Recoil Particles fromPo210and Their Ionization in Argon and Helium

The average energy $W_r$ to make an ion pair for the recoil particles ejected by the disintegration of ${\mathrm{Po}}^{210}$ has been measured in argon and helium by a modification of a very old method. The ratio of this value for the recoil particles to the corresponding $W_{\alpha }$ value for the ${\mathrm{Po}}^{210}$ alpha particles was found to be 4.1 and 4.5 in helium and argon, respectively. The ratio for argon is in good agreement with an earlier determination by Madsen for an argon-air mixture. The maximum radial distance traversed by the recoil particles was found to be 7.7×${10}^{-3\mathrm{}}$ cm and 52×${10}^{-3\mathrm{}}$ cm in argon and helium, respectively, under standard conditions.