The theory of luminescence in radioactive luminous compound

Previous work on this subject is reviewed, and the results of brightness measurements on zinc sulphide compounds containing radium are given for periods up to 4,000 days. From a comparison of the brightness curves of compounds made with the same luminescent material, but with different radium concentrations it is shown that the brightness-time relationship is of the form B=rf(rt), where r is the radium content. The experimental facts available for developing and testing a theory of the luminescence are summarised, and a number of different theories, including a modified form of the recovery theory, are considered. Finally it is shown that the observed brightness curves are in excellent agreement with Rutherford's original theory of the destruction of active centres, provided this be combined with a simple hypothesis as to the cause of the progressive increase in the light absorption of the material which has been found experimentally. This leads to the following form of the brightness-time relationship - log {B/(b+B)} +kt+a=0, where a, b and k are constants, of which the last two are proportional to the radioactive concentration for any given grade of luminescent material. From the results obtained on two grades of compound it is concluded that (a) the rate of destruction of active centres is six to nine times that of the ionisation of inactive molecules, and (b) in the new material used for the compounds measured, 20 to 50 per cent. of the molecules are in the active state.