Measurements of Key Reactions for the Production and Destruction of Elements in Stars

The evolution of the universe has left an imprint in the form of the chemical elements. Understanding the cosmic origins of the elements remains a major challenge for science. The lightest elements are believed to have been produced in the Big Bang and indeed their abundances provide evidence for the Big Bang. All elements heavier than Li were most likely produced inside stars. The elements are then injected into the interstellar medium by stellar winds and explosions leading to generations of cosmic re-cycling. The abundances of elements we see today in our own solar system, distant stars, and meteorites as well as the observation of still decaying radionuclides provide us with clues about how the elements came to be produced in a variety of different processes and stellar environments. However, to unravel these mysteries of how elements are formed we need to understand the nuclear reactions producing and destroying the elements. Since explosive environments play a major role in this process - where temperatures and densities are high, and timescales are short - it is often the reactions and properties of unstable nuclei that are of key importance.
Some of the greatest open questions relate to the production of the heavy elements (beyond iron) where reactions with neutrons play a critical role. During the Fellowship I will directly study certain key reactions (many for the first time) taking place in stars, using highly sensitive techniques at new international accelerator facilities. These measurements aim at understanding the stellar environments in which the elements are produced and how the neutrons themselves originate. In the latter case, this will also lead to important new information relevant for cancer treatments using Neutron Capture Therapy.
The fact that element formation is still happening in the cosmos is evident from the observation of cosmic gamma-ray emitters in our galaxy. Cosmic gamma-ray emitters are radioactive nuclei which decay after a time that is much smaller than the typical lifetime of a star (lifetimes of cosmic gamma-ray emitters can range from months to millions of years). When they decay, high energy photons (gamma rays) are emitted which can be detected with satellite telescopes. Within the Fellowship I will measure the key neutron-induced reactions influencing the final abundances of the cosmic gamma-ray emitters aluminium-26 (26Al) and iron-60 (60Fe) observed as remnants of supernova explosions. A supernova in the vicinity of the solar system is thought to be responsible for the injection of 60Fe material onto the surface of the earth 3 million years ago, now observed as isotopic enrichment in ocean sediment samples.
Although neutrons play the dominant role in the production of heavy elements, there are certain rare, proton-rich, isotopes that cannot be produced this way, and their origin remains unknown. New processes and astrophysical sites have have been proposed to resolve this. One suggested process (the nu-p process) now being intensively pursued, invokes a mechanism driven by intense neutrino winds produced in the core of the exploding star. A key hydrogen burning reaction will be studied for the first time using the world-leading ISOLDE radioactive beam facility at CERN which, if strong enough, could block the production of heavier proton-rich elements in this process by producing a closed cycle of nuclear reactions. Heavy ion storage rings facilities injected with proton-rich radioactive isotopes offer a promising new method for studying reactions producing heavy proton-rich nuclei. I will become involved in the design of such experiments for the future TSR@ISOLDE facility at CERN.