K.F. Canter

Theory versus experiment in low-energy positron diffraction by Cu{111}

Abstract Four experimental spectra measured in a low-energy positron diffraction (LEPD) experiment on Cu{111} are satisfactorily matched by intensity calculations. The calculations were carried out with a computer program developed for LEED (low-energy electron diffraction) but using a potential consisting of negative Coulomb contribution, no exchange term and the correlation correction used normally for electrons. The present experimental data are not refined enough to show that positrons do not feel an exchange potential.

Positronium emission from metal surfaces

Positronium (Ps, a positron–electron bound pair) emission as a result of slow positron (30–1000 eV) bombardment of Cu, Al, Ni, and Ag surfaces has been investigated as a function of incident positron energy, sample temperature, and surface contamination. The fraction of 30 eV positrons that were reemited as Ps at 380 K with <5% surface contamination ranged from 0.43 to 0.47. The emission fraction is found to increase with both increasing surface contamination and incident positron energy. Both thermally activated and nonthermally activated Ps emission is suggested by the data. A one‐step thermal activation model applied to the data yields activation energies of 0.44±0.04, 0.46±0.04 and 0.75±0.05 eV for Al(100), Ag(polycrystalline), and Ni(100), respectively.

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.

Is there a positron mobility edge in gaseous helium

Abstract Positron lifetime spectra are obtained in gaseous 3He and 4He. A delayed peak results at τR, the time required for positrons to slow down to the positron energy threshold ER for localization into a self-trapped state. τR is found to exhibit a temperature, density, and isotopic dependence which suggests that ER is determined by the interaction of the positron with many He atoms at a time.

Experimental investigation of a DC electric field on positron self-trapping in helium gas

Abstract Positron lifetime spectra taken in helium gas at 5.5 K and 129 standard densities are compared with Monte Carlo simulations of Farazdel and Epstein. This comparison establishes that the energy threshold for positron self-trapping and the decay rate of the self-trapped state are not affected by an electric field of 52 V/cm. The comparison also suggests a small change in the positron diffusion prior to the self-trapping.

Experimental investigation of positron self-trapping near the vapour-liquid critical point of hellium-4☆

Abstract Positron self-trapping in helium near the vapor-liquid critical point is investigated using the positron annihilation lifetime technique. A 13% decrease in the slowing-down time for positrons to reach the self-trapped state is observed at the critical temperature 5.190 K relative to the slowing-down time at 5.200 K.