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Explosive nucleosynthesis on neutron-stars in binary systems


   Department of Physics

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  Dr C Diget  No more applications being accepted  Awaiting Funding Decision/Possible External Funding

About the Project

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,γ)13N(p,γ)14O(β+ν)14N(p,γ)15O(β+ν)15N(p,α)12C which encompasses isotopes up to 15O and 15N, as well as the second Hot-CNO: 15N(p,γ)16O(p,γ)17F(p,γ)18Ne(β+ν)18F(p,α)15O(β+ν)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. The present project aims at a determination of one of the key breakout reactions, 18Ne(α,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(α,p)21Na reaction rate, which has been a central target for the nuclear astrophysics research community for decades. This reaction will for the first time be studied through alpha-transfer with a radioactive ion beam of 18Ne, available at the ion beam facility SPIRAL (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 γ-ray detection. During the experimental programme, links with astrophysical modellers of neutron stars will furthermore be explored, allowing direct application of the observed reaction rates in an advanced theoretical framework.

Project objectives:

The key objectives of the project will therefore be to prepare and carry out the experiment to measure alpha capture on 18Ne, as well as to analyse the results and their astrophysical impact, in collaboration with researchers from across Europe.

Training and methodologies:

As the PhD project will be at the core of the 18Ne experimental programme from preparation through to impact analysis, the project will offer a wide range of specialist training opportunities across detector development, data analytics, and astrophysical simulations. As part of the experimental programme, the PhD training will involve participation in a wide range of experiments utilising these facilities, in preparation for taking a leading role in their own experiment on 18Ne alpha transfer. The experimental work in preparation of the campaign will for example involve development of detector components, such as a diamond-detector beam monitor. Following the experiment, the project focus will switch to the analysis of the multi-particle coincidence data, utilising the wide range of detector types utilised in the experiment: highly-segmented silicon detectors, high-purity germanium detectors, diamond detectors, plastic scintillators, as well as ion-chambers. This analysis will utilise both the ROOT data analysis framework (CERN analysis package, written in C++), as well as GEANT4 for detailed simulations of detector response. This analysis will be complemented by reaction model calculations to describe the transfer reaction and to determine the astrophysical reaction rate. There will furthermore be opportunities for assessing the impact of results in binary stellar systems.

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