Discovery of Natural Radioactivity

The French scientist Antoine-Henry Becquerel (1852— 1908), while experimenting in 1896 with fluorescence pro­duced by X rays, discovered that uranium salts emit uncessingly a tvpe of radiation that is able to blacken photo­graphic plates, this phenomenon being called radioactivity. The husband-and-wife team of Pierre Curie (1859—1906) and Marie Sclodovvska-Curie (1867—1934) continued these investigations and discovered much stronger radioactive elements — radium and polonium, the latter being named after Marie Sclodowska-Curie's native country, Poland.

Investigations of the properties of radioactivity showed the strength of the radiation to be independent of chemical combination and of any external physical influences such as temperature, pressure, exposure to light, and X-ray or cathode-ray bombardment. Thus, radioactivity became recognized as a property of the element itself, or more properly, of the nucleus of the atom.

It was soon found that the gold-leaf electroscope was more sensitive for measuring radioacti­vity than the photographic plate. This device, adapted for the study of radioactivity, is shown in the accompanying figure. Here the sup­porting rod for the thin metal foil projects through an insulating collar in the bottom of the metal chamber and terminates in a metal plate. The shell of the electroscope surrounds this plate, and the radioactive material to be tested is placed at a known distance below the plate. The radioactive emissions ionize the air within the instrument and the electroscope, originally charged, is observ­ed to discharge.

With the electroscope it was discovered that three kinds of radiation are given off, but that not every radioactive material emits all three. The strongest radiation, named alpha rays, was found to give measurable effects up to sample-to-electroscope distances in air of one to three inches, depending upon the material under test. At slightly greater distances the discharge rate decreased markedly; the results suggested specific ranges in air for these rays. With the sample located beyond the terminal range of the alpha rays, a weaker emanation, called beta radiation, was still observed. The effect of these rays was lessened by interposing metal foils a millimeter or two thick between the sample and the elect­roscope. A third type of radiation, named gamma radiation, was then measured; it was much weaker than the other two but very penetrating, and could only be stopped by absorption in several centimeters of lead.

Experiments conducted in a uniform magnetic field established the following facts: alpha particles carry a po­sitive charge twice as large as the negative charge on the electron and have a mass about four times that of the hydro­gen atom; beta particles are negatively charged with exactly the charge of the electron but have a mass which may be many times that of the electron. Diffraction experiments show gamma rays to be electromagnetic waves having wavelengths shorter even than X rays.

The mass and charge found for alpha particles suggest­ed that these might be helium nuclei— that is, helium atoms with the two valence electrons removed.