Theory of the luminosity produced in certain substances by α-rays

In the preceding paper, Mr. Marsden has examined the decay of the luminosity excited byα -rays in zinc sulphide, willemite, and barium platinocyanide, when subjected to an intense bombardment byα -particles. He has shown that the luminosity decreases with continued bombardment to a very small fraction of its initial value. For a given bombardment, the rate of decay of luminosity is about the same for zinc sulphide and willemite, but is especially rapid in barium platinocyanide. The action of theα -particles on phosphorescent zinc sulphide is of special interest and importance on account of the marked scintillations observed, and the fact that eachα -particle under suitable conditions produces a visible scintillation. Mr. Marsden has brought out the essential fact that the actual number of scintillations observed for a constant source ofα -rays changes very little with continued bombardment, but the brightness of the scintillations rapidly diminishes. It is well known that theα -particles exert a marked dissociation effect in complex molecules on which they fall. For example, theα -rays from radium or its emanation, dissolved in water, dissociate the water molecules, producing hydrogen and oxygen at a rapid rate. I have shown elsewhere (‘Radio-active Transformations,’ p. 253), that the magnitude of this effect is in agreement with the view that eachα -particle dissociates as many molecules of water as it produces ions in its path in air. The loss of energy of theα -particle in passing through a gas is mainly used up in producing ions in the gas. The laws of absorption ofα -particles, which have been so carefully worked out by Bragg, show that no definite distinction as regards absorption can be drawn between a solid and a gas. It is reasonable to suppose that theα -particle produces ions in a solid as well as in a gas, and that the absorption of theα -particle is due mainly to the energy used up in this process. If the solid matter is composed of complex molecules, the latter will be dissociated by theα -particles. As Marsden has pointed out, pure zinc sulphide does not exhibit the scintillation effect, but this only appears in zinc sulphide to which certain impurities have been added. Since the amount of impurity present is of the order of 1 per cent., it is probable that only a small fraction of the total number of molecules give rise to the scintillation effect. These “active centres,” as they will be called, will on the average be uniformly distributed among the inactive molecules.