18.2 NATURAL RADIOACTIVITY
In 1896, Becquerel accidentally discovered that uranium salt crystals emit an invisible radiation that can darken a photographic plate. He also observed that the radiation had the ability to ionize a gas. Subsequent experiments by other scientists showed that other substances also emitted radiations. The most significant investigations of this type were conducted by Marie Curie and her husband Pierre. They discovered two new elements which emitted radiations. These were named polonium and radium. This process of emission of radiations by some elements was called natural radioactivity by Marie Curie. Subsequent experiments performed by Henry Becquerel suggested that radioactivity was the result of the decay or disintegration of unstable nuclei.
The spontaneous emission of radiation by unstable nuclei is called natural radioactivity. And the elements which emit such radiations are called radioactive elements.
Three types of radiation are usually emitted by a radioactive element. In order to show that three types of radiations are emitted by a radioactive element, a small amount of radioactive material is placed in a lead block with a narrow slit. The radiations coming out of the radioactive element are passed through a strong magnetic field. These three types of radiations are deflected in different directions by the magnetic field (Fig.18.3). The radiations which are deflected towards the left are called alpha particles. Those are deflected towards the right are called beta particles while the radiations which are undeflected by the magnetic field are called gamma rays.