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2-1: The Discovery of Radioactivity

In 1896, the natural radioactivity was discovered by A. H. Becquerel (France, 1852 - 1908). He found that the uranium element can expose a photographic dry plate, even if it is separated from the uranium or covered with a sheet of black paper. He also found that a kind of "ray" from the uranium can be sensed by an electroscope. Namely this "ray" has an electric charge. Nowadays, this "ray" is called a radiation or radioactivity, and a substance emitting the radiation is called a radioactive material.

[The Discovery of Radium and Polonium]
In 1898, M. Curie (Poland, France, 1867 - 1934) and P. Curie (France 1859 - 1906) confirmed that the radioactivity does not depend on the chemical state of the relevant atom but relates to the atom itself. They noticed that the radioactivity of pitchblende is stronger than uranium, although the former is the ore of the latter, and then they discovered new elements, radium and polonium, out of pitchblende.
By the way, you should note that the term "radioactivity" was given by M. Curie.

F. Soddy (UK, 1877 - 1956) found that the radium emits radiation to decay and change into another element, radon (1903). This is the first discovery of the radioactive decay of an element.
After the discovery of the radioactivity or the radioactive decay of an element, people have realized that the atom is not an ultimate particle but it consists of smaller basic elements. Namely, an atom has still an internal structure. Therefore it became a very interesting problem what the structure of an atom is.

[Types of Radioactivity]
It has been clarified that there are three types of radioactivity, i.e. alpha, beta and gamma rays.
By testing how the radioactive rays curve in an electric field or a magnetic field, their charges were investigated. It was seen that the charge of alpha rays is +2e where e is the elementary charge, and its charge-to-mass ratio is a half of that of hydrogen ion. It was also clarified that beta rays are nothing other than electrons. It was made clear that gamma rays have no charge and are very similar to X rays which was accidentally discovered by W. C. Roentgen (Germany, 1845 - 1923). It was later clarified that both X rays and gamma rays are electromagnetic waves of very short wavelength; usually, the wavelength of gamma rays is shorter than that of X rays.
A schematic drawing of the orbits of alpha, beta and gamma rays in a uniform magnetic field are shown in the following figure, where the magnetic field is applied in the direction perpendicular to this picture from front side to back side.

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