Stars forge elements through nuclear reactions, and this group will measure those reactions in laboratories on Earth. The research addresses a fundamental gap in our understanding of where the chemical elements come from. New astronomical observations—including gravitational waves from neutron star mergers—have revealed the sites where elements are made, but nuclear physics must supply the reaction rates and nuclear properties needed to model these processes. Without these measurements, we cannot explain why the universe contains the particular mix of elements it does. This is fundamental science with no immediate practical application. The group will measure exotic, neutron-rich isotopes produced in particle accelerators, using a detector system called AIDA to track their decay, and ion traps to weigh them precisely. They will also measure low-probability fusion reactions at an underground facility shielded from cosmic rays, and use a storage ring to study reactions that occur in supernovae and novae. One experiment will improve measurements of a key Big Bang reaction, potentially constraining the existence of exotic particles beyond the Standard Model. If successful, the work will provide the nuclear data needed to interpret astronomical observations and test models of element formation—from the Big Bang to exploding stars.
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The elements we see around us today are still being forged by nuclear reactions in stars and some were first produced as early the primordial big-bang. Understanding the different astrophysical origins and production mechanisms of the elements is a fundamental challenge in science. Remarkable new astronomical observations, including those of neutron star mergers by Gravitational Wave, and electromagnetic radiation measurements, provide a challenge to nuclear physics to determine the key nuclear properties required to understand and model element production at these astrophysical sites. In the case of neutron star mergers and the production of heavy elements beyond Iron, this involves explosive reactions on isotopes with 20-30 neutrons more than the stable isotopes we see around us today. Here on the earth we are now able to produce these exotic isotopes for the first time using modern heavy ion accelerators. The Edinburgh Group has led the development of a key detection system, AIDA, to perform these measurements on these isotopes as they begin their long decay journey back to stability. In a new development within the Group we will be using ion traps to momentarily incarcerate these exotic isotopes to precisely measure their masses, a fundamental property that determines how far from stability the production of heavy elements proceeds. Heavy elements can also be produced over a long period of time by neutron fusion reactions during quiescent phases of stellar evolution, tracking closely to the line of stability. In this case, the reaction probabilities need to be measured directly in nuclear reaction measurements with intense neutron beams. The origin of the neutrons in stellar environments occurs by very low energy fusion reactions between charged particles and requires quantum tunnelling to proceed. These reactions have very low reaction probabilities and will be measured at a new underground accelerator facility LUNA MV where the background from cosmic rays is low. At the same facility, we will explore the reaction rate between Carbon nuclei that determines whether massive stars explode as supernovae or whither away into white dwarfs. We will measure a reaction occurring during core collapse supernovae explosions that controls the amount of gamma-rays observed from the subsequent decays of radioactive nuclei after the explosion using a storage ring where the radioactive ions repeatedly traverse a Helium gas target and the reactions are measured with a new detector system, CARME, developed by the Group. This system will also be used to measure reactions occurring in novae explosions that control the production of elements ejected into the cosmos and isotopic ratios measured in pre-solar grains found in meteorites. We will also explore the evolution of nuclear shell structures far from stability, including phenomena at magic numbers, representing particular stable quantum configurations. These structures leave behind their fingerprints in the abundances of the elements. Finally we return to the original elemental origin, the Big Bang. Here astronomical observations, of the microwave background radiation and light element abundances, now supercede the precision of nuclear reaction measurements required to model the Big Bang so we will measure a key reaction for the Big Bang using CARME on the storage ring representing a completely new approach to such measurements, where we hope to improve the precision and eliminate certain systematic sources of error. Such improved measurements can for example limit the possible existence of exotic particles beyond the Standard Model of Particle Physics.
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