Many-body theory of positron interactions with atoms and molecules

Lead Research Organisation: Queen's University of Belfast
Department Name: Sch of Mathematics and Physics

Abstract

Positrons are the antiparticles of electrons. They are produced in abundance in our Galaxy, and are readily obtained on Earth using accelerators or radioactive isotopes. When positrons come into contact with their matter counterparts, the pair annihilate in a pyrotechnic flash, releasing all their energy as pure light. This emitted light is detectable, and is strongly characterised by the environment the electron was in immediately prior to annihilation, making positrons a unique probe. As such, they have important use in medical imaging in PET (Positron Emission Tomography) scans, diagnostics of industrially important materials, and understanding the distribution of antimatter in the Universe.

When low-energy positrons interact with normal matter, such as atoms, they pull strongly on the electrons and may even cause one of the electrons to `dance' around the positron, forming so-called positronium (as the positron and electron may annihilate, this may ultimately be a `dance to the death'). Such effects are known collectively as `correlations'. Correlations have a very strong effect on positron collisions with atoms and molecules. In particular, they can enhance the rate of positron annihilation by many orders of magnitude. They also make the accurate description of the positron-atom system a challenging theoretical problem. Proper interpretation of material science experiments, however, rely heavily on calculations that must fully account for the correlations. For example, to accurately interpret Positron Induced Auger Electron Spectroscopy, a powerful technique used to study defects and corrosion in materials, one requires the exact relative probabilities of annihilation with core electrons of various atoms. Moreover, accurate description of the positron-molecule system is required to help explain the origin of the strong annihilation signal from the galactic centre; to develop new spectroscopic PET-scanning methods for medical imaging, drug development and industrial diagnostics; and to advance antimatter-matter chemistry. Crucial in all cases is the ability of theory to accurately calculate the response of the atomic and molecular structure to the positron.

A powerful method of describing the positron-atom or molecule system, which allows for the study and inclusion of correlations in a natural, transparent and systematic way, is many-body theory. In this method, complicated mathematical expressions that describe processes of interest, e.g., positron annihilation with an atomic electron, are replaced by series of relatively simple and intuitive diagrams, each of which represents a distinct correlation process. This programme of research proposes to develop new state-of-the-art diagrammatic many-body theory, and recently emerged revolutionary computational methods for high-precision calculations of positron annihilation with individual electrons in complex atoms and molecules. These computational methods will allow for the summation of millions of diagrams, completely unfeasible using the best existing brute-force methods, providing a powerful framework that can yield precision calculations in addition to keen insight. Moreover, the application of the methods will naturally extend to other important atomic and molecular properties and processes, required for tests of fundamental physics and development of quantum technologies.

The unique and unrivalled calculational capability that this programme will develop will enable the most accurate interpretation of industrially important materials science experiments at recently launched international facilities; help provide fundamental insights into antimatter in the Galaxy; explain existing experimental results that remain crying out for theoretical explanation; advance Positron Emission Tomography technology and antimatter-matter chemistry; and overall, illuminate this intricate dance of matter and antimatter.

Planned Impact

This programme of research will develop state-of-the-art theoretical and revolutionary computational methods for high-precision calculations of positron (antimatter) interactions and annihilation with atoms and molecules. In addition to their importance for fundamental physics, such calculations are essential to develop industrially important material science techniques and to develop PET (Positron Emission Tomography), important in medical imaging, drug development and diagnostics of flow processes in industrial plants.

Although the immediate focus is on development of fundamental science and knowledge generation and exchange between fundamental researchers, in addition to the numerous academic beneficiaries, my programme of research promises strong positive short and long term impacts on the UK economy and society:

It has been agreed with the host institution that as part of the programme I will co-supervise a PhD student. The student will be exposed to a wide range of important theoretical and computational techniques, whose application span numerous subfields of theoretical physics. This will encourage a new pipeline of researchers in antimatter physics, essential to ensure the UK stays at the international forefront of the field. The student will also gain high-level IT skills, and important experience in problem solving, team-working, data analysis, project planning etc, all desirable and transferable to diverse career pathways and essential to the future economy of the UK.

In the short term, the ability to calculate positron annihilation with core electrons of atoms across the periodic table, which is a key objective of the programme, promises to enable the most accurate interpretation of industrially important material science experiments. The research will benefit those who use such techniques, allowing them to improve their diagnostics and to steer and develop new positron-based material science technologies. Advancing the development of new materials has the potential to positively impact on the UK economy and society, by furthering fundamental science, directly improving the quality of life of UK citizens owing to the exploitation of the new materials, and attracting international investment (for research and through commercialisation) that will contribute to the UK economy.

The ability to perform accurate calculations of positron annihilation on molecules is crucial to develop new methods of PET (Positron Emission Tomography). This imaging technique has important use in medical imaging, drug development and also for diagnostics of flows in industrial plants. By extending current PET technology to avail of the detailed information contained in the light signal emitted in an annihilation event, new spectroscopic PET technologies can be established and developed. These methods could, e.g., enable important non-invasive diagnostics of human tissue, to provide improved pathology studies and clinical management of patients. This, coupled with the advances in matter-antimatter chemistry the project will develop, will also help advance drug development. Developments in such areas have obvious benefits to quality of life, and have the potential to attract international investment that will contribute to the UK economy.

I am passionate about communication of science through outreach and public engagement, and I will undertake a strong and varied programme of such activities throughout the duration of the Fellowship. These activities include developing freely available and accessible podcasts on my research, giving a lay talk at the `Belfast Science Cafe', and developing an antimatter-themed educational installation at the award winning `Belfast W5 - science and discovery centre'. Overall, these activities will increase the general public's understanding and awareness of antimatter, will inspire the next generation of UK scientists, and will reach out to and inform those who wouldn't usually seek out science.

Publications

10 25 50
 
Description Positron interaction with a many-electron atoms is a very challenging theoretical problem, due to the role played by the correlated motion of the positron and atomic electrons. Such correlations have an overwhelming effect on the positron-atom scattering and annihilation, producing orders-of-magnitude enhancements of the relevant characteristics. The approach I have developed has enabled for the first time to fully account for these effects and predict the scattering cross sections, annihilation rates and spectra, and positron thermalisation rates in complete agreement with experimental data for noble-gas atoms, that remained poorly understood for decades. For example, the work has enabled the first simultaneous probing of the energy dependence of the positron scattering cross sections, annihilation rates and annihilation gamma spectra, has yielded the best description of long-standing experimental results to date, and has resolved outstanding "puzzles" in the field: e.g., establishing that the significant discrepancy between gas-cell and trap-based measurements of the "thermal" positron annihilation rate in Xe is a result of the rapid annihilation of low-energy positrons leading to a non-Maxwellian quasi-steady-state positron momentum distribution. This understanding and the methods I have developed have important implications for the much more difficult problem of positron annihilation in molecules, and for numerous applications of positron annihilation in condensed-matter spectroscopies and positron emission tomography (PET). The success in solving the positron-atom problem has also opened the prospect of developing a similarly accurate theory for the interaction of positronium (an electron-positron bound complex) with many-electron atoms. Indeed, in a recent groundbreaking work I demonstrated the power of this approach by performing the first accurate calculations of pickoff annihilation rates (where the positron annihilates with an atomic electron) for positronium collisions on helium and neon.
Exploitation Route 1) The fundamental understanding of the role of correlations in the positron-atom system, and the calculation of enhancement factors that parametrize these can enable the most accurate interpretation of positron-based atomic and condensed matter experiments, including materials science techniques such as positron annihilation induced Auger electron spectroscopy. The insight provided will also inform other sophisticated theoretical and computational approaches to the problem, by enabling them to include the dominant interactions at play. (2) The new fundamental understanding of positron cooling in atomic and molecular gases is expected to help to enable the development of more efficient positron traps and accumulators (required for e.g., the antihydrogen experiments at CERN) and for next generation high-energy-resolution cryogenic positron beams (required for positron-based vibrational spectroscopy of molecules). (3) The new many-body theory of positronium interactions with atoms that has been developed enables the acceleration of the understanding of this difficult system, and provides a platform from which important applications such as PET (positron emission tomography) can be developed by exploiting the understanding of the fundamental interactions that will be acquired.

Following the success of the Postdoctoral Fellowship, I will build on this research through a European Research Council Starting Grant (2019--2024).
Sectors Healthcare,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology

 
Description On award of the Fellowship the host institution (Queen's University Belfast) provided additional support for my project in the form of a PhD student (who began work in Oct 2016) and three MSci students (one in 2015/2016 academic year, who is now undertaking PhD study at a leading UK University, and two in 2016/2017: all received first class marks). The students have developed working knowledge of a broad range of topics in theoretical physics, and the PhD student has recently presented their work at a national conference and has a number of papers in preparation. In addition to their academic development, they have gained problem-solving, team-working, programming and project-planning skills, all desirable and transferrable to diverse career pathways and essential to the future of the UK economy. Economy and society: The Fellowship has enabled me to continue to be active in science outreach. I presented my research to members of the general public at the Northern Ireland Science Festival, and attended the annual BT Young Scientist exhibition (2017 and 2019) in Dublin as a representative for the Institute of Physics. Holding the EPSRC Fellowship enabled me to join the Institute of Ireland Committee, and the Institute of Physics UK Atomic and Molecular Interactions group (AMIG) committee. In these roles I have contributed to production of policy documents (e.g., Institute of Physics Programme for Government in Northern Ireland). I was elected treasurer to IOP Ireland in 2018, and manage a budget of >£100k that covers the gamut of IOP Ireland's activities.
First Year Of Impact 2019
Impact Types Societal,Economic,Policy & public services

 
Description (ANTI-ATOM) - Many-body theory of antimatter interactions with atoms, molecules and condensed matter
Amount € 1,318,419 (EUR)
Funding ID 804383 
Organisation European Commission 
Sector Public
Country European Union (EU)
Start 02/2019 
End 01/2024
 
Title Computer software: "ANTICOOL: Simulating positron cooling and annihilation in atomic gases", a Fortran program. 
Description "ANTICOOL: Simulating positron cooling and annihilation in atomic gases" is a Fortran computer package for ab initio calculation of lifetime spectra, time-varying annihilation rates and positron annihilation gamma spectra in atomic gases via Monte Carlo simulation based on quantum mechanical cross sections. The source is hosted on the Computer Physics Communications Library. 
Type Of Technology Software 
Year Produced 2017 
Open Source License? Yes  
Impact The computer code ANTICOOL, developed in 2017, has already been used to produce the results of two papers [Phys. Rev. Lett. 119, 203404 (2017) and Phys. Rev. Lett. 119, 203404 (2017)]. The source code is due to be made freely available on the Computer Physics Communications library. 
URL https://www.sciencedirect.com/science/article/pii/S0010465515000910
 
Description Presentation at the 2016 Northern Ireland Science Festival's "Tomorrow's future today" event, Ulster Museum, Belfast. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Ran a dedicated stand at the Northern Ireland Science Festival's "Tomorrow's future today" event, Ulster Museum, Belfast, that included a large poster explaining my research on positron and antimatter interactions with atoms and molecules. Discussed the concepts of my work and the subject with members of the general public over the course of 1 week during the hugely successful and well attended Northern Ireland Science Festival. This undoutedly raised awareness of antimatter research in medical imaging, characterising industrially important materials, and also fundamental research. Overall, it was a fantastic opportunity to engage with the general public and demystified antimatter.
Year(s) Of Engagement Activity 2016