Active target technology development for nuclear astrophysics

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. 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 isotopes 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 will develop new detector technology that will use the unstable beams available at ISAC. The project will explore the use of GEMs (Gas Electron Multipliers) to amplify the very weak signals produced in the detector by the products of the nuclear reactions of these unstable isotopes on helium. The GEMs will need to operate reliably at low pressures and be able to amplify the signal consistently across the detector for an extended period of time. In order to study reactions with a very low probability, the detector will also need to be able to identify the reactions of interest from the large amount of background noise and we will develop some clever hardware and software tricks to do this.

Once operational, the active target detector will be used to study several reactions which are key to our understanding of these exploding stars and so will help to explain where all the elements are created.

There is considerable interest in the performance of GEMs in a variety of applications, from LHC physics to nuclear medicine. The project will allow this detector technology to be developed and the expertise gained to be made available to the wider UK and European user communities. It is hoped that the project will provide a mechanism for closer links with the astrophysics and particle physics communities to identify common goals and challenges. In particular we will be studying the stability of the GEM gain at low pressures, in cylindrical geometry as a function of position (homogeneity across GEM surface), beam intensity (sensitivity to betas and distributed charge) and ion mass (localised space charge from heavy ion Bragg peak). Any developments in e.g. GEM design, pulse shape analysis, etc necessary to deliver the required detector capability would potentially drive the improvement of GEM performance, stability and reliability in other applications.

From a wider perspective, the training provided to researchers benefits UK industry as many of our students go on to work in related fields, such as nuclear medicine, the energy industry, environmental monitoring. Indeed, our highly skilled doctoral researchers are sought after by a wide range of companies, including financial institutions and computer firms. In particular the hands-on experience of gas detectors, electronics, data acquisition systems, radiation safety, and vacuum technology, coupled with the computational skills of data analysis, pulse shape algorithms, FPGAs and programming would give the PDRA appointed a unique set of skills and expertise that would be highly valued by any technology-based company.