Ionization by Alpha Particles in Mixtures of Gases

The presence of minute impurities greatly increase the ionization produced by Po alpha particles in helium. Systematic studies of this effect show an increase in ionization up to 40 percent for approximately 0.1 percent of argon. Similar results have been obtained with CO₂, Kr, Xe, H₂, N₂, and C₂{H}₄$ as the contaminant. Preliminary experiments with mercury vapor in helium confirm the large effect already reported by others. A similar increase in ionization in pure argon is obtained by the addition of C₂{H}₂$ or ${C}₂H₄. These increases in ionization seem to be caused by the production of ions, when metastable atoms in the parent gas undergo collisions with molecules of the impurity. In mixtures of H₂, N₂, or A with helium, the excess in ionization is observed to decrease by a few percent as the pressure of the gas mixture is increased from 48 to 110 cm of mercury. For no other contaminant gases tested thus far in helium is such a pressure change in ionization observed.