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.

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).

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 piece of uranium seemed of its own accord to produce radiation that fogged photographic plates, electrified gases, and sometimes even burned physicists and the energy needed to do these things evidently came from no place at all. The metal just sat there, its atoms quietly working away, continuously beaming out penetrating rays in seeming disregard for the conservation of energy.

What prompted Becquerel to develop the plates?

But why had Becquerel bothered to develop those plates, which he thought were faintly exposed at best? His behavior has been explained as thoroughness: Jean Becquerel has suggested that his father planned to resume his experiments and wished to use fresh plates, so why to develop the old ones anyway? The explanation (proffered by G.E.M. Jauncey in a 1946 paper in the American Journal of Physics) is "impatience after awaiting four days for the sun to shine". Yet other reasons, suggested, are "simple thrift or an overriding curiosity". We can dismiss the belief that Becquerel planned to resume his experiments on that Sunday: Meteorological records indicate that the day was less sunny than the average of the ceding four days

A better explanation for Becquerel's activity is that he wanted to have sufficient material to report at the next day's session of the academy. In previous experiments he had already found, or so he believed, that weak illumination triggered his crystals somewhat. Perhaps he thought that these newly prepared plates had been exposed to some diffuse daylight, if not a short period of sunlight, before he placed them in the dark drawer. Thus, even if he could not describe many additional experiments, he might furnish evidence of the connection between the intensity of the photographic image and the intensity and duration of phosphorescence.

Another page from Marie Curie's doctoral thesis describing the set-up for measuring the ionisation power of "uranium rays" The method employed consists in measuring the conductivity acquired by air under the action of radioactive bodies; this method possesses the advantage of being rapid and of furnishing figures which are comparable. The apparatus employed by me for the purpose consists essentially of a plate condenser, AB (Figure 1). The active body, finely powdered, is spread over the plate B, making the air between the plates a conductor. In order to measure the conductivity, the plate B is raised to a high potential by connecting it with one pole of a battery of small accumulators. P, of which the other pole is connected to earth. The plate A being maintained at the potential of the earth by the connection CD,an electric current is set up between the two plates. The potential of plate A is recorded by an electrometer, E. If the earth connection be broken at C, the plate A becomes charged, and this charge causes a deflection of the electrometer. The velocity of the deflection is proportional to the intensity of the current, and serves to measure the latter. But a preferable method of measurement is that of complensating the charge of plate A, so as to cause no deflection of the electrometer. The charges in question are extremely weak; they may be compensated by means of a quartz electric balance, Q, one sheath of which is connected to plate A and other to the earth. The quartz lamina is subjected to known tension, produced by placing weights in a plate,T; the tension is produced progressively, and has the effect of generating progressively a known quantity of electricity during the time observed. The operation can be so regulated that, at each instant, there is compensation between the quantity of electricity that traverses the condenser and that of the opposite kind furnished by the quartz. In this way, the quantity of electricity passing through the condenser for a given time, i.e., the intesity of the current, can be measured in absolute units. The measurement is independent of the sensitiveness of the electrometer. (Source Resonance, March 2001) (fig.6). That he found the plates as blackened as they would have been had the crystals phosphoresced continuously, and that he recognized the significance of his surprising observation, shows that the discovery of radioactivity was not simply a happy accident but also a product of genuine scientific talent. Becquerel's example is comforting to us: His genius emerged because he mistakenly believed in a connection between the penetrating rays and phosphorescence, and because he felt compelled to speak at the academy's meeting.

Though a major step, this event does not deserve to be called the discovery of radioactivity. The discovery was a process, not an instantaneous occurrence, for even at this point Becquerel had not sufficiently localized the phenomenon. No doubt Becquerel was a skilled and ingenious experimenter. However, in this early research he was not sufficiently meticulous to exclude extraneous influences and to see that some of his experimental results could bear more than one explanation. Thus, he often concluded that his experiments proved uranium rays to posses a certain physical property, only to have it shown later that the effect was due to another cause. Indeed, his investigations are particularly interesting for their many false trails, unreproducible results and misinterpreted effects.

Yet,his erroneous conclusions inexorably led him to further experiments, which often revealed the true nature of the phenomenon. This uneven progress is perhaps the most striking facet in the story of the discovery of radioactivity. But it must be understood that few scientists are able to avoid false trails.

He recognized that the next step must be to determine if any light at all was necessary to stimulate the crystals. Working in a dark room, he placed different minerals atop photographic plates in an opaque cardboard box. When developed five hours later, the plates showed strong images in samples in which the crystals lay directly on the emulsion and less intense images in those in which the crystals were separated from the emulsion by sheets of aluminum and glass. Besides showing attenuation, the samples involving aluminum and glass also indicated that chemical action was not the explanation for the photographic smudges. Nor could the smudges result from the luminous radiation, because the phosphorescence of uranium salts is perceptible only for about 0.01 second, too short a time to expose a plate.Becqurel therefore suggested that phosphorescent bodies might give off an invisible emission that lasts much longer than the visible radiation.

Even before Rö;ntgen's discovery of X-ray, it had become almost a standard procedure for scientists exploring various types of radiation to perform some of the experiments that Rö;ntgen conducted to determine the properties of X-rays. Becquerel followed suit, as was only logical, because he believed that his own rays were similar to X-rays. He only had to substitute a layer of uranium salts for a cathode-ray tube, for example, to show that the separate gold leaves of an electroscope were made to fall. Having established this electrical property, he next examined whether the rays were reflected and refracted and he claimed they were. This conclusion, however, would be corrected by Rutherford some three years later.

Through March and the succeeding months of 1896, Becquerel found that those crystals kept in darkness retained their ability to expose a photographic plate. Surely, he felt, this was a remarkable example of long-lived phosphorescence. But he was at a loss to explain the equally intense images produced by non-phosphorescent uranous sulfate. This discovery led him on a new path of investigation. Since uranium nitrate ceases to luminesce when dissolved or melted in its water of crystalization, Becquerel, in darkness, heated a crystal in a sealed glass tube, protecting it even from the light of the alcohol flame. He then allowed it to recrystallize in darkness. All phosphorescence had been destroyed in this process, yet the salt still produced results on a photographic plate as strong as crystals exposed to light. Indeed, Becquerel admitted the anomalous behaviour of his samples: All salts of uranium emitted the invisible radiation, while other phosphorescent bodies did not. Finally, he tried a disk of pure uranium metal and found that it produced penetrating radiation three to four times as intense as that he had first seen with potassium uranyl sulfate. With this last announcement, on May 18, 1986, Becquerel's discovery of radioactivity was complete, although he continued with ionization studies of his penetrating radiation until the following spring. The new rays emerged from the element uranium, and with the implicit consequence that this was an atomic phenomenon, it may be said that the process of the discovery of radioactivity was essentially over. It was a process that took several months, notable for a number of conclusions that were later overturned!

Enter Marie and Pierre Curie
Marie Curie (1867-1934) leaped into this exciting new field. She soon discovered at roughly the same time that Becquerel and Ernest Rutherford (1871-1937)(fig8) did that the radiations given off by uranium were composed of more than one type. Some rays were bent one way by a magnetic field; others were bent another way. Rutherford named the positively charged rays alpha rays and the negatively charged ones beta rays (also known as alpha particles and beta particles). Exactly what these rays or particles were composed of, no one knew, but by 1898 Marie Curie suggested a name for these radiations radioactivity and that is the name that stuck. And in 1900, Paul Ulrich Villard discovered a third, unusually penetrating type of ray in radioactive radiation, one that did not bend at all n a magnetic field, which he named the gamma ray. The use of Greek letters to name these rays simply meant that their identity was unknown, as with the X in X-ray. The Law of Exponential Decay Rutherfod and Soddy observed in 1902 that the activity of a radioactive element was diminishing in an exponential or (logarithmic) manner. This implied that the rate of decay of an active species, that is, the number of atoms that disintegrate in a unit interval of time, is proportional to the total number of atoms of that species present at that time. If we suppose that at a given instant, there are N atoms present of a particular radioelement, the rate of disintegration is represented by dN/dt. Since the rate of disintegration is proportional to the total number of atoms N, the relationship between the two, following methods of simple calculus, can be written as

- dN/dt = N

where l is is a constant which Rutherford and Soddy called the "radioactive constant". It is now referred to as the disintegration constant or the decay constant of the element under consideration. The negative sign is due to the fact that the number of atoms of the radioactive element decreases with time, and hence the rate dN/dt is a negative quantity. The value of l depends on the property of a given radioelement and is independent of the physical condition or state of chemical combination. In an equivalent exponential form, the above equation yields the result,

N t = N 0 e - lt

where N 0 is the number of atoms present at any arbitrary zero time and N t is the number remaining after the lapse of a further time t. Another constant introduced by Rutherfod in 1904, called the "half-life" is the time required for the radioactivity of a given amount of the element to decay to half its initial value, that is, when half of the N0 atoms present at the zero time have decayed.

Marie Sklodowska, a Polish girl came to Paris at the Sorbonne University to study physics and mathematics and qualified with honours and distinction. She married Pierre Curie (1859-1906) of the same university in 1895. Pierre was already famous for his discovery of piezo electricity - a property shown by some crystals such as quartz of developing an electrical voltage between opposite ends when subjected to pressure. Marie Curie used the discovery of her husband (see Box) to measure radioactivity. Radioactive rays, like X rays, ionized any gas they passed through (including air) making it capable of conducting electricity. She found that she could measure the current so conducted with a galvanometer and offset it with the potential of a crystal under pressure. By measuring the amount of pressure it took to balance the current, she could obtain the reading of the intensity of the radioactivity. She systematically tested radioactive salts and succeeded in showing that the degree of radioactivity was in proportion to the amount of uranium in the radioactive material thereby narrowing the source of the radioactivity in her samples down to uranium. Then in 1898 she made yet another find: the heavy element thorium was also radioactive. It was already known that natural pitchblende is three or four times more active than uranium. Even more interesting is the fact that as Marie was working to separate uranium out of pitchblende, she found that the residues she produced had a much higher measurement of radioactivity than the uranium content alone could account for. Since the other minerals present in the ore were not radioactive, that could mean only one thing. Some other radioactive element, in amounts too small to detect, must also be present! By this time, Marie's work had developed so much potential that her husband Pierre joined her to help with the backbreaking, tedious work of crystallizing the elements from the ores. Though himself a fine scientist with a successful career, he set his own work aside and spent the remaining seven years of his life assisting her, recognizing both her extraordinary gifts as a scientist and the importance of the path she was following. By July 1898 the two had succeeded. Working together, they had isolated a tiny amount of powder from the uranium ore from the fraction that contained bismuth. It was a new element, never before detected, with a level of radioactivity 400 times higher than uranium. They named the new element polonium, after Marie's home country. But something still seemed strange. The ore still gave off more radioactivity even than the uranium and polonium combined could account for. There must still be something else. In December 1898 they found the answer: another, even more radioactive element obtained from the fraction that contained barium which was 900 times more active than uranium. This one they named radium (from Latin radius meaning ray). Marie and Pierre could not really offer a good description of new element radium because the amount they were able to derive from the ore they had was so minuscule. They could measure its radiations, and Eugene Demarcay, a specialist in elemental line spectra, was able to provide the spectral characteristics. (Different elements give off different wavelengths of electromagnetic radiation or light, and these can be observed as discrete lines.) The next project was to produce a large enough quantity of radium that they could weigh it and measure it and see it. For this, they required a much bigger laboratory and financial resources which Sorbonne University could not provide. Undaunted by the circumstances, they set to work in a make-shift laboratory housed in a neighbouring abandoned court-yard. Through the courtesy of the Academy of Sciences, Vienna, they managed at a reasonable cost, stacks of the required ore pitchblende. The new laboratory was damp with a leaky glass roof, walls made of card-boards, a few tables knocked together as the work tables, a gas stove and no exhaust to remove noxious fumes arising from the work upto 20 kg of the ore every day. It was a back breaking, hazardous and almost suicidal adventure with no help coming from any quarters. They spent their life savings to obtain large masses of waste ore from a nearby mine, and they began the monumental task. They spent four years, during which Marie lost 5 pounds, purifying and repurifying the ore into small amounts of radium, say, about 0.1 gram. Marie Curie wrote her doctoral dissertation on the subject in 1903, for which she, Pierre and Henri Becquerel shared the Nobel Prize in physics that year. In 1906, two years after receiving an appointment as professor of physics at the Sorbonne, Pierre Curie was run over by a horse-drawn truck at the age of 47. Marie was appointed in his place and she became the first woman to teach physics at the Sorbonne. Eight years after Pierre's death (1914), she received another Nobel for her discovery of two new elements, viz. Polonium and Radium, this time in chemistry and this time alone, Pierre - her partner and collaborator - no longer at her side. Years later, in 1935 to be precise, their daughter Irene and her husband Frederic Joliot-Curie - the second husband and wife team - were awarded the Nobel Prize in chemistry for their discovery of artificial radioactivity.

Parent Transforms Into a Daughter

When Ernest Rutherford (1871-1937) arrived on scholarship at the Cavendish Laboratory from his native in New Zealand, he was 24 years old, a large dark haired man with strong opinion, plenty of ambition and no money. He was tireless in every quest and loved his role as one who put endless questions to nature. A consummate experimentalist, Rutherford had a talent for designing experiments and an uncanny ability to pick out one significant fact from a mass of confusing detail. Rutherford began by examining the Becquerel rays from uranium. Indeed, until about 1904, the emission from radioactive elements received far more attention than the emitters. Passage of the radiation through foils revealed one type that was easily absorbed and another with greater penetrating ability, which Rutherford named alpha and beta. In 1898, he took up the professorship of physics at the McGill University, Montreal, the authorities of which were convinced by Sir J.J. Thomson's testimonial that said, "I have never had a student with more enthusiasm or ability for original research than Mr. Rutherford". While in Cambridge, Rutherford's work in radioactivity was solely with uranium salts; in Montreal his first inclination was to examine thorium substances, since the activity of these substances had been noticed only a few months earlier. With passage of time, the number of radio elements had increased. Rutherford added several more to the list. Rutherford was joined by Frederick Soddy (1877-1956)(fig.10) in May 1900. Soddy was the youngest son of a London merchant and was raised in the Calvinist tradition by his dominant half-sister. Before coming to McGill, he was engaged in independent chemical research at Oxford. Soddy joined with Rutherford in a series of investigations which produced the theoretical explanation of radioactivity. Rutherford and Soddy proposed in 1902 the theory of "radioactive disintegration". They suggested that the atoms of radio elements, unlike those of inactive elements, undergo spontaneous disintegration with the emission of alpha or beta particles and the formation of atoms of new elements. In their words: "The disintegration of the atom and the expulsion of a charged particle leaves behind a new system lighter than before and possessing physical and chemical properties quite different from those of the original parent element. The disintegration process once started, proceeds from stage to stage with measurable velocities in each case". It appeared to Rutherford and Soddy that the activity was diminishing in what the mathematicians call an exponential (or logarithmic) manner. This would mean that the rate of the decay of an active species, i.e. the number of atoms which disintegrate in a unit interval of time, is proportional to the total number of atoms of that species present at that time. Given the disintegration is taking place continuously, the number of atoms present is changing, and so also is the rate of disintegration (see Box: How the radioactive elements decay).

In 1903, Soddy joined Sir William Ramsay (1852-1916)(fig.9), the great chemist best known for the discovery of the inert gaseous elements. They experimentally confirmed the prediction of Rutherford and Soddy that disintegration of radium would continously produce helium (or what was termed alpha particle earlier). It may be of interest to note that helium was first detected in the Sun way back in 1868 by the French Astronomer Pierre J. Janssen (1824-1907) while studying a total solar eclipse in the tobacco fields of Guntur, Andhra Pradesh. Yet another important contribution by Soddy was that of the existence of "isotopes". It turned out that decay of several species of radioactive elements disintegrated to products which had identical chemical properties but they differed in their atomic weights. Soddy called them isotopes (from the Greek, iso-same and topos-place). Today we know that such atoms have the same number of protons (i.e. the atomic number) but may have different number of neutrons. Further studies established that the phenomenon of radioactivity involved emission of alpha particles (helium nuclei), beta-rays (electrons) or gamma rays (electromagnetic radiation like X-rays, but of very short wave-length) transforming the "parent" nucleus into a "daughter" nucleus.

Alpha particles help Unravel the Structure of the Atom

The discovery of radioactivity shattered the age old ideas about the indivisibility of atoms. It also helped develop our understanding about their structure. In 1908, Rutherford had returned to England, to the University of Manchester, where a young German physicist named Hans Geiger teamed up with him. Together, they bombarded thin pieces of gold foil with alpha particles from radium. Based on his discovery of the electron, Sir J.J. Thomson (1856-1940) had suggested in 1898 that atoms were spheres of positively charged matter with negatively charged electrons embedded in them, something like "raisin in poundcake". However, the results of the scattering of alpha particles from the gold atoms (1911) suggested that atoms consisted of a tiny positive nucleus with electrons circling outside it - somewhat similar to our solar system. Most of the atom's mass would be contained in the nucleus, and an equal number of negatively charged electrons would be found in motion around it. Rutherford's idea about the atomic nucleus was indeed extra-ordinary, for which he earned the title "the Newton of physics".

Making Stable Atoms Unstable

In 1934, Irene Joliot Curie (1897-1956) and Frederic Joliot (1900-1958) discovered "artificial" radioactivity. In the course of a study of the effect of alpha particles from the naturally occuring radioelement polonium (discovered) by Irene's parents Marie and Pierre Curie), on the nuclei of aluminium in particular, Irene and Frederic found that after the source of alpha particles was removed, the aluminium foil on which alpha particles were irradiated became radioactive and followed the exponential law of radioactive decay described earlier. The same phenomenon was observed with boron and magnesium as well. The "transmuation" of boron, magnesium and aluminium by alpha particles had given birth to new radioelements! These new elements not found in nature, would then be the radioactive species which decays with the emission of a positron (i.e. a positive electron) or what is also called beta decay. An alpha particle bombarded on an aluminium atom would transform it into a phosphorus atom, which being unstable, would in turn emit a positron to transform into a stable, naturally occuring isotope of silicon. The discovery of artificial radioactivity opened up altogether a new field. Man-made Radioisotopes found applications in industry, biology, health, agriculture, archaeology and many other areas of human activity.
Legacy of radioactivity

The discovery of radioactivity evolved into the study of nuclear physics almost a century ago. The applications that have flowed from the work of Becquerel and others are primarily nuclear medicine, nuclear reactors, and nuclear weapons. In the field of medicine, diagnostic procedures such as tracer techniques, therapeutic applications like the treatment of cancer by radiation, have proved to be highly valuable. For example, gamma rays from an isotope of cobalt are used in the treatment of cancer. The radioactivity of isotopes of elements used as tracers enables the scientists to locate them, even inside a living body. One isotope of carbon, known as carbon-14 (i.e. with 6 protons and neutrons) has a sufficiently long half-life (that is, the time in which half of the initial number of carbon-14 atoms would decay in a given sample containing this isotope). It is present in the bones and woods. When an archaeologist digs them up, a comparison of radioactivity of carbon-14 present in them with that in the living specimens allows him to estimate their age.

But, the future of the nuclear reactors and weapons appears to be problematic. This is so, because the nuclear reactors are always fraught with possible meltdowns (remember the Three Mile Island and the Chernobyl disasters?), and the skyrocketing costs. Nuclear weapons, too, seem to share similar concern. No doubt, after a hundred years of the discovery of radioactivity, we now face even newer challenges through its all pervading influence.

References

1. The discovery of radioactivity Lawrence Badash Physics Today, February 1996 .A well researched article written in a lucid style. The present article draws heavily on it.
   
2. The Second Creation Robert P. Crease and Charles C. Mann Affiliated East-West Press Pvt. Ltd. 1986 A wonderful resource book. A must for anyone interested in knowing about the makers of the twentieth century physics.
   
3. The History of Science from 1895-1945 Ray Spangenburg and Diane K. Moser Universities Press (India) Ltd. 1999 Highly readable. A set of five volumes on history of science from the ancient Greeks until 1990s.
   
4. Uranium Glenn Seaborg Encyclopaedia Britannica (1957) Vol. 22
   
5. Sourcebook on Atomic Energy Samuel Glasstone Affiliated East-West Press Pvt. Ltd. 1969 A superb and invaluable resource on history, atomic and nuclear phenomena, and applications of atomic energy.
   
6. Resonance (March 2001), page 97 A monthly journal of science education published by Indian Academy of Sciences, Bangalore.

Radioactivity:

Important terms used in connection with radioactivity are given below. The terms given do not necessarily appear in the present article.

Alpha particle : Charged particles emitted from a radioactive atom. Each charged particle consists of two protons and two neutrons. Atom : This is the smallest unit of an element. It contains a nucleus with neutrons and protons, surrounded by orbiting electrons. Atomic mass : The mass of an atom usually expressed as atomic mass unit (amu).

Beta particle : (often designated beta rays) Charged particles emitted from a radioactive atom. These particles are identical except for their charge. The charge is classified as positive (positron) or negative (electrons or negatron).

Carbon-14 : A naturally occuring radio isotope of carbon having a mass number of 14 and half-life 5780 years. Used in Radio carbon dating for determination of age of ancient objects.

Cathode rays : Electrons originating at the cathodes of gaseous discharge devices. These electrons are often focused in a small area such as a tube and intensified on a surface. The most familiar form of a cathode-ray tube is the television picture tube.

Conductivity : The ratio of electric current to the electric field in a material. Passage of electric charge which can occur a variety of ways such as passage of electrons or ionized atoms.

Curie : A unit of radioactivity, defined as that quantity of any radioactive nuclide which has 3700 X 1010 disintegrations per second.

Deuterium : The isotope of element hydrogen with one neutron and one proton in the nucleus.

Electrons : A negative charged particle that orbits the nucleus of an atom. It is lighter in weight than a proton or neutron.

Elements : An element is a substance made up of atoms with the same atomic number. 75% of the elements are metals and the others are nonmetals. A few examples are oxygen, iron, gold, chlorine, and uranium.

Fluorescence : Electrons absorb energetic radiation (for example ultraviolet light) raising an electron to a higher "Bohr" orbit. The energized electron soon drops down in a series of steps through lower energy states and in the process releases photons at lower energy states corresponding to visible light. The bright color occurs because the photons are concentrated in a narrow range of wavelengths.

Geiger counter : A radiation counter that uses a Geiger-Mésarller tube in appropriate circuits to detect and count ionizing particles, each particle crossing the tube produces ionization of gas in the tube which is roughly independent of the particle's nature and energy resulting in a uniform discharge across the tube. Also known as Geiger-Mésarller Counter.

Geiger-Mésarller tube : A radiation-counter tube usually consisting of a gas-filled cylindrical metal chamber containing a fine-wire anode at its axis. Also known as Geiger-Mésarller Counter tube.

Half-life : The period of time it takes for half the nuclei of a radioactive element to undergo decay to another nuclear form.

Heavy water : A compound of hydrogen and oxygen containing a higher proportion of the hydrogen isotope deuterium than does naturally occuring water.

Ionization chamber : A particle detector which measures the ionization produced in the gas filling the chamber by the fast-moving charged particles as they pass through.

Isotope : An atom having the same number of protons in its nucleus as other varieties of the element but has a different number of neutrons.

Magnetic field : All magnetic fields are created by moving electric charge. The single moving electron around a nucleus is a tiny electric current. These orbiting electrons create magnetic fields and their net effect is to provide the atom with a magnetic field.

Neutron : A particle with no charge that is located in the nucleus of an atom.

Nuclear physics : A branch of physics that includes the study of the nuclei of atoms, their interactions with each other, and with constituent particles.

Nucleus : The central part of every atom that contains protons and neutrons.

Nuclide : A species of atom characterized by the number of protons, number of neutrons, and energy content in the nucleus, or alternatively by the atomic number, mass number, and atomic mass. To be regarded as a distinct nuclide, the atom must be capable of existing for a measurable life time. Also known as nuclear species.

Pitchblende : A brown to black fine grained, amorphous, variety of uraninite which has a dull luster and contains small quantities of uranium. Also called pitch ore or nasturan.

Phosphorescence : Luminescence that persists after a light source has been removed. Materials such as phosphors or phosphorogens are activated from a light source to emit the light in the form of photons of light.

Polonium : A naturally radio active chemical element, Po, atomic number 84. It is used in photographic film to reduce the static charge.

Proton : A positively charged particle that is located in the nucleus of an atom.

Radiation effects : The harmful effects of ionizing radiation on humans and other animals, such as production of cancer, cataracts, and radiation ulcers, loss of hair, reddening of skin, sterilization, nausea, etc.

Radioactive contaminant : A radioactive material which has spread to places where it may harm persons, spoil experiments, or make products or equipment unsuitable or unsafe for consumption by living beings, or for some specific purpose.

Radioactive decay : The spontaneous transformation of a nuclide into one or more different nuclides, accompanied by either the emission of particles from the nucleus, nuclear capture or ejection of electrons. Also known as radioactive transformation, radioactive disintegration or radioactivity. Radioactive element : An element all of whose isotopes spontaneously transforms into one or more different nuclides, giving off various types of radiations, examples include uranium, radium and thorium.

Radioactive emanation : A radiactive gas given off by certain radioactive elements, all of these gases are isotopes of the element radon. Also known as emanation.

Radioactive waste : Liquid, solid, or gaseous waster resulting from mining of radioactive ore, production of reactor fuel materials, reactor operations, processing of irradiated reactor fuels, and from use of radioactive materials in research, industry, and medicine.

Radioactive waste disposal : The disposal of waste radioactive materials and equipment contaminated by radiation: the two basic disposal methods are concentration for burial underground or in the sea, and dilution for controlled dispension: reprocessing of reactor fuel is a major source of radioactive waste.

Radioactivity : A behaviour of an element in which nuclei are undergoing change and emitting particles. This occurs naturally in approximately fifty elements. It can be produced artificially.

Radioactivity equilibrium : A condition which may arise in the decay of a radioactive parent with short-lived descendants, in which the ratio of the activity of a parent to that of a descendant remains constant.

Radiochemistry : Area of chemistry concerned with the study of radioactive substances.

Radio isotope : An isotope which exhibits radioactivity. Also known as radioactive isotope.

Radio isotope assay : An analytical technique including procedures for separating and reproducibly measuring a radioactive tracer.

Radiometric dating : A technique for measuring the age of an object or sample of material by determining the ratio of the concentration of a radio isotope to that of a stable isotope in it: for example, the ratio of carbon-14 to carbon-12 reveals the approximate age of bones, pieces of wood, and other archaeological specimens. Also known as radioactive age determination or radiogenic dating.

Radium : A naturally radio active chemical element, Ra, that has an atomic number 88. It is used as a source of neutrons alpha particles.

Thorium : A naturally radio active chemical element, Th, that has an atomic number 90. It is used as fuel in nuclear reactors called the "breeder" reactors. Thorium is available in India on the Kerala coast in the form of monzile sand.

Tracer : A foreign substance, usually radioactive, that is mixed with or attached to a given substance so the distribution or location of the latter can later be determined; used to trace chemical behaviour of a natural element in an organism. Also known as tracer element.

Transmutation : A nucleus process in which one nuclide is transformed into the nuclide of a different element.

Tritium : The hydrogen isotope having mass number 3. It is one form of heavy hydrogen.

Uranium : A chemical element, U, that has an atomic number 92. It reacts with nearly all nonmetals and is used as fuel for nuclear reactors, available in the mineral form at Jeduguda in Bihar.

X rays : Invisible electromagnetic radiation with wavelengths shorter than visible light. X rays are produced when high energy charged particles collide with other charged particles or with atoms.