Henri Becquerel

Antoine Henri Becquerel (Template:IPAc-en Template:Respell;<ref>Template:Cite web</ref> Template:IPA; 15 December 1852 – 25 August 1908) was a French experimental physicist who shared the 1903 Nobel Prize in Physics with Marie and Pierre Curie for his discovery of radioactivity.<ref name=Nobel1903>Template:Cite web</ref><ref>Template:Cite web</ref>

Education and career

Antoine Henri Becquerel was born on 15 December 1852 in Paris, France. His grandfather, Antoine César Becquerel, father, Edmond Becquerel, and later his son, Jean Becquerel were all notable physicists.<ref name=PaisInward/>Template:Rp

Becquerel attended the Template:Lang, before studying engineering at Template:Lang (1872–1874) and Template:Lang (1874–1877).<ref name=":0">Template:Cite web</ref> In 1888, he received his D.Sc. from the Sorbonne; his thesis was on the plane polarisation of light, with the phenomenon of phosphorescence and absorption of light by crystals.<ref>Template:Cite thesis</ref>

In 1878, Becquerel became an assistant at the Template:Lang, where in 1892 he was appointed Professor of Applied Physics. In 1894, he became chief engineer in the Department of Roads and Bridges. He became a professor at Template:Lang in 1895.<ref name=":0"/>

Discovery of radioactivity

Image of Becquerel's photographic plate which has been fogged by exposure to radiation from a uranium salt. The shadow of a metal Maltese Cross placed between the plate and the uranium salt is clearly visible.

Becquerel's discovery of spontaneous radioactivity is a famous example of serendipity, of how chance favours the prepared mind. Becquerel had long been interested in phosphorescence, 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 following Wilhelm Röntgen's discovery of X-rays in late 1895. During the experiment, Röntgen "found that the Crookes tubes he had been using to study cathode rays emitted a new kind of invisible ray that was capable of penetrating through black paper".<ref name=":6">Template:Cite web</ref> Becquerel learned of Röntgen's discovery during a meeting of the French Academy of Sciences on 20 January where his colleague Henri Poincaré read out Röntgen's preprint paper.<ref name=PaisInward>Template:Cite book</ref> Template:Rp Becquerel "began looking for a connection between the phosphorescence he had already been investigating and the newly discovered x-rays"<ref name=":6" /> 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 some uranium salts for his experiments.<ref name=PaisInward/>

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. Both William Crookes and Becquerel's 18-year-old son, Jean, witnessed the discovery.<ref name=PaisInward/>Template:Rp

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.<ref name=":5">Template:Cite journal</ref> There followed a period of intense research into radioactivity, including the determination that the element thorium is also radioactive and the discovery of additional radioactive elements polonium and radium by Marie Skłodowska-Curie and her husband Pierre Curie. The intensive research of radioactivity led to Becquerel publishing seven papers on the subject in 1896.<ref name=":0" /> Becquerel's other experiments allowed him to research more into radioactivity and figure out different aspects of the magnetic 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."<ref>Template:Cite web</ref>

As simultaneity often happens in science, radioactivity came close to being discovered nearly four decades earlier in 1857, when Abel Niépce de Saint-Victor, who was investigating photography under Michel Eugène Chevreul, observed that uranium salts emitted radiation that could darken photographic emulsions.<ref>Niepce de Saint-Victor (1857) "Mémoire sur une nouvelle action de la lumière" (On a new action of light), Comptes rendus ... , vol. 45, pages 811–815.</ref><ref>Niepce de Saint-Victor (1858) "Deuxième mémoire sur une nouvelle action de la lumière" Template:Webarchive (Second memoir on a new action of light), Comptes rendus ... , vol. 46, pages 448–452.</ref> By 1861, Niepce de Saint-Victor realized that uranium salts produce "a radiation that is invisible to our eyes".<ref>Template:Cite web</ref> 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.<ref name=":7" /> 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 ... ).<ref name=":7">Edmond Becquerel, La lumière: ses causes et ses effets, vol. 2 (Paris, France: F. Didot, 1868), page 50.</ref>

Experiments

Becquerel in the lab

Describing them to the French Academy of Sciences on 27 February 1896, he said:

One wraps a Lumière 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.<ref>Template:Cite journal</ref><ref>Comptes Rendus 122: 420 (1896), translated by Carmen Giunta. Accessed 02 March 2019.</ref>

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.<ref>Template:Cite journal</ref><ref>Comptes Rendus 122: 501–503 (1896), translated by Carmen Giunta. Accessed 02 March 2019.</ref>

Later life and death

In 1900, Becquerel measured the properties of beta particles, and he realized that they had the same measurements as high speed electrons leaving the nucleus.<ref name=":0" /><ref name=":3">Template:Cite web</ref> The following year, he discovered that radioactivity could be used for medicine; he left a piece of radium in his vest pocket, and noticed that he had been burnt by it. This discovery led to the development of radiotherapy, which is now used to treat cancer.<ref name=":0" />

Becquerel died on 25 August 1908 in Le Croisic at the age of 55.<ref name=":0"/> He died of a heart attack,<ref name=PaisInward/>Template:Rp but it was reported that "he had developed serious burns on his skin, likely from the handling of radioactive materials."<ref>Template:Cite news</ref>

Recognition

Awards

Country	Year	Institute	Award	Citation	Template:Reference column heading
Template:Flagdeco United kingdom	1900	Royal Society	Rumford Medal	"For his discoveries in radiation proceeding from uranium"	<ref>Template:Cite web</ref>
Template:Flag	1903	Royal Swedish Academy of Sciences	Nobel Prize in Physics	"In recognition of the extraordinary services he has rendered by his discovery of spontaneous radioactivity"	<ref name=Nobel1903/>

Memberships

Country	Year	Institute	Type	Template:Reference column heading
Template:Flag	1902	American Philosophical Society	International Member	<ref>Template:Cite web</ref>
Template:Flagdeco United kingdom	1908	Royal Society	Foreign Member	<ref>Template:Cite web</ref>

Commemorations

The SI unit of radioactivity is named after Becquerel.<ref>Template:Cite web</ref> A crater on the Moon, as well as a crater on Mars, are named after him.<ref>Template:Cite web</ref><ref>Template:Cite web</ref> Becquerelite, a uranium mineral, is named after him.<ref>Template:Cite web</ref> Minor planet 6914 Becquerel is named in his honour.<ref>Template:Cite book</ref>

External links

• Template:Nobelprize including the Nobel Lecture, "On Radioactivity, a New Property of Matter", 11 December 1903
• Becquerel short biography Template:Webarchive and the use of his name as a unit of measure in the SI
• Annotated bibliography for Henri Becquerel from the Alsos Digital Library for Nuclear Issues
• Henri Becquerel, SI-derived unit of radioactivity
• "Henri Becquerel: The Discovery of Radioactivity", Becquerel's 1896 articles online and analyzed on BibNum [click 'à télécharger' for English version].
• Template:Cite EB1911
• Template:Cite webTemplate:Cbignore