Explosive nucleosynthesis on neutron-stars in binary systems - 18Ne reactions

Context of the research: Over recent years, the detections of gravitational waves from the merger of a binary neutron-star system have highlighted their significance in nucleosynthesis in our universe [B.P. Abbott et al. Phys. Rev. Lett. 119, 161101 (2017) LIGO Scientific Collaboration and Virgo Collaboration]. The present project will experimentally investigate the nucleosynthesis involved in one of the likely precursors to these mergers, a common-envelope system in which the first neutron star is embedded into the envelope of its binary (giant) companion star before the second neutron star is created. Explosive nucleosynthesis on the surface of neutron stars is in its initial phase fuelled by the Hot-CNO cycle, in which protons are converted to alpha-particles through catalytic cycles, such as the first Hot-CNO cycle: 12C(p,gamma)13N(p,gamma)14O(beta+nu)14N(p,gamma)15O(beta+nu)15N(p,alpha)12C which encompasses isotopes up to 15O and 15N, as well as the second Hot-CNO: 15N(p,gamma)16O(p,gamma)17F(p,gamma)18Ne(beta+nu)18F(p,alpha)15O(beta+nu)15N, which reaches as far as 18Ne. During these processes, significant levels of waiting-point nuclei, such as 15O and 18Ne, are built up in the hydrogen-burning layer, awaiting breakout from the Hot-CNO cycles. Project aims and objectives: The present project aims at a determination of one of the key breakout reactions, 18Ne(alpha,p)21Na, which in the right conditions can lead to the rp-process, potentially producing some of the most neutron-deficient isotopes available on Earth. This reaction may take place both on precursors to neutron-star mergers and in the similar Type I X-ray Bursts, which also takes place on the surface of neutron stars in binary systems. Specifically, the project will focus on an experimental determination of the 18Ne(alpha,p)21Na reaction rate, which has been a central target for the nuclear astrophysics research community for decades. This reaction will be studied through alpha-transfer with a radioactive ion beam of 18Ne at the GANIL Laboratory in France. Here we will make use of advanced detector facilities, in particular the MUGAST silicon-detector array for detection of light-ion ejectiles from the reaction and the EXOGAM2 HPGe array for gamma-ray detection. During the experimental programme, the astrophysical impact will furthermore be explored in collaboration with astrophysical modellers. The key objectives of the project will therefore be to (a) prepare and (b) carry out the experiment to measure alpha capture on 18Ne, as well as (c) to analyse the results and (d) to assess their astrophysical impact. Applications and benefits: The 18Ne project will offer a wide range of impact from detector development to astrophysical impact simulations. The experimental work in preparation of the campaign will for example involve development of detector components, such as a diamond-detector beam monitors and alternative modes of beam monitoring for high-intensity radioactive beams. The detector development has potential impact both within fundamental nuclear (astro) physics research, for equivalent (radioactive) ion beam monitoring, as well as in applications of advanced nuclear detection in characterisation of charged-particle radiation in medical physics and/or the nuclear energy sector. There will furthermore be opportunities within the project for assessing the impact of results in astrophysical binary stellar systems, leading to an improved understanding of how binary systems may have contributed to the production of chemical elements in the universe. This will be carried out in collaboration with an international team of researchers from across Europe, particularly linked to the GANIL facility (France), and may thereby strengthen the strategic partnerships between UKRI-STFC and GANIL.