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Resonant clustering

Periodic Reporting for period 1 - ResClust (Resonant clustering)

Reporting period: 2016-03-01 to 2017-02-28

The nucleus lies at the heart of the atom, comprising a dense clump of protons and neutrons, collectively known as nucleons. As such, a stable element found on earth often consists of several isotopes, each with a different mass number of nucleons and, therefore, slightly different mass. The classical view of a nucleus is that of a homogeneous, spherical ‘bag’ of protons and neutrons. However, under some conditions the nucleons can clump together into groups or clusters. The extent to which nucleons inside the nucleus can clump together to form clusters in light systems, e.g. carbon, oxygen or neon, directly influences the astrophysical production, rate of decay and even the types of processes that can occur. Many stable light nuclei have equal numbers of protons and neutrons and naturally exist on earth. Such nuclei have been the focus of many studies into clustering. However, the scope of clustering away from stability – where the number of protons and neutrons is not the same – has yet to be fully investigated. This is the so-called exotic clustering regime which nuclei often live for only a few seconds before disintegrating. Studies of such systems will aid our understanding of, for example, some of the most energetic events in the universe – X-rays bursts – for which nuclear clustering is believed to enhance the onset reaction.

The aim of the project was to begin addressing the deficiency of data on exotic nuclei by implementing novel measurement apparatus for key short-lived isotopes using resonant elastic scattering – a method to map out nuclear energy levels by scattering and fusing two nuclei together before disintegrating into the initial components after a tiny fraction of a second – and at the same time demonstrate improvements to this spectroscopic tool that can be applied to a wide range of nuclear structure studies. By scanning out lots of energies during the fusion or scattering process, the internal structure of the compound or fused nucleus can be examined in detail. The new experimental apparatus was developed to perform such studies in addition to software for the subsequent data analysis. Both of these can be used by the nuclear physics community to perform similar experimental investigations that will help tie down the dominant astrophysical processes across a range of different stellar temperatures and elucidate the role that nuclear clustering plays. Put simply, the structure of the fused nucleus gives information about how probably it is to be formed at different temperatures in stars. Thus, how energy is released in different stars and other stellar objects can be inferred.
Preparatory work for the project consisted of the development and testing of the experimental apparatus. It was divided into two principal tasks. The first was the development of a scattering chamber with the characteristics required by the experimental method proposed – specifically, accommodating a large area detector array for recording the scattered, charged nuclei and to be able to contain up to one atmosphere of a helium gas target. As most nuclear reaction experiments take place in vacuum, a complication when using helium gas as the target is that it must be securely separated from the high-vacuum of the particle accelerator beam tube. The second task was to propose and perform these resonant scattering experiments using the newly developed experimental and data analysis capabilities to explore nuclear-cluster structures in exotic nuclei that are relevant to element formation and explosive astrophysical events.

A first experiment was proposed by the ER together with Dr Wheldon and performed before the fellowship began using a smaller detector set-up and was fully analysed and the results submitted for publication during the grant period. The experiment consisted of studying the cluster structure of the unstable neon-19 nucleus (with 10 protons and 9 neutrons) by using a radioactive oxygen-15 beam specially developed by the ER for this investigation. This scattering reaction involving neon-19 is important as it is believed trigger the onset of X-ray bursts. The results (submitted to Physics Letter B) provide evidence that supports a significant role for nuclear cluster structure in enhancing this crucial stellar process.

The second system studied was the unstable oxygen-15 nucleus; important for understanding how clustering evolves as the neutron-to-proton ratio in the nucleus varies. The oxygen isotopes provide an excellent laboratory in which to study such effects due to the well established results in heavier isotopes. By investigating oxygen-15, the behaviour in this first neutron-deficient oxygen isotope can be explored. Using beam of carbon-11 – an unstable form of carbon with one neutron fewer – specially developed and produced at the EXOTIC facility at LNL (Italy) by the ER, made such a direct experimental measurement possible. The carbon-11 beam was successfully produced with a high intensity and good quality and preliminary results from this experiment have been presented at two conferences.

An important component of all of these measurements is precise knowledge of the stopping how the beam nuclei slow down in helium gas. To provide these data a separate experiment was carried out at LNS, Catania (Italy) and the analysis completed. Such measurements prevent systematic uncertainties arising from the slow-down of the beam in the gas and are also useful to other physicists.
In order to further study the evolution of nuclear clustering to more neutron-deficient isotopes, an experiment to investigate oxygen-14 has been proposed and approved at Texas A&M University, USA complementing the data obtained during the fellowship.

The results of the two experiments were presented in three international conferences:
XXXX Symposium on Nuclear Physics, Moroles, Mexico, January 2017; IoP Nuclear Physics Conference, Birmingham, UK, April 2017; 11th Nuclear Structure and Dynamics 2016, Naples, May 2016.
The study of the two systems investigated in this project, neon-19 and oxygen-15, has enabled our knowledge of the clustering of unstable nuclei to be deepened. Clustering in this region of neutron deficient nuclei was poorly understood. Moreover, the study of the neon-19 cluster structure has important implications in nuclear astrophysics since it is a key nucleus for understanding the nucleosynthesis process in explosive astrophysical environments and the key reaction in triggering the onset of X-ray bursts.

A new experimental capability was developed to perform such studies as well as software for the data analysis. Both can be used by the wider nuclear physics community to perform similar experimental investigations on exotic nuclear systems to further our understanding of the role of clustering and its effects on stellar processes and allowing more precise data to be obtained in which background contamination is also reduced. This latter point has been achieved by carefully timing the interactions between the beam and the helium gas target.

During this one-year fellowship, the work has been well received by the scientific community: three proposals at international facilities were approved; three contributions to international conferences have been presented, two of which were invited; one journal article has been submitted, two conference proceedings are being published and further journal manuscripts are in preparation.
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