New Directions in Antimatter Chemistry and Physics

508 Pages·2002·26.81 MB·English

by  Franco A. Gianturco (eds.), B. M. Jelenković, J. J. Bollinger, A. B. Newbury, T. B. Mitchell, W. M. Itano (auth.)

NEW DIRECTIONS IN ANTIMATTER CHEMISTRY AND PHYSICS
New Directions in Antimatter
Chemistry and Physics
Edited by
Clifford M. Surko
Professor of Physics,
Physics Department,
University of California, San Diego,
U.S.A.
and
Franco A. Gianturco
Professor of Chemical Physics,
Dipartimento di Chimica,
Università di Roma “La Sapienza ”,
Rome, Italy
KLUWER ACADEMIC PUBLISHERS
NEW YORK, BOSTON, DORDRECHT, LONDON, MOSCOW
eBook ISBN: 0-306-47613-4
Print ISBN: 0-7923-7152-6
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Contents
Preface ix
I. Positron Sources and Beams
1. A laser-cooled positron plasma
B. M. Jelenkovic, J. J. Bellinger, A. B. Newbury,
T. B. Mitchell and W. M. Itano 1
2. Trap-based positron beams
R. G. Greaves and C. M. Surko 21
3. Intense radioisotope sources for spin polarized positron beams
F. Saito, T. Hyodo, Y. Nagashima, T. Kurihara, N. Suzuki
Y. Itoh, and A. Goto 35
II. Antihydrogen, Bose-Condensed Positronium, and Positrons
in Materials
4. Collisions of H and
Alexander Dalgarno, Piotr Froelich, Svante Jonsell,
Alejandro Saenz, and Bernard Zygelman 47
5. Positron physics in a new perspective: Low-energy
antihydrogen scattering by simple atoms and molecules
E. A. G. Armour and C. W. Chamberlain 53
6. The Bose-Einstein condensation of positronium in submicron
cavities
D. B. Cassidy and J. A. Golovchenko 83
7. Cooling and quenching of positronium in porous material
Haruo Saito and Toshio Hyodo 101
8. New experiments with bright positron and positronium beams
A. P. Mills, Jr. and P. M. Platzman 115
9. Positron states in materials: density functional and
quantum monte carlo studies
Bernardo Barbiellini 127
vi
10. Depth-profiled positron annihilation spectroscopy of
thin insulating films
D. W. Gidley, K. G. Lynn, M. P. Petkov, M. H. Weber
J. N. Sun, and A. F. Yee 151
III. Positron and Positronium Interactions with Atoms
11. The scattering of positrons and positronium by atomic
targets
H. R. J. Walters, Jennifer E. Blackwood and Mary T.
McAlinden 173
12. Positronic atoms
J. Mitroy, M. W. J. Bromley and G. G. Ryzhikh 199
13. Perspectives on physics with low energy positrons:
fundamentals, beams and scattering
Michael Charlton 223
14. Positron chemistry by quantum monte carlo
Massimo Mella, Simone Chiesa, Dario Bressanini, and
Gabriele Morosi 235
15. Antimatter compounds
D. M. Schrader and J. Moxom 263
16. Positronium-atom/molecule interactions: momentum-transfer
cross sections and
Y. Nagashima, F. Saito, N. Shinohara, and T. Hyodo 291
17. Correlations between cross sections and threshold energies
for positronium formation and direct ionization
J. W. Humberston, P. Van Reeth and G. Laricchia 303
IV. Positron and Positronium Interactions with Molecules
18. Scattering of positronium atom off atomic hydrogen and
helium targets
A. S. Ghosh and Prabel K. Sinha 323
19. Atomic and molecular physics using positron traps and
trap-based beams
C. M. Surko 345
Contents vii
20. Experimental studies of positron scattering using a weak
radioactive isotope source
O. Sueoka 367
21. Future opportunities in positron-atom (molecule) scattering
Stephen J. Buckman 391
22. Theory of positron annihilation on molecules
Gleb Gribakin 413
23. Bound states of positron with molecules
M. Tachikawa, I. Shimamura, R. J. Buenker and
M. Kimura 437
24. Low-energy positron dynamics in polyatomic gases
F. A. Gianturco, T. Mukherjee, T. Nishimura and
A. Occhigrossi 451
25. A test calculation on of model potentials for correlation
and polarization effects in positron scattering from molecules
Robert R. Lucchese, F. A. Gianturco, P. Nichols, and
Thomas L. Gibson 475
26. On the contribution of polarization-correlation forces to
high annihilation rates in positron-molecule collisions
Márcio T. do N. Varella, Claudia R. C. de Carvalho
and Marco A. P. Lima 493
Author Index 503
Index 505
Preface
This volume is the outgrowth of a workshop held in October, 2000 at the
Institute for Theoretical Atomic and Molecular Physics at the Harvard-
Smithsonian Center for Astrophysics in Cambridge, MA. The aim of this
book (similar in theme to the workshop) is to present an overview of new
directions in antimatter physics and chemistry research. The emphasis is on
positron and positronium interactions both with themselves and with
ordinary matter. The timeliness of this subject comes from several
considerations. New concepts for intense positron sources and the
development of positron accumulators and trap-based positron beams
provide qualitatively new experimental capabilities. On the theoretical side,
the ability to model complex systems and complex processes has increased
dramatically in recent years, due in part to progress in computational
physics. There are presently an intriguing variety of phenomena that await
theoretical explanation. It is virtually assured that the new experimental
capabilities in this area will lead to a rapid expansion of this list.
This book is organized into four sections: The first section discusses
potential new experimental capabilities and the uses and the progress that
might be made with them. The second section discusses topics involving
antihydrogen and many-body phenomena, including Bose condensation of
positronium atoms and positron interactions with materials. The final two
sections treat a range of topics involving positron and positronium
interactions with atoms and molecules.
In the area of experimental capabilities, positron physics has historically
been hindered severely by the lack of intense, cold and bright positron
sources. One article in the first section presents a new design for an intense
source. Other articles in the same section describe new developments in the
use of Penning traps to create ultra-cold and intense, pulsed and continuous
positron beams. These developments present qualitatively new
opportunities to study a range of phenomena ranging from fundamental
atomic and molecular physics to the characterization of materials and
material surfaces.
The articles in Section II on antihydrogen speak for themselves. There are
presently two experimental efforts at CERN to create and trap antihydrogen
atoms. If successful, this will represent the first stable antimatter in the
laboratory. It is quite likely that these efforts will blossom into several
important, long-term research directions. The two antihydrogen articles
discuss fundamental processes involving the interaction of these antiatoms
with ordinary matter. Not only is this an important theoretical question, but
it also has potentially important consequences for the development of
ix
x
practical schemes to cool antihydrogen sufficiently quickly to be able to trap
the atoms in present-day magnetic traps before they are lost to annihilation.
Another article in this section describes an experiment designed to create
Bose-condensed positronium. This is a very ambitious project, but one that
may now be feasible due, in part, to recent advances in positron traps and
beams. Even in lieu of the ultimate goal of producing Bose condensation,
this experimental research direction is likely to lead to other fascinating new
possibilities. These topics fall under the general category of positron-matter
interactions at high densities (e.g., the limit in which the de Broglie
wavelengths of the particles becomes comparable to the interparticle
spacing). Important questions in this regime include the formation of
molecules and the phase diagram of the correlated electron-positron gas in
the quantum regime (e.g., BEC positronium represents one phase in this
diagram). Other articles in Section II discuss the theoretical and practical
aspects of positron interactions in materials.
The last two sections in this volume describe positron interactions with
atoms and molecules. While this is a subject with a long history, a
surprisingly large number of fundamental questions remain to be
understood. This is due in large part to the fact that the experimental tools
that have been commonplace in studying ion, electron, photon, and neutral
atom interactions are only now becoming available to study the analogous
positron and positronium interactions. Many of the problems in this area
relate to low-energy interactions of positrons with matter and can be
considered to be fundamental to the establishment of a quantitative
antimatter-matter chemistry.
Section III focuses on phenomena involving atoms. One important question
regards the existence and nature of positron-atom bound states. Described
in this section are accurate calculations of the ground state energy levels of
these complexes. These calculations leave little question that such states do
exist. Thus the challenge in this area is now in the experimentalist's camp.
Other phenomena of interest include understanding the details of
positronium formation in positron-atom collisions and a variety of other
positron and positronium scattering processes, including the application of
recently developed experimental tools to study electronic excitations and
resonances in atoms.
Section IV focuses on positron and positronium interactions with molecules.
Presently there is keen interest in this area, driven by the recent advances in
both theory and experiment. Newly developed experimental techniques
have permitted the study of low-energy annihilation processes further
illuminating experimental findings, dating back three decades, that modest
changes in the chemical structure of molecules can result in orders of
Preface xi
magnitude changes in the annihilation rate. While far from understood, this
and related phenomena have recently been analyzed by a number of
theoretical groups. Several possible mechanisms have been proposed and
are under active discussion. The quantitative predictions that are now
becoming available will undoubtedly stimulate a new round of experiments,
some of which are outlined in the experimental articles in this section.
Other topics of interest include understanding the dynamics of the post-
annihilation system including molecular fragmentation and the distribution
of final states.
The situation is more or less the reverse in the area of low-energy positron
scattering, where there have been a range of predictions for phenomena
involving molecules that remain still to be tested. New experimental
techniques appear to be on the verge of being able to study these aspects of
our understanding. This topic of positron-molecule scattering also relates
directly to the question of positron annihilation on molecules in the sense
that low energy scattering experiments can, in principle, measure the zero-
energy scattering length. In this way, one can obtain a quantitative measure
of the the low-energy positron-molecule interaction that, at least in some
theories, is predicted to give rise to positron bound states and virtual
resonances.
This volume contains many stimulating ideas that are likely to inspire new
research efforts into the chemistry and the physics of low-energy antimatter
and matter-antimatter interactions. It also presents an up-to-date picture of
the scientific landscape as viewed by the international community of
physicists and chemists, both experimentalists and theoreticians, who are
tackling the broad range of problems in this area. In closing, we wish to
thank the people responsible for this new look at the field. We are indebted
to Kate Kirby and members and staff of the Institute for Theoretical Atomic
and Molecular Physics at the Harvard-Smithsonian Center for Astrophysics
for hosting the workshop at which this project began to take shape. We
thank both the authors and reviewers of the articles in this volume for their
cooperation. Finally we thank Ms. Judy Winstead for her generous
involvement and the substantial amount of work required to successfully
merge the contributions from over twenty authors into the final manuscript
that produced this volume.
Cliff Surko
La Jolla
Franco Gianturco
Rome
Chapter 1
A LASER-COOLED POSITRON PLASMA
†
J. J. Bollinger, A. B. Newbury ,
T. B. Mitchell‡, and W. M. Itano
§
Time and Frequency Division, National Institute of Standards and Technology
Boulder, CO
Abstract We present results on trapping and cooling of positrons in a Penning
trap. Positrons from a 2 mCi source travel along the axis of a
6 T magnet and through the trap after which they strike a Cu reflec-
tion moderator crystal. Up to a few thousand positrons are trapped
and lose energy through Coulomb collisions (sympathetic cooling) with
laser-cooled By imaging the laser-induced fluorescence, we
observe centrifugal separation of the ions and positrons, with the
positrons coalescing into a column along the trap axis. This indicates
the positrons have the same rotation frequency and comparable den-
sity as the ions, and places an upper limit of
approximately 5 K on the positron temperature of motion parallel to
the magnetic field. We estimate the number of trapped positrons from
the volume of this column and from the annihilation radiation when the
positrons are ejected from the trap. The measured positron lifetime is
> 8 days in our room temperature vacuum of Pa.
1. INTRODUCTION
This paper presents experimental results on the capture, storage and
cooling of positrons in a Penning trap that simultaneously contains laser-
cooled ions. The experimental work follows previous discussions
and simulations of trapping and sympathetic cooling of positrons via
Coulomb collisions with cold ions [1,2]. Cold positron plasmas are
useful as a source for cold beams of high brightness [3, 4, 5, 6]. Many
of the chapters in this volume, for example the chapters by Greaves
* Also at Institute of Physics, University of Belgrade, Yugoslavia.
†
Permanent address:Ball Aerospace, Boulder, CO 80301
‡
Permanent address: Dept. Phys. and Astron., Univ. of Delaware, Newark, DE 19716
§
Contribution of the National Institute of Standards and Technology. Not subject to U.S.
copyright.
1
C.M. Surko and F.A. Gianturco (eds.),
New Directions in Antimatter Chemistry and Physics, 1–20 .
© 2001 Kluwer Academic Publishers. Printed in the Netherlands.