TOTAL CROSS-SECTION MEASUREMENTS FOR POSITRONS AND ELECTRONS SCATTERED BY SODIUM AND POTASSIUM ATOMS

Absolute total-scattering cross sections (Q(T)'s) have been measured for positrons and electrons colliding with sodium and potassium in the 3-102-eV range, using the same apparatus and experimental approach (a beam-transmission technique) for both projectiles. Comparing the present measured Q(T)'s for positron and electron scattering by each separate alkali-metal atom shows that (1) they are very similar in shape and magnitude over the entire energy range investigated, (2) they tend to merge near (and above) the relatively low energy of about 40 eV, and (3) the positron Q(T)'s become higher than the corresponding electron values as the projectile energy is decreased below about 40 eV. These positron and electron Q(T) comparison measurements differ markedly from the situation for room-temperature gases, but are supported by recent theoretical calculations both for the positron and electron Q(T) comparisons and for their respective absolute values.