Positronium - Matter Interactions

The observed imbalance between matter and antimatter in our universe is one of the greatest mysteries in science. Positrons are the antimatter counterpart to electrons with which they annihilate releasing gamma-rays. In addition to their importance in our fundamental understanding of nature, studies of their interactions with ordinary everyday matter allow us, for example, to investigate crystal structures and to obtain functional images of human organs using the medical technique of positron emission tomography (PET). In many collisions of positrons with matter, positronium (Ps) is formed.

Positronium is a composite made of a positron and electron, analogous to the hydrogen atom with the positron replacing the proton. So often is Ps formed, that e.g. 80% of gamma-rays detected in PET, and 95% of all gamma-rays released from the galactic centre of the Milky Way are the result of Ps decay. Once formed, it has ample time to interact with matter before annihilating because its lifetime, although of the order of nanoseconds, is still millions of times longer than typical scattering times. For these reasons, knowledge of how Ps itself interacts with matter is important not only for collision physics but also, for example, so that we may improve radioprotection in PET, or learn about the environment in which positrons annihilate in outer space.

Since both experimental and theoretical studies with Ps projectiles are difficult, corresponding data are scarce or non-existent. Recently, however, we have discovered that positronium scatters from a wide variety of atomic and molecular targets in a similar manner to that of a bare electron at a given projectile velocity. This is not what was expected! And, whilst the reason is unclear, its ramifications at a fundamental and practical level might be far reaching. It looks like the positron in Ps is heavily screened (or "cloaked") from the target by its partner electron and we wish to understand why. In the process, through the proposed research, we shall charter the outcome of such collisions and glimpse into the formation of Ps resonances and possibly Ps compounds. From a practical viewpoint, the outcome of the project may feed directly into models of how Ps interacts with matter, encompassing interstellar clouds and the human body.

The outcomes of the proposed studies are expected to impact primarily on the academic sector through the supply of unique data and to aid the understanding of atomic interactions involving antimatter. However, it is envisaged that our results will also serve as an input, for example, into simulations of Ps collisions with matter including industrially important materials and biological tissue.

Thus potential beneficiaries of the proposed research outside the academic sector may include those working, for example, in the materials and nuclear medicine industries, and of course their customers and patients. In particular for Positron Emission Tomography, an improved understanding of the microscopic distribution of the energy deposits along the positron tracks may result in improvement of imaging-resolution and more accurate radiation dosimetry. In the longer term, an advance of this type, leading to the development of better protocols for radioprotection and cancer radiotherapy, could benefit public health and hence society.

Economic benefits are also expected to arise for the public and private sectors through the training of talented young scientists in performing highly-skilled research and developing concomitant transferrable skills (e.g. numerical and computational, problem-solving, creative, communication, leadership) which are highly valued by businesses and organizations. Past group members have found employment in national and international laboratories, government departments, software development, scientific consulting and the finance sector, as well as in academia.

Research with antimatter continues to excite the imagination of the general public and mass media (from the matter-antimatter asymmetry to the recent discovery of antimatter-production in thunderstorms on Earth). It thus represents a powerful vehicle for stimulating further interest and promoting understanding in physics and science, especially among young people. In the framework of the departmental "Science for the Public" initiatives, we shall endeavour to promote public awareness and engagement with our research by delivering popular lectures, talks at schools and interested societies and through popular publications (e.g. last November our work featured in New Scientist and Physics World).