S. Berko

Positron annihilation in high critical temperature superconductors

Abstract Two dimensional angular correlation of electron-positron annihilation radiation (2D ACAR) experiments are discussed in terms of calculated positron wavefunctions and ACAR spectra for high critical temperature superconductors. It is emphasized that positron wavefunction effects are larger in these materials than, for example, the previous class of A15 superconductors. Calculations indicate that the chain derived ridge section and the S centered hole pocket may be observable in YBa 2 Cu 3 O 7 , in agreement with recent experiments. The plane derived Fermi surfaces are expected to be extremely difficult to observe in this material. On the other hand, calculations show that positrons are a good probe of the Cu-O layers in Tl 2 CaCu 2 O 2 .

Positron and Positronium Momentum Spectroscopy in Solids: An Overview

After a brief introduction to the experimental techniques used in positron annihilation spectroscopy and an outline of the underlying theory, we describe the applications of the angular correlation of annihilation radiation method to the study of the momentum density of electrons in solids, and in particular of the Fermi surfaces of some pure metals and alloys. The use of low energy positron beams for surface studies will also be briefly discussed, with emphasis on the newly developed angle-resolved positronium emission spectroscopy.

Fermi surface studies of non-dilute disordered alloys by positron experiments

The technique of positron annihilation experiments as applied to the study of Fermi surfaces in metals and alloys is briefly reviewed. The angular correlation of the two annihilation photons is directly related to the momentum distribution of the positron-electron system; breaks in this distribution reflect the size and shape of the Fermi surface. Recent two-dimensional angular correlation measurements designed to study the Fermi surface of Cu−Al and Cu−Zn alloys are presented, and are compared with theoretical predictions.

A positronium beam and positronium reflection from LiF

At the Brookhaven National Laboratory we have constructed a positronium (Ps) beam by transmitting monoenergetic, low energy positrons through a gas cell containing either Ar or He which provide an electron to form positronium. A description of the positron beam and of the Ps formation mechanisms are found in these Proceedings (see M. Weber, et al. and B. L. Brown). The positrons were obtained by magnetically deflecting positrons in the straight section of the positron beamline (see Fig. 1) into a beamline which contained the gas cell and a Ps detection chamber. By having two beamlines we are able to switch from an experiment which uses positrons (a study of the angular correlation of annihilating radiation--ACAR) to one which uses Ps atoms without breaking vacuum, nor moving equipment. This, however, put a constraint on the placement of the Ps beamline because it could not interrupt the annihilation gamma ray in its long flight from the target chamber to a gamma ray position imaging detector (Anger camera). At present this constraint has resulted in a degradation of the positron beam intensity and energy resolution in the Ps beamline. Efforts are presently underway to eliminate this problem.

Review of Precision Positronium Experiments

In the years since its discovery by Martin Deutsch, [1] positronium has been studied by many workers not only because of the intrinsic interest generated by this exotic atom containing antimatter, but also because in some ways positronium being the lightest of all atoms is also in principle the simplest to interpret theoretically. The measurement of the structure of simple atoms provides one of the best opportunities for testing the predictions of quantum electrodynamics, the theory which most accurately accounts for the interactions of electrically charged particles and electromagnetic radiation. For example, atomic hydrogen is the simplest nuclear atom, and its ground state hyperfine splitting is one of the most accurately measured quantities [2]. It is unfortunate, however, that theoretical uncertainties stemming from the finite size, mass, and polarizability of the proton have so far precluded a meaningful comparison with the measurement to better than a few parts per million. In an attempt to be free of such nuclear effects, experimenters have persued the study of the non-nuclear atom positronium which is describable to a very good approximation using only the electron, positron, and photon fields.