Antoine Henri Becquerel was born in Paris in 1852 into a family of great scientists. Both his grandfather and father were successively professors of physics at the Musee d’Histoire Naturelle in Paris. When it was decided that Henri should also follow a scientific career it could not have imagined that a few weeks of his works at the beginning of 1896 would result in the discovery of radioactivity. This discovery, with others, led to a revolution in physics which has had an enormous effect on the world.
In 1878, at the age of 26, Henri became an assistant at the Musee, and performed a lot of experiments in association with his father, who by this time had become Professor of Physics there. It was as a result of some of this work that 10 years later Henri received his doctorate degree. Much of his tim was occupied investigating the absorption of light by crystals, and the effects of magnetism on beams of light and on the phosphorescence of sulphides and compounds of uranium.
By the year 1895 Henri Becquerel had followed his father as professor of physics at the musee, and also professor of applied physics at the Conservatoire des Arts et Metiers in Paris. Up to this time he been known to be an outstanding physicist, but the discovery which was to make him famous was made during the months of January and February 1896, only.
Henri Becquerel, up till then, had continued his experiments in phosphore-science. In the first days of 1896 he received word in Paris that Rontgen had discovered that X-Rays excite fluorescence in certain substances. Henri Becquerel immediately decided to see whether phosphorescent substances emitted rays, similar to X-rays, by placing the substance on a photographic plate which was wrapped in black paper, and exposing them to sunlight. Only when using salts of uranium did he obtain fogging of the plate, showing that radiations came from these salts only.
It was then, almost by chance, that Henri Becquerel was led to realise that these radiations were of unknown origin.
Because the weather had become cloudy, Henri Becquerel put all his material into a drawer and awaited a sunny day. As a check, he then developed the photographic plate and found that the fogging had again occurred, even though the uranium salt could not get have phosphoresced since it had been in darkness for some days.
To see whether the effect depended on the uranium salts having previously been exposed to sunlight at all, Henri Becquerel next prepared the salts in darkness. He also preformed the experiment in darkness, but found the same result – the fogging did not depend on phosphorescence.
After further work, and by noting that his newly discovered rays could pass through metal plates and still fog the photographic plate, though less intensely, Henri Becquerel found that all salts of uranium, and the metal uranium itself, continuously give off invisible rays. He had discovered that uranium is radioactive.
It is now known that Henri Becquerel discovered on type of radioactivity – beta-ray activity, which is die to high-speed electrons leaving the nucleaus of the atom – in this case, the uranium atom.
Within a period of about six weeks Henri Becquerel had enough evidence to announce his discovery of spontaneous (or natural) radioactivity to the Acedemy of Science in Paris. This he did in February, 1896.
Pierre and Marie Curie now began their work on radioactivity elements in close association with Henri Becquerel and in 1903 Henri Becquerel received on Nobel prize, and Pierre and Marie Curie shared another.
Nowadays it is generally considered that Henri Becquerel discovered radioactivity by chance, but it is truer to say that he as looking for an effect so similar to radioactivity that he must have discovered it sooner or later, and he was so great a scientist that he quickly realised the importance of his evidence.
Henri Becquerel, after further important work in radioactivity, died in 1908 at Croissic in Brittany, but will be remembered – in the words of Nobel prize award in 1903 – “for the discovery of spontaneous radioactivity”