The Ionization in Argon and in Air by Single Alpha-Particles as a Function of Their Energy

An extended series of measurements has been made to determine the possible variation of $W$, the energy to make an ion pair in pure argon, with the energy of the ionizing alpha-particle. The ionization in argon relative to the ionization produced by a comparison polonium alpha has been measured for the alpha-emitters ThX, ThC, ThA, RaC′, ThC′. For alpha-particles in this energy region from 5 to 9 Mev, no deviation of $W$ from the average $W$ for the polonium alpha could be found amounting to as much as ½ percent. This is in marked contrast to the alpha-ionization in air from the work of Stetter. More indirect measurements in the region 1-5 Mev indicate no measurable change in $W$ for alpha-particles in argon, while values derived from the Stetter curve for air show a marked increase of $W$ as zero alpha-energy is approached. Evidence from direct measurements is given to show that the energy to make an ion pair in argon for the ${\mathrm{Li}}^{7*}$ ion emitted from the reaction ${\mathrm{B}}^{10}(n,\alpha ){\mathrm{Li}}^{7*}$ is apparently the same as the average for the polonium alpha-particle.