields, such as nuclear astrophysics. Amongst the large variety of new observations, it is worth to remark: new nuclear topologies (halo nuclei, cluster structures), new decay modes, or the breakdown of traditional shell closures and the appearance of new magic numbers. Because of the rich abundance of new physical observations which it entails, the development of RIB has caused the advent of one of the most exciting times in nuclear structure since the 1960's.
The object of this project is to study the physics of nuclei where several of these new features concur and combine, i.e. in particular neutron-rich isotopes of light elements, such as 14Be and 8He. In these systems, the tail of the neutron-density distribution extends far beyond that of the tightly bound nucleons, forming what has been named a “nuclear halo”. Such nuclei, and nuclei in their vicinity, constitute a rich source of theoretical as well as experimental challenges. In many cases, these systems are loosely-bound or unbound, and cannot be understood without including in their description the coupling to continuum states. Furthermore, many experimental results required the abandonment of the picture where the nucleons are bound in a mean field of the nuclear force, and the adoption of a description in terms of interacting clusters of nucleons.
In the last decades, the goodness of a mean field approach for the description of these systems is questioned by the small number of nucleons involved. Nuclei in this region may be thus better described by ab-initio calculations (now capable of reaching nuclei up to carbon) and few-body cluster models. Nuclear theory has made an intense effort to provide a good description of these light, exotic systems, yet currently it has not reached a satisfactory level and more experimental effort is paramount to provide data necessary to guide the developments. More generally, such systems also provide the rare opportunity to study neutronic matter separate from protons, and to study the physics of Efimov states, of which some halo nuclei, called Borromean, can be understood as a particular case. An example of a Borromean nucleus is 14Be, made by a 12Be core and 2 halo neutrons. In analogy to the three interlocking rings in the Borromeo family coat of arms, all 3 units must be present for the system to be bound.
More specifically, this project focussed on the spectrum of light, neutron-rich nuclei, in particular at energies beyond the break-up threshold, which reveals information about continuum coupling. To add a necessary piece to the puzzle of the structure of 14Be, low-lying states in unbound 13Be were investigated. These states were studied by populating the isobaric analog resonance in 13B through the resonant scattering of 12Be nuclei on protons. A 12Be beam was post-accelerated for the first time at REX-ISOLDE, CERN, and studied with MAYA, an active-target in which the detection gas, isobutane, contained the protons that were the target of the reaction. The 12Be data is being analysed by Dott.ssa Sara Sambi, a Ph.D. student in our group. An important problem was represented by the contamination of 12C4+ ions in the beam, about 100 times more intense than 12Be.
A performance check of the setup and the analysis method was performed investigating known resonances in 13N, and was the topic of the Master Thesis of Mr. M. Khoderi. Preliminary results were presented at different meetings, and a final publication is expected within 1 year.
Additional avenues of research which were pursued were the study of the beta-delayed light-ion emission channels in the disintegration of 14Be and 8He, or near-lying isotopes. In October 2012 a proposal (E648) to study 14Be at GANIL was presented to the GANIL PAC. Despite acknowledgments of the group quality and expertise, the committee suggested to resubmit the proposal to the next PAC, including new purposely made theoretical predictions. A proposal to study 8He has been delayed for the same reason.
Due to this set-back, and the closing of the ISOLDE facility for whole of 2013 and most of 2014 for the HIE-ISOLDE upgrade, the Fellow also pursued additional avenues of investigation. At GANIL, In the same laboratory, he presented a Letter of Intent to investigate 0+ states in 48Ca populated via alpha transfer, using the AGATA gamma-ray spectrometer. The topic exhibits many analogies with studies performed on lighter nuclei, where clusters play a major role in some systems. The LoI was positively received and a proposal will be submitted at the next PAC.
On the topic of shell-evolution, and to gain training on beta-decay studies, the Fellow has also been actively involved in two experiments at the Nishina Center for Accelerator-Based Science, at RIKEN laboratory, Japan. Using the most intense beams worldwide from fragmentation, very neutron-rich nuclei like 136,138Sn and 132Cd were produced and studied using the EURICA gamma-ray spectrometer and the WASABI array of Silicon detectors. The neutron-delayed beta-decay of 132Cd and the beta decay of 131Cd shed new light on the proton-subshell closure along the chain of N=82 isotopes. On this topic, a publication co-authored by the fellow has already been submitted to Nature. In a second experiment, the excited states in 74Ni were populated via Coulomb excitation and studied using the DALI array. The data are currently being analyzed by Italian colleagues at Legnaro National Laboratory. Finally, the fellow also devoted part of his time to complete the analysis and the interpretation of the single-neutron transfer reaction 78Zn(d,p)79Zn, performed at REX-ISOLDE using the Minball Ge spectrometer and the T-REX Silicon detector array. The draft publication is almost ready for submission to Physical Review Letters.
The training received during the Marie Curie Fellowship in resonant scattering, reaction models, active targets, digital electronics and read-out, and data-analysis procedure for multi-parameters systems, has allowed him to acquire several interdisciplinary competencies both from the physical as well as on technical side. Thanks to this, and the new connections made as part of his training, he found a permanent research position at the Advanced Science Research Center at Japan Atomic Energy Agency in Tokai-mura, Japan.