Explosive Nucleosynthesis in Novae and X-ray Bursts

Nuclear astrophysics is one of the many applications of nuclear physics and arguably one of the most exciting. It tries to explain where all the elements around us, the oxygen in the air, the iron in our blood, the silicon in computer chips, come from. Where and how were they formed? On top of this, nuclear astrophysics tries to understand how nuclear reactions affect the life and death of all stars. How do such tiny things influence such massive objects as stars? Most stars get their energy by burning stable elements, such as the carbon and oxygen we are familiar with, over long periods of time. The energy is produced by nuclear reactions, turning one element into another. However, not all types of carbon, for example, are the same. Different types have different numbers of neutrons (but the same number of protons) and are called isotopes. Some isotopes of an element are unstable or radioactive and will change or decay into a different element, after a certain amount of time. In some stars which are very hot, the nuclear reactions happen so quickly that unstable isotopes will react with other isotopes before they have time to decay. Often, these hot stars will explode in spectacular displays of stellar fireworks, such as novae and supernovae. So to understand these exploding stars we need to be able to study the nuclear reactions with unstable isotopes that play a role. Astronomers can study these exploding stars by looking at the light that shines from them. From this light they can tell what elements were produced in the explosion and this gives nuclear physicists information on which nuclear reactions could be important. When certain unstable isotopes are produced, they decay and give off a particular type of light at very high energies called gamma rays. If these gamma rays are seen, then we know, not only that a particular element was produced, but also that it was a particular isotope and by knowing how far away the star is, we also know how much was produced. Scientists can then compare these observations with the predictions of computer models to see if we understand how these exploding stars work. These models need information on how quickly these unstable isotopes are created and destroyed by nuclear reactions and that is where the nuclear physics comes in. In the last few years, advances in technology have allowed scientists to accelerate these short-lived unstable istopes so that they can be used to study these reactions. Laboratories have been built to provide such unstable beams for studies and new laboratories are being developed that can produce more variety of unstable beams and higher intensities. One such laboratory is at TRIUMF in Vancouver, in Canada and is called ISAC. The proposed research programme will use the beams available at ISAC to study two of these special isotopes, 18F and 18Ne. We will study how quickly these isotopes are destroyed in such stellar explosions by different nuclear reactions.