6.The Nature of Gamma Rays.
[Read before the Wellington Philosophical Society, 12th July, 1911.]
Dr. E. Von Sohweidler pointed out in 1905 that an effect such as ionization by a rays due to a finite number of independent events would be subject to fluctuations. The mathematical theory of the different experiments which have been made with light, a and β rays, has been developed by Mr. N. R. Campbell.
One of us began some preliminary experiments in 1908 at the Cavendish Laboratory to detect discontinuous effects with γ rays. Two forms of apparatus have been used in our experiments. In the first two similar cylindrical ionization-vessels were placed close together with their axes directed to the source of the γ rays — some radium. If the γ rahs have
a spherical wave-front, the two similar vessels, being symetrically placed with respect to the source, should be equally effected by the γ rays, though the results of the equal effects may not be the same. If, on the other hand, the γ rays are any type of corpuscular radiation (in the New-tonian sense) made of a finite number of particles, the effect in the ionization-vessels would be unequal over short periods of time. To compare the number of ions produced in the two vessels, the electrodes were connected to an electrometer, one vessel being positively the other negatively charged. The positive and negative currents from the two cans were balanced as closely as possible for long periods of time, and so there was no large steady drift of the electrometer. The quartz fibre electrometer (Proc. Camb. Phil. Soc., vol. 15, p. 106, 1909) showed fluctuations in this balance. In the second apparatus a box-shaped ionization-can with a central plane electrode was used. The positive ions formed in one half of the can were received on one side of the flat electrode, the negative ions from the other half of the can on the other side of the electrode. By making the can airtight and thoroughly drying the contained air, complete “saturation” was produced with a field of only 8 volts per cm. Large fluctuations were observed when the ionization currents from the two halves were balanced, the source of γ rays being placed outside the can in the plane of the central electrode. This experimental result would be explained if (1) the γ rays from radium are projected particles, or (2) if the number of ions produced in air by a constant source of rays is subject to fluctuations.
We are continuing the experiments with a view to determining what part each of these factors plays in producing the fluctuations observed.
The radium used in these experiments was lent by the Royal Society of London.