Positron atomic physics – A look to the future

This article is a brief overview of opportunities and challenges in the area of low-energy positron and positronium atomic physics. The ideas presented here come from discussion during the final, summary session of the XIII International Workshop on Low Energy Positron and Positronium Physics, Campinas Brazil, July 2005. 2006 Elsevier B.V. All rights reserved. As is the custom, the final session of the workshop was devoted to a group discussion regarding the challenges and opportunities in the area of low-energy positron and positronium physics. The session began with brief presentations by Michael Bromley (San Diego), Mike Charlton (Swansea), Gleb Gribakin (Belfast) and Nella Laricchia (London) on their views of the future and continued with a group discussion. The following is a brief overview of key points raised by the group. A more detailed report of the workshop, including this summary session, will be published elsewhere [1]. 1. Positron sources, technology and facilities Presently, most positron atomic physics experiments are done using Na sources. Typical Na-based beams using solid neon-moderators (i.e. the most efficient moderators available) have fluxes in the range from 10 to 10 positrons per second. For the past few years, almost all of these sources have come from one supplier. It was pointed out that the field would benefit by the availability of alternative means to obtain positron sources. On the world scene, there are a few intense positronbeam facilities. Examples in Europe are the reactor-based beam at the University of Delft, and a new facility at the new reactor in Munich. Both operate with positron fluxes 0168-583X/$ see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.01.030 * Tel.: +1 858 534 6880; fax: +1 858 534 6574. E-mail address: [email protected] in excess of 10 s . Generally, the positron atomic physics community has not as yet exploited these resources that, at present, are used predominantly for condensed matter and materials science studies. There is a general sense that the opportunities presented by the availability of these intense beams should be more fully utilized for positron atomic physics. In terms of new facilities, there are experiments in progress to make a more intense source using small accelerators and N (Washington State and U.C. Riverside). It was suggested that a long-term goal for intense positron facilities should be the establishment of beam lines made as ‘‘turn-key’’ as possible to facilitate a broad range of users and experiments. One important direction for an intense positron facility would be the development of a Ps beam line. The current method of choice for forming Ps beams involves the relatively inefficient process ( 10 3 Ps atoms per positron) of charge exchange in a gas cell, and thus Ps experiments would benefit greatly by the availability of a much more intense positron-beam. The field has benefited by the recent development of cold, trap-based positron-beams. This technique produces a magnetized beam (i.e. in fields of strength 0.01–0.1 T; energy resolution 20 meV). While this technique is superior to electrostatic beams, for example, for measurement of integral cross sections, it has disadvantages for other applications such as measurement of small cross sections and differential inelastic cross sections. A question was raised as to whether high-brightness electrostatic beams with similar energy resolution could 2 C.M. Surko / Nucl. Instr. and Meth. in Phys. Res. B 247 (2006) 1–4 be developed. This does not appear to be out of the question using brightness-enhancing techniques currently under development. In this same vein, if such an electrostatic beam were available; it could, for example, be used with the COLTRIMS technique to measure multiple final-state projectiles resulting from a collision, and the ‘‘magnetic angle changer’’ technique to study scattering over a wide range of angles. Both of these latter techniques have made tremendous contributions in the conventional areas of electron and ion scattering. The group was enthusiastic about the new positron atomic physics and materials science facility currently being established at the Australian National University in Canberra. As currently planned, the facility will have a magnetically guided, trap-based beam, using a Na/neon-moderator positron source. The development of colder positron-beams (e.g. resolution 1 meV, FWHM), now in progress, would be a welcome development. Applications include more precise studies of threshold phenomena and searches for narrow resonances. In a similar vein, techniques to produce larger, denser and colder positron plasmas would facilitate increased production of low-energy antihydrogen and study of the Ps2 molecule, BEC Ps and electron–positron plasmas. 2. Positron and positronium interactions – experiment There are many areas where further and/or more refined experiments are needed. Examples include positron-impact ionization (including positronium formation), which would benefit from study of the differential cross sections, for example, to provide more stringent tests of theory. Careful studies of annihilation and Ps formation up to and through the threshold would be of interest to test the precise theoretical predictions that are now available. There is a discrepancy between prediction and experiment for the peak in the positronium formation cross section in helium that should be further investigated. The formation of Ps on inner-shell orbitals has been the subject of considerable speculation and requires definitive experimental study. In the area of electronic excitation of molecules, sharp, near-threshold increases in the cross sections for excitation of the lowest-lying electronic states have been observed in N2 and CO. The origin of these features is not understood and currently the subject of considerable theoretical interest. There are also significant discrepancies between theory and experiment in the positron analogue of the ‘‘e ! 2e’’ problem (i.e. correlation of final-state particles in direct ionization). These discrepancies and more refined electron–positron correlation experiments should be pursued. Comparative studies of positronand electron-impact cross sections, such as those planned for the new Canberra positron facility, are likely to be very insightful. While simple targets are of great interest, study of more complex systems would also be of value. Examples include the noble gas atoms, and related molecular sequences such as CH4, CF4, CCl4, CBr4. This would, for example, aid in the development and testing of theories, including their ability to describe strong electron–positron correlation effects in scattering, annihilation and positron binding. Such studies would also provide information about positron interaction with the vibrational degrees of freedom and their effect in enhancing annihilation rates. The area of Ps scattering with atoms and molecules has been much less well explored. There are many open issues in this area, such as the details of inelastic collisions, Psbreakup, target ionization and doubly inelastic processes.