It will be observed that the value of the absorption constant divided by the density is very nearly the same for such different substances as glass, mica, ebonite, wood, iron and aluminium. The divergences from the law are great, however, for the other metals examined, viz. copper, silver, lead and tin. In tin the value of λ divided by the density is 2·5 times its value for iron and aluminium. These differences show that a law for the absorption of the β rays depending only on the density does not hold for all substances. With an exception in the case of tin, the value of λ divided by the density for the metals increases in the same order as their atomic weights.
The absorption of the β rays by matter decreases very rapidly with increase of speed. For example, the absorption of cathode rays in Lenard’s experiment (loc. cit.) is about 500 times as great as for the uranium β rays. The velocity of the β rays of uranium was found by Becquerel to be about 1·6 × 1010 cms. per sec. The velocity of the cathode rays used in Lenard’s experiment was certainly not less than ⅒ of this, so that, for a decrease of speed of less than 10 times, the absorption has increased over 500 times.
85. Number of electrons stopped by matter. An account will now be given of the experiments made by Seitz[[136]], to determine the relative number of electrons which are stopped in their passage through different thicknesses of matter. The experimental arrangement is shown in [Fig. 31].
Fig. 31.
The radium was placed outside a glass vessel containing an insulated brass plate P, the connection of which with a wire leading to the electrometer could be made or broken by a simple electromagnetic device. The β rays from the radium R, after passing through openings in a brass plate A, covered with thin aluminium foil, were absorbed in the plate P. The glass vessel was exhausted, and the charge communicated to P by the β rays was measured by an electrometer.
In a good vacuum, the magnitude of the current observed is a measure of the number of β particles absorbed by the upper plate[[137]]. The following table shows the results obtained when different thicknesses of tin foil were placed over the radium. The second table gives the ratio I/I₀ where I₀ is the rate of discharge observed before the absorbing screen is introduced. The mean value of the absorption constant λ was deduced from the equation
where d is the thickness of matter traversed.
The values included in the brackets have not the same accuracy as the others. There is thus a wide difference in penetrating power of the β particles emitted from radium, and some of them are very readily absorbed.