POSITRON BEAM SPECTROSCOPY OF ICE FILMS

The abundance and many unusual properties of water have made it the subject of a large number of experimental investigations over many decades. While on Earth most solid water is crystalline, amorphous solid water is the most abundant phase of water elsewhere in the galaxy. Its physical properties are strongly influenced by growth conditions - for example, water vapour deposited on a very cold surface forms low-density amorphous ice, believed to be a major component of comets, planetary rings, and interstellar clouds. It is also a model system for studying deeply supercooled liquids. Above -138 degrees C amorphous ice transforms to stable crystalline ice, and the reactivity of the ice surface is sensitive to its structure. Consequently, its physical and chemical properties are of considerable interest to physical chemists, astrophysicists, planetary scientists, and cryobiologists. There is still controversy about the fundamental properties of ice - for example, temperature at which it forms a glass, whether crystallization begins in the bulk or at the surface, and the nature of the porosity of deposited ice films. On this last point, pores smaller than 2nm may not be detected if they are isolated rather than interconnected. We plan to provide new insights into as many of these issues as possible. The application of Variable-Energy Positron Annihilation Spectroscopy (VEPAS) to these profoundly interesting systems has been encouraged by pilot measurements which indicated that there may be many new phenomena to be uncovered by this technique.A positron - the anti-particle of the electron - is implanted into a sample with a depth distribution determined by its incident energy and is eventually annihilated by an electron. Doppler broadening of the annihilation radiation line at mc-squared, is caused by the motion of the electrons at the various annihilation sites and is thus associated with each structural feature of the material. Positrons are highly sensitive to open volume point defects in a material, ranging from missing single atoms to small clusters of up to ~20 missing atoms. Further, in ice films the positron-electron bound state positronium can be formed. If there are pores or cavities in the sample of diameters of a few nm or more, positronium atoms can reside in them for periods long enough to allow annihilation into three gamma rays to occur - events which we can detect. The larger or more interconnected the pores, the greater the fraction of Ps decaying into three gammas, providing a sensitive probe of these relatively large open volumes.While other techniques have provided some insights into surface changes and pore properties, it is expected that VEPAS - with its mixture of positron and positronium spectroscopies - will provide a more direct method for the depth-sensitive characterization of atomic structure and of pore evolution for a range of ices grown under a variety of conditions. The technique, although appearing to be perfectly suited to these studies, has not been used to date to probe the structure of ice films, making the proposed measurements entirely novel.

The impact of our new results on these various fields will be realised by direct and indirect communication with researchers in the fields listed above under 'Academic Beneficiaries' - we shall send copies of our published papers directly to the relevant researchers, and present our work at conferences. The PI and the Bath positron group has a good record of communication with researchers in other fields over the past decade, including physicists, chemists and electronics engineers in the UK (at Dundee, Newcastle, King's College London, Manchester, Southampton and Bath), Canada (McMaster University), China (Zhejiang and Wuhan Universities), and Australia (ANU Canberra). The project will provide significant training opportunities to the RA and student who will be involved. These include the technical training associated with use of high vacuum equipment, nuclear instrumentation, particle and radiation detectors, and cryogenic equipment, as well as training in reporting and communicating research. The PI and the Bath positron group not only maintain a high international reputation - which contributes to the scientific standing of the UK - but also has an excellent record in training highly-qualified people who have gone on to make significant contributions to UK science, business, industry and society as a whole. For example, in recent years four graduates of the group are now University physicists, four are employed in high-tech industries in the UK, one joined EPSRC, one has a successful career in the City of London, and one joined the legal profession. The RA will take back his enhanced level of experience and expertise to his home institution, thereby having an impact on its future capabilities. The success of the proposed research will emphasise the utility and effectiveness of positron beam spectroscopy to provide a new probe of ice films. The introduction of this technique will have an impact not only on the field of positron physics but also on ice science, providing a new way to probe the structure and kinetics of these systems. The University of Bath engages widely in outreach activities and organises many events to advance the public understanding of science - for example, many talks in schools and colleges and a series of public lectures aimed at the general public. The PI has participated in such activities, presenting an overview of his work to student groups and to visiting Physics teachers. It is expected that the proposed research would represent an appropriate topic for outreach events. The National HE STEM programme, a three-year HEFCE-funded scheme to increase and widen participation principally in Chemistry, Engineering, Mathematics and Physics and enhance the skills and knowledge base of the workforce in these areas, has as its regional director Dr Laughton of the Bath Department of Physics; it is therefore expected that the work proposed could play a role in this outreach activity. If there are any, admittedly unexpected, results of potential value to industry or business, then the University's Research Development and Support Office exists to help in securing ongoing research funding and assist commercial and public sector organisations access cutting-edge knowledge and expertise at the University of Bath.