In the last years of the 19th century many scientists focused on the research of the properties of X rays. Henry Becquerel was among them. In 1896 he carried out an experiment which aimed at examining whether various materials stimulated by ultraviolet light to fluoresce also became the source of some molecules or waves (the scientists linked the emergence of X rays with the fluorescence of glass stimulated by cathode rays). In his experiment he used a photographic plate wrapped in two sheets of black paper. The paper was opaque to light rays. Becquerel put on thus prepared plate a substance, which was to be stimulated to fluoresce. He used crystals of uranium-potassium sulphate. He exposed them to sun rays for several hours. After the experiment he processed the photographic plate. He saw an outline of a fluorescent substance on the processed plate.
In subsequent experiments he placed various object (e.g. a coin) between crystals and the plate. That rendered negative images of the objects on the plate. Based on the experiments carried out, Becquerel came to a conclusion that the investigated substance emitted radioactivity which penetrated through paper and exerted chemical action on the photographic plate.
However, a few days after the first experiment had been carried out, Becquerel discovered that the photograph became blackened also when crystals containing uranium were not stimulated to fluoresce by light radiation. Negative images appeared also when the experimental set was kept in darkness. While investigating this phenomenon, the scientist carried out a few experiments in which he tried to precisely isolate uranium crystals from the influence of rays of different lengths. As it turned out, the substance still emitted radiation. Soon afterwards Becquerel discovered that various substances containing uranium admixtures emitted such radiation whether they had the capability to fluoresce or not. Hence the source of the observed phenomenon was uranium. Becquerel discovered the phenomenon of radioactivity.
Becquerel's discovery, which was actually purely accidental, brought about turmoil in the whole scientific community. Researchers tried to find out which substances emitted radiation and what the properties of that phenomenon were.
In the years 1895-1896 Thomson and Rutheford focused their research on the phenomenon of ionisation of gases exposed to rays discovered by Becquerel. During this experiment Rutheford discovered that there were actually two kinds of the radiation - one, called alfa, was easily absorbed even by sheets of paper; the other, called beta, could penetrate even through thick metal sheets (e.g. through 0.25 centimetre thick aluminium).
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In the years 1898-1899 scientists
Marie Sklodowska-Curie, Pierre Curie (1859-1906) and
G. Bemont discovered new radioactive elements - radium and polonium.
Rutherford and Owens discovered yet another radioactive element - thorium. Rutherford i Frederic Soddy (1877-1956) noticed that a certain gaseous substance called "gaseous emanation" emanated from radium. That gas did not show any chemical activity. Hence it was similar to noble gases. After an examination with a spectroscope it turned out that the gas included helium. |
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In subsequent years scientists undertook researches of both kinds of radiation. It turned out that the hardly penetrative alfa rays consisted of positively charged molecules (they are deflected in a magnetic field in the same direction as other positive molecules). It turned out that the q/m ratio of those molecules equalled about 4.8*107 coulombs per kilogram. The ratio was twice smaller than the q/m ratio of hydrogen ions. Scientists came to a conclusion that alfa molecules consisted of helium atoms charged with a double positive charge - such molecules have the mass equal 4*(hydrogen mass) and positive charge 2*(elementary charge). Soon afterwards Rutheford and Royds carried out the following experiment: they introduced a strongly radioactive body into an extremely thin capillary tube (the walls were only 0.01 mm thick). After tight closing the capillary tube was placed inside a more spacious tube containing vacuum. Alfa molecules - but not the "gaseous emanation" - could penetrate through the thick walls of the capillary tube. After a few days helium would appear in the external tube. Its sole origin must have been alfa molecules, which lost positive charge (electrons were attached) and changed into helium atoms.
Much more penetrative beta molecules can be deflected in electric and magnetic fields in such a way that they must have negative charge. After subsequent researches scientists came to a conclusion that beta molecules were simply electrons (simultaneously researchers gathered observations which pointed out to the existence of new laws, which were later described by the theory of relativity).
The third kind of radiation emitted by radioactive bodies was also discovered - gamma radiation. It turned out that the radiation was actually electromagnetic waves of a wavelength generally smaller than 10*-11 meter.
Soon after the discovery of the phenomenon of radioactivity many scientists undertook the research of radioactive elements. They discovered that uranium, thorium and actinium gave - through subsequent disintegrations - birth to a numerous group of radioactive elements. Those elements were set by researchers into sequences in such a way that each element originates from the one that precedes it in the table and is transformed into the substance following it in the table. The last element of all sequences is a permanent element (namely lead). In the radioactive sequence, there often appeared elements of the same chemical properties but of different atomic masses (this clue pointed out to the existence of isotopes).
In subsequent researches scientists linked the kind of emitted radiation to individual disintegrations.
