Relative Yields of Ions Produced by α- Particles in Air and Water Vapour

Abstract

IN continuation¹ of the examination of the radiological properties of water, the yield of ion-pairs in water vapour under the action of α- particles has been compared with the yield in air. A sector-shaped parallel-plate ionization chamber containing either water vapour at 95° C. or air has been exposed to a semi-collimated beam of 5·0-MeV.α- particles derived from a polonium source. A low collecting field (about 50 volts/cm.) is sufficient to produce complete saturation in both gases at pressures of 10–20 cm. of mercury, and several determinations of the relative yield of ions¹ have been made. These have a mean value IH₃o/I_air =1·139 ± 0· 006. Making allowance for possible systematic errors, the corresponding ratio for the energy expenditure per ion pair is taken to be the reciprocal of this figure, $W_{{\text{H}}_{\text{2}}\text{O}}$ = 0· 878 ± 0·02. If the value of W_air for 5-MeV. α-particles is 34·7 ± 0·5 eV.², the value for water vapour is therefore $W_{{\text{H}}_{\text{2}}\text{O}}$ = 30·5 ± 0·8 eV.