Image of Becquerel's photographic plate which has beenfogged by exposure to radiation from a uranium salt. The shadow of a metalMaltese Cross placed between the plate and the uranium salt is clearly visible.
Becquerel's discovery ofspontaneous radioactivity is a famous example ofserendipity, of how chance favours the prepared mind. Becquerel had long been interested inphosphorescence, the emission of light of one colour following the object's exposure to light of another colour. In early 1896, there was a wave of excitement followingWilhelm Röntgen's discovery ofX-rays in late 1895. During the experiment, Röntgen "found that theCrookes tubes he had been using to studycathode rays emitted a new kind of invisible ray that was capable of penetrating through black paper".[8] Becquerel learned of Röntgen's discovery during a meeting of theFrench Academy of Sciences on 20 January where his colleagueHenri Poincaré read out Röntgen's preprint paper.[5]: 43 Becquerel "began looking for a connection between thephosphorescence he had already been investigating and the newly discovered x-rays"[8] of Röntgen, and thought that phosphorescent materials might emit penetrating X-ray-like radiation when illuminated by bright sunlight; he had various phosphorescent materials including someuranium salts for his experiments.[5]
Throughout the first weeks of February, Becquerel layered photographic plates with coins or other objects then wrapped this in thick black paper, placed phosphorescent materials on top, placed these in bright sun light for several hours. The developed plate showed shadows of the objects. Already on 24 February he reported his first results. However, the 26 and 27 February were dark and overcast during the day, so Becquerel left his layered plates in a dark cabinet for these days. He nevertheless proceeded to develop the plates on 1 March and then made his astonishing discovery: the object shadows were just as distinct when left in the dark as when exposed to sunlight. BothWilliam Crookes and Becquerel's 18-year-old son,Jean, witnessed the discovery.[5]: 46
By May 1896, after other experiments involving non-phosphorescent uranium salts, Becquerel arrived at the correct explanation, namely that the penetrating radiation came from the uranium itself, without any need for excitation by an external energy source.[9] There followed a period of intense research into radioactivity, including the determination that the elementthorium is also radioactive and the discovery of additional radioactive elementspolonium andradium byMarie Skłodowska-Curie and her husbandPierre Curie. The intensive research ofradioactivity led to Becquerel publishing seven papers on the subject in 1896.[6] Becquerel's other experiments allowed him to research more into radioactivity and figure out different aspects of themagnetic field when radiation is introduced into the magnetic field. "When different radioactive substances were put in the magnetic field, they deflected in different directions or not at all, showing that there were three classes of radioactivity: negative, positive, and electrically neutral."[10]
As simultaneity often happens in science, radioactivity came close to being discovered nearly four decades earlier in 1857, whenAbel Niépce de Saint-Victor, who was investigating photography underMichel Eugène Chevreul, observed that uranium salts emitted radiation that could darken photographic emulsions.[11][12] By 1861, Niepce de Saint-Victor realized that uranium salts produce "a radiation that is invisible to our eyes".[13] Niepce de Saint-Victor knew Edmond Becquerel, Henri Becquerel's father. In 1868, Edmond Becquerel published a book,La lumière: ses causes et ses effets (Light: Its causes and its effects). On page 50 of volume 2, Edmond noted that Niepce de Saint-Victor had observed that some objects that had been exposed to sunlight could expose photographic plates even in the dark.[14] Niepce further noted that on the one hand, the effect was diminished if an obstruction were placed between a photographic plate and the object that had been exposed to the sun, but " …d'un autre côté, l'augmentation d'effet quand la surface insolée est couverte de substances facilement altérables à la lumière, comme le nitrate d'urane … " ( ... on the other hand, the increase in the effect when the surface exposed to the sun is covered with substances that are easily altered by light, such as uranium nitrate ... ).[14]
Describing them to the French Academy of Sciences on 27 February 1896, he said:
One wraps aLumière photographic plate with a bromide emulsion in two sheets of very thick black paper, such that the plate does not become clouded upon being exposed to the sun for a day. One places on the sheet of paper, on the outside, a slab of the phosphorescent substance, and one exposes the whole to the sun for several hours. When one then develops the photographic plate, one recognizes that the silhouette of the phosphorescent substance appears in black on the negative. If one places between the phosphorescent substance and the paper a piece of money or a metal screen pierced with a cut-out design, one sees the image of these objects appear on the negative ... One must conclude from these experiments that the phosphorescent substance in question emits rays which pass through the opaque paper and reduce silver salts.[15][16]
But further experiments led him to doubt and then abandon this hypothesis. On 2 March 1896 he reported:
I will insist particularly upon the following fact, which seems to me quite important and beyond the phenomena which one could expect to observe: The same crystalline crusts [of potassium uranyl sulfate], arranged the same way with respect to the photographic plates, in the same conditions and through the same screens, but sheltered from the excitation of incident rays and kept in darkness, still produce the same photographic images. Here is how I was led to make this observation: among the preceding experiments, some had been prepared on Wednesday the 26th and Thursday the 27th of February, and since the sun was out only intermittently on these days, I kept the apparatuses prepared and returned the cases to the darkness of a bureau drawer, leaving in place the crusts of the uranium salt. Since the sun did not come out in the following days, I developed the photographic plates on the 1st of March, expecting to find the images very weak. Instead the silhouettes appeared with great intensity ... One hypothesis which presents itself to the mind naturally enough would be to suppose that these rays, whose effects have a great similarity to the effects produced by the rays studied by M. Lenard and M. Röntgen, are invisible rays emitted by phosphorescence and persisting infinitely longer than the duration of the luminous rays emitted by these bodies. However, the present experiments, without being contrary to this hypothesis, do not warrant this conclusion. I hope that the experiments which I am pursuing at the moment will be able to bring some clarification to this new class of phenomena.[17][18]
In 1900, Becquerel measured the properties ofbeta particles, and he realized that they had the same measurements as high speedelectrons leaving thenucleus.[6][19] The following year, he discovered thatradioactivity could be used for medicine; he left a piece ofradium in his vest pocket, and noticed that he had been burnt by it. This discovery led to the development ofradiotherapy, which is now used to treat cancer.[6]
Becquerel died on 25 August 1908 inLe Croisic at the age of 55.[6] He died of a heart attack,[5]: 49 but it was reported that "he had developed seriousburns on his skin, likely from the handling of radioactive materials."[20]
^abcdePais, Abraham (2002).Inward bound: of matter and forces in the physical world (Reprint ed.). Oxford: Clarendon Press [u.a.]ISBN978-0-19-851997-3.
