Ionization from Fe Atoms Incident on Various Gas Targets

The ionization ratio $\beta$, which is defined here as the ratio of the number of ion-electron pairs $N_I$ formed in a thick gas target to the number of atoms $N_A$ injected into the gas at a given velocity, has been determined for Fe atoms impinging on target gases of He, Ne, Ar, Kr, ${\mathrm{N}}_2$, ${\mathrm{O}}_2$, C${\mathrm{O}}_2$, and air. The Fe atoms are generated in the gas by the vaporization of solid, submicron-diameter iron particles that are injected into the gas at high initial velocities ($15<v<\mathrm{}45\mathrm{}$ km/sec). The solid particles are nearly completely vaporized while suffering small deceleration, and $N_A$ is derived from the measured initial mass of the particle. Atoms evaporated from the particle travel through the gas at approximately the injection velocity of the particle. $N_I$ is determined by means of a parallel-plate ionization chamber. For a single collision, $\beta$ is closely given by the ratio of the ionization cross section ${\sigma }_I$ to the momentum-loss cross section ${\sigma }_D$. In the energy range covered by the experiment, ${\sigma }_D$ is a slowly varying function of energy and may be replaced by a constant multiple of ${\sigma }_G$, the geometrical cross section. If the number of collisions where ionization is energetically possible is small, ${\sigma }_I$ can be estimated from $\beta$. For the particular case of Fe→${\mathrm{N}}_2$ we obtain ${\sigma }_I\approx 0.6$ ${\sigma }_G$ at an Fe-atom velocity of 40 km/sec. This is in qualitative agreement with the value reported by other experimenters using more conventional atomic-beam techniques.