. R ADIOACTIVITY A. H. Becquerel discovered radioactivity in purely by accident.
While studying the fluorescence and phosphorescence of compounds irradiated with visible light, Becquerel observed an interesting phenomenon. After illuminating some pieces of uranium-potassium sulphate with visible light, he wrapped them in black paper and separated the package from a photographic plate by a piece of silver. When, after several hours of exposure, the photographic plate was developed, it showed blackening due to something that must have been emitted by the compound and was able to penetrate both black paper and the silver. Experiments performed subsequently showed that radioactivity was a nuclear phenomenon in which an unstable nucleus undergoes a decay.
This is referred to as radioactive decay . Three types of radioactive decay occur in nature : (i) ( -decay in which a helium nucleus He is emitted; (ii) -decay in which electrons or positrons (particles with the same mass as electrons, but with a charge exactly opposite to that of electron) are emitted; (iii) " -decay in which high energy (hundreds of keV or more) photons are emitted. Each of these decay will be considered in subsequent sub-sections. .
. Law of radioactive decay In any radioactive sample, which undergoes ( , or " -decay, it is found that the number of nuclei undergoing the decay per unit time is proportional to the total number of nuclei in the sample. If N is the number of nuclei in the sample and N undergo decay in time t then N N t or, N / t = # N , ( . ) where # is called the radioactive decay constant or disintegration constant .
The change in the number of nuclei in the sample * is d N = – N in time t . Thus the rate of change of N is (in the limit t ) d – d N N t * N is the number of nuclei that decay, and hence is always positive. dN is the change in N , which may have either sign. Here it is negative, because out of original N nuclei, N have decayed, leaving ( N – N ) nuclei.