An experimental investigation of the mechanism of scintillation of helium, helium-argon, and helium-neon mixtures excited by $\alpha$ particles has been performed. No detectable decrease in light yield was observed at pressures less than 3 atm when the applied electric field was increased in steps to $\frac{E}{p}\sim 1.0$ V/cm Torr, where $E$ is the electric field and $p$ is the gas pressure. The pulse shape of helium scintillation light consists of a slow component and a spike appearing on the leading edge of the pulse. The main component of light intensity is represented by the formula $\exp (-\frac{t}{{\tau }_m})-\exp (-\frac{t}{{\tau }_f})$, where $t$ is the time in sec, ${\tau }_m=10\mathrm{}\times {10}^{-6\mathrm{}}$ sec, and ${\tau }_f=0.3\mathrm{}{p}^{-2.2\mathrm{}\pm 0.3}$ sec ($p$ in Torr). Most of the emitted photons had a wavelength of less than 1050 ÅA. On the basis of these results, it is concluded that the emitted photons are released in the decay of excited helium molecules formed as a result of a three-body collision between a metastable and two ground-state helium atoms. A characteristic large drop in the light yield for a mixture of a small proportion of argon in a major fraction of helium can be explained by the production of argon ions by metastable helium atoms (Penning process). The same characteristic large drop for a mixture of small concentration of neon in helium seems to be caused by the excitation transfer from helium to neon atoms. The cross section ${\sigma }_f$ for the formation of an excited helium molecule by a three-body collision is estimated to be $50\times {10}^{-23\mathrm{}}p$ ${\mathrm{cm}}^2$ at 300°K, where $p$ is in Torr. From the value of ${\sigma }_f$, the cross section ${\sigma }_i$ of the helium-argon Penning process is calculated as 41×${10}^{-16\mathrm{}}$ and 14×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ by using the ratios of ${\sigma }_i$ to ${\sigma }_f$ estimated by Jesse and Sadauskis and by Northrop and Gursky, respectively.

• Received 3 April 1967

DOI:https://doi.org/10.1103/PhysRev.165.225