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Antoine-Henri Becquerel
Discovery of Radioactivity
Dr V B Kamble

Antoine-Henri Becquerel (1852-1908) is known for his discovery of radioactivity, for which he received the Nobel Prize for Physics jointly with Marie Curie (1897-1934) and Pierre Curie (1859-1906) in 1903 and the contributions he made to that field. He was a member of the Academy of Sciences, became its President, and was elected to the far more influential post of permanent Secretary. He held three chairs of Physics in Paris - at the Museum of Natural History, at the cole Polytechnique,and at the Conservatoire National des Arts et Méésartiers' - and attained high rank as an engineer in the National Administration of Bridges and Highways.

Henri's father, Alexandre-Edmond Becquerel, and his grandfather, Antoine César becquerel,were renowned physicists, both members of the Academy of Sciences and each in his turn professor of Physics at the Muserum of Natural History. Henri Becquerel was born on December 15, 1852, and was educated at the Lycee Louis-le-Grand,Ecolésar Polytechnique (1872-1874) and at the Ecolésar des Ponts et Chaussees (1874-1877), where he received his engineering training. On leaving the Polytechnique, he married Lucie-Zoe-Marie Jamin, daughter of J.C. Jamin, academician and professor of Physics in the Faculty of Sciences in Paris. Before the end of his schooling, he had begun both his private research and his teaching career at the Polytechnique.His wife died in March 1878, a few weeks after the birth of their son Jean.Becquerel succeeded to the post of his father at the Museum, and from then on,his professional life was shared among the Museum, the Polytechnique, and the Ponts et Chaussees.

Becquerel's early research was almost exclusively in optics.His first extensive investigations (1875-1882) dealt with the rotation of plane-polarised light by magnetic fields. He next turned to infra-red spectra, making visual observations by means of the light released from certain phosphorescent crystals under infra-red illumination. He then studied the absorption of light in crystals.With these researches, Becquerel obtained his doctorate from the Faculty of Sciences of Paris (1888) and election to the Academy of Sciences (1889). In 1890, he married his second wife. Following the death of his father in 1891,he succeeded in the following year to his father's two chairs of Physics at the Conservatoire National des Arts et Métiers and at the Museum. Thus, in the beginning of 1896, at the age of forty three, Becquerely was established in the rank and responsibility, his years of active research behind him and all that for which he is now remembered still undone!

There are few scientific discoveries whose circumstances are known as minutely as those around the almost accidental finding of radioactivity. On January 7, 1896, the great French mathematician Jules-Henri Poincare (1854-1912) received a letter containing several astonishing photographs of the bones in someone's hand. The bones belonged to Wilhelm Conrad Rööntgen (1845-1923), a scientist Poincare had never visited. The letter explained that the pictures had been taken with the aid of a new discovery, X-rays that Ròöntgen had turned up the previous month, and that he was publicizing his findings by mailing off prints all over Europe. Publicized they were: The photographs created a sensation across the globe(fig.3). Within three weeks, little Eddie McCarthy of Dartmouth, New Hampshire, became a local celebrity when his broken arm was set by physicians armed with X-rays images of the fracture(fig.4). It is easy to imagine Poincare's amazement-photographs of the inside of a human being! -and he quickly asked two local doctors if they could duplicate Röntgen's work. On January 20, they showed their own X-ray photographs to the assembled members of the French Academie des Sciences. The reaction was immediate and extreme. In the next fortnight, five members of the Academie presented papers on the new phenomenon.

Antoine-Henri Becquerel (1852-1908), too, was sitting in the audience when the X-ray photographs were shown. He was fascinated by the strange ghostly images and the mysterious emanations that produced them.Both he and his father had studied the phenomenon of phosphorescence-the museum laboratory was filled with lumps of stone and wood that shone in the dark. The glow of X-ray emission put Becquerel in mind of the light in his study; although he had not done much active research in the last few years, he thought immediately of putting some phosphorescent rock on photographic paper to see if it would darken it in the same way as one of Röntgen's X-ray sources. It would not be all that much work. Born on December 15, 1852, Becquerel was the third in the line of Becquerel who held the chair of applied physics at what is today called the National Museum of Natural History in Paris. Like his grand father Antoine-Cesar Becquerel, and father Alexandre-Edmond Becquerel, before him, he was a member of the French Academy of Sciences and attended its weekly meetings. During the meeting of January 20, 1896, he felt that the X-rays appeared to emanate from the area of a glass vacuum tube made fluorescent when struck by a beam of cathode rays. Poincare wondered aloud if such radiation was emitted by other luminescent bodies.

Becquerel was immediately challenged by this question. In fact, he was ideally suited to answer it. Not only was he expert in the investigation of various luminescent effects, a common activity in physical laboratories of the 19th century, but he had studied the phosphorescence of some uranium compounds in particular. He also was skilled in laboratory applications of photography. And, like most physicists, he sought a better understanding of the nature of matter, so perhaps the mechanics of phosphorescence would bring him closer to reaching this final, philosophical goal.


What is Radioactivity?

Imagine that you are holding a water melon in your hands. All of a sudden, for no apparent reason, one of its seeds comes flying out through the thick skin. At the same time, you find that the water melon has turned into a musk melon. Before you realise what happened, the musk melon throws out a seed and turns into an apple.As you are looking at the apple wondering to bite into it or not, a seed shoots out of it, and now what you have in your hand is an orange. By the time you try comprehend this unbelievable chain of event, the orange throws out a seed and becomes a emon. Surely, you would not like to eat a lemon, so you wait for it to turn into a berry or into a grape. You keep waiting, but nothing happens. The lemon remains a lemon. You may think that may be the a magician is trying to keep you awaiting from eating, or that there is a hitherto unknown power which is responsible for the entire chain of events. Fortunately, we never find one fruit changing into another kind of fruit the way it is described here.However, you may like to note that a very similar process is called Radioactivity.

Atoms are the smallest constituents that make up of elements, and hence all matter. At the centre of each atom, there is a much smaller nucleus that contains even tinier particles called Protons and Neutrons. The nucleus of a "Radioactive"atom - throw out one or more of these tinier articles, sometime particles other than proton or neutron thereby changing into a different element, or sometimes electromagnetic radiation. Such a nucleus is said to decay, or break apart when the decay of a nucleus occur. One type of atom is changed into a different type., and hence one element into another.

and will consider his watch to be slow!

An expert on uranium phosphorescence:

In the second half of the 19th century, Henri's father, Alexandre-Edmond Becquerel (1820-1891), was the leading authority in Europe on the subject of the phosphorescence of solids. It was an important field, made prominent by Robert Wilhelm Bunsen's (1811-1899) and Gustav Robert Kirchhoff's (1824-1887) recent spectroscopic analyses. Incidentally, "Fluorescence" is defined as the emission of light only during stimulation by external radiation. "Phosphorescence" persists after the external radiation ceases. "Luminescence" is the umbrella term. Edmond was drawn to the investigation of uranium salts because of their exceptionally bright phosphorescence and their interesting spectra. One of his contributions was to show that the uranic series of salts is phosphorescent and that the uranous series is not.His son, Henri, began publishing on phosphorescence in 1883, and wrote twenty papers on this and related areas of study over the next 13 years, being attracted especially to the effects of infrared radiations. Like his father, Henri was fascinated by uranium salts, and he examined their absorption bands in both infrared and visible regions.

Although uranium and its compounds interested the Becquerels, the study of these substances remained in something of a scientific backwater throughout the 19th century. Uranium had been discovered in 1789 by a German analytical chemist, Martin Klaproth, while he had been examining pitchblende from Saxony. Its name was chosen in honor of William Herschel's discovery of the planet Uranus in 1781 (a practice continued in the 20th century with the naming of neptunium and plutonium). Not until 1841, however, was it recognized that Klaproth had obtained only the oxide. Eugene Peligot, a noted French chemist,then succeeded in separating the metal.

Attention was again directed to uranium when Dmitri Ivanovich Mendeleev (1834-1907) formulated his periodic table in 1869 and showed it to be the heaviest element. But in an age of burgeoning chemical production, few applications for it were found. Compounds were tried as toning agents in photography, as dyes or stains for leather and wool and as mordants for silk and wool, and attempts were made with the metal to form an alloy with steel. The greatest use was in the ceramic and glass industries, in which uranium was valued for making coloured glazes and coloured clear glass. By varying the percentage of the salt used, one could get yellow, orange, brown, green or black.

Photography entered the laboratory around the middle of the 19th century, being used to complement the microscope, telescope and balloon (for aerial photography),and to capture events such as sound waves, flying bullets, drop splashes, the motion of animals and lightning. Röntgen's encounter with X-rays, which evoked tremendous public interest, relied heavily on photography for its fame. By far the greatest scientific use of this tool came in the century's last two decades,which suggests the impact of dry, gelatin emulsion plates. By 1896 Becquerel would probably have had at his disposal dry photographic plates of relatively good quality, uniform emulsion and long shelf life. Luminescence, uranium, photography Becquerel was in the right place at the right time. But he still might have failed to recognizes radioactivity as a phenomenon separate from phosphorescence if he had not been an accomplished physicist (Figure).


Why does an apple remain an apple?

Most atoms are not radio active, fortunately. Their neuclei are "stable",i.e. they do not decay. This is why an ordinary object, such as an apple or a watermelon with millions and millions of atoms with stable neuclei, always remains the same. Incidentally, we call a radioactive nucleus unstable because it can decay. When it does decay, both the number of protons and neutrons can change. Will the resulting nucleus be stable? Well, it may be stable or it too may be radioactive. If it is radio active, it may be further decay to form yet another new nucleus, which could be radioactive as well. This is how a "decay series" could occur. Each kind of chemical element in the series changes into the next kind. Ultimately, the series may end with an element such as lead, i.e. not radio active. The atoms in this element are stable, they do not decay. Thus after a sufficiently long time one may find that radium or uranium has completely changed into lead!

A model of scientific method:

Becquerel's working hypothesis was that a body had to luminesce to emit penetrating radiation such as Röö;ntgen had found. His technique was to wrap a photographic plate in light tight black paper, position the mineral on the plate, and leave the experiment on his window sill where sunlight would stimulate the mineral to glow. At a meeting of the Academy of Sciences on 24 February 1896, he claimed success, reporting that several materials in particular, phosphorescent crystals of potassium uranyl sulfate emitted rays that penetrated thick black paper and exposed the photographic plate. This exposure was little more than a smudge. To refine the results and to make them more attractive to others Becquerel also placed coins and other thin, metallic objects under the crystals, producing interesting silhouettes and showing their penetrating power. It must, however,be stated that X-rays, which produced far sharper photographic images in less time were overwhelmingly more popular.

On Wednesday and Thursday, 26 and 27 February, 1896, Becquerel prepared several arrays of crystals and photographic plates. The Parisian winter, however, brought half a week of overcast skies, forcing Becquerel to postpone the experiments;he felt that he needed strong sunlight. The plates rested in a dark drawer until Sunday, 1 March, when Becquerel developed them, "expecting to find very weak images. To the contrary", he wrote in his memoirs, "the silhouettes appeared with great intensity".

The following day, on 2 March 1896, Becquerel reported to the Academy of Sciences that the potassium uranium sulfate crystals could be stimulated to emit the new rays by diffuse daylight hrough a thin cloud cover, as well as by reflected and refracted direct sunlight. He also described using different thicknesses of copper foil to examine the absorption of the rays. But the most astounding result that Becquerel offered was that stimulation of the crystals by sunlight immediately before or during the experiment was apparently not necessarry.

The Uranium, it seemed, was spitting out X-rays all by itself. This, too, was not entirely correct. In fact, the lump of potassium uranyl sulfate was emitting a whole spectrum of radiation, of which only a small portion was X-rays. Nonetheless, the discovery caused a sensation, in part because it was so easy to duplicate. Almost every laboratory in the world had construction paper,photographic plates, and chunks of uranium ore. Within weeks, scientists across the Continent were looking in astonishment at the blurred black patches on their photographs making Becquerel a celebrity (Figure).

Within weeks, news of Becquerel's findings had spread to Germany, Great Britain, Italy, and the United States, further exciting researchers already stirred by the discovery of X-rays. Tests of the two phenomena were often conducted on the same workbench. The consequences of each discovery, however, were far different. X-rays were found to be simply pulses of light light of an intensity and power never before seen, but light nonetheless. Radioactivity, on the other hand, was something entirely new, something that did not fit anywhere. The existence of radioactivity metal that somehow shot out energy! was a direct attack on the most ardent beliefs of Becquerel and his colleagues. When the strange behaviour of uranium was first noted, Becquerel wrote in his memoirs, "There was no reason to presume that the phenomenon was [anything but] a new example of a known type of energy transformation. Contrary to every expectation, the first experiments demonstrated the existence of an apparently spontaneous production of energy" . They had spent many years, those nineteenth century scientists, establishing the law of conservation of energy: Energy was neither created not destroyed. But every single pi