| Product | Maximum range of α particles in air | Velocity of α particles |
|---|---|---|
| Radium | 3 cms. | ·82 V₀ |
| Emanation | 3·8 or 4·4 cms. | ·87 or ·90 V₀ |
| Rad. A | 4·4 or 3·8 „ | ·90 or ·87 V₀ |
| Rad. C | 6·7 „ | 1·00 V₀ |
It is difficult to determine from the experiments whether the range 3·8 cms. belongs to the rays from the emanation or from radium A. The mean velocity of the α particles is thus ·90 V₀, and the maximum variation for the individual products does not vary more than 10 per cent. from the mean value.
The results of Becquerel, discussed in [section 92], at once receive an explanation on the above results. The α particles, expelled from radium in radio-active equilibrium, have all ranges lying between 0 and 6·7 cms. of air. The velocity of the α particles which are able to produce a photographic impression varies between ·64 V₀ and V₀. The particles which have only a short range in air are projected with a smaller velocity than those which have a greater range. The former are in consequence more bent by a magnetic field. It is thus to be expected that the apparent curvature of the path of rays in a uniform magnetic field will be greater close to the radium than at some distance away.
Range of phosphorescent action in air. Some experiments were also made to see whether the action of the α rays in producing luminosity in substances like zinc sulphide, barium platinocyanide, and willemite, ceased at the same distance as the ionizing action.
A very active wire was placed on a moveable plate, the distance of which from a fixed screen of phosphorescent substance could be varied. The distance at which the phosphorescent action ceased could be determined fairly accurately. Different thicknesses of aluminium foil were then placed over the active wire, and the corresponding distance at which the luminosity disappeared was measured. The results are shown graphically in [Fig. 107], where the ordinates represent the distance of the phosphorescent screen from the active wire, and the abscissae the number of layers of aluminium foil, each ·00031 cms. thick.
Fig. 107.
It is seen that the curve joining the points is a straight line. 12·5 thicknesses of foil absorbed the rays to the same extent as 6·8 cms. of air, so that each thickness of aluminium corresponded in absorbing power to ·54 cms. of air. For a screen of zinc sulphide, the phosphorescent action ceased at a distance of air of 6·8 cms., showing that the photographic and phosphorescent ranges of the α rays in air were practically identical.
The experiments with barium platinocyanide and willemite were more difficult, as the β and γ rays from the active wire produced a luminosity comparable with that produced by the α rays. Fairly concordant results, however, were obtained by introducing a thin sheet of black paper between the active wire and the screen. If the luminosity was sensibly changed, it was concluded that the α rays still produced an effect, and in this way the point of cessation of phosphorescent action could be approximately determined. For example, with eight thicknesses of foil over the active wire the additional thickness of air required to cut off the phosphorescent effect of the a rays was 2·5 cms. for willemite, and 2·1 cms. for barium platinocyanide.
The corresponding distance for zinc sulphide was 2·40 cms., a value intermediate between the other two.