Introduction

Experimental work in positron scattering by atoms and molecules is generally very difficult in comparison with that in electron scattering. Even in the case of a strong radioactive isotope, the intensity of a positron beam is several orders of magnitude lower than that of an electron beam. Since positron scattering experiments started in 1972, many measurements using a weak radioactive isotope (strength typically less than lOOjiCi) have been performed in total cross section (TCS) and inelastic experiments [1]. The merit in the experimental methods using a weak radioactive isotope is that slow positrons emitted from a moderator are effectively used in the measurements by applying a time-of-flight (TOF) technique with low background as described later.

In this article, the experimental studies of positron (or electron) scattering performed by our group using a weak radioactive isotope source are mainly

New Directions in Antimatter Chemistry and Physics, 367-390. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.

described. Although this technique is the traditional one, it is still an important approach even now. The experimental procedure is described, and trial methods for improvements are also indicated on methods of narrowing the energy width and beam transportation from a magnetic field to a non-magnetic field. Most of the previous TCS experimental data for polyatomic molecules have been published in the book by Kimura et al.[2], however, new TCSs data for positron and electron scattering are presented here for several such molecules. Other topics discussed include systematic analyses, the comparative TCS data for positrons and electrons at energies lower than 4.0eV, and at energies higher than 50eV, and the problem of Ps formation near the threshold energy. Especially, detailed analysis is given of the relation of TCS to the polarization interaction between the target gas and the projectile using the comparative method of positron and electron TCS data at energies higher than 50eV. Electronic excitation and ionization cross sections by positron impact on 02 molecules [3] and He, Ne and Ar atoms [4] are described. A novel method for a vibrational excitation experiment using a wide energy width beam is also introduced briefly. Some discussions of future works and experimental plans are presented briefly.

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