Упражнения к лекции 3. Технический перевод и его рабочие источники. The discovery of natural radioactivity

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

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Quantitative measures of intensity can be made timing the discharge rate for different distances between the sample and the plate.

With the electroscope it was discovered that three kinds of radiation are giveir 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 centimetersof 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 bur 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; gamma rays exhibit neither charge nor mass. 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. This surmise was confirmed by an experiment in which a spectrum discharge tube was exhausted until the electric discharge through it ceased, and then a small amount of alpha-emitting radioactive substance was placed just'outside a thin window in the tube. When a potential difference was again applied to the tube a few days later, it glowed with a characteristic yellow - orange color, and the glow was found upon examination with a spectroscope to comprise the well known spectrum lines of helium. This test showed alpha particles from the radioactive material to have penetrated the window of the discharge tube, to have picked up two electrons each, and to become neutral helium atoms.