Elements and Atoms: Chapter 17
Discovery of Radioactivity: Becquerel

Antoine Henri Becquerel (1852-1908; view photo at Access Excellence, at the National Health Museum) was the third of four Becquerels to hold a chair of physics at the Paris Museum of Natural History and to belong to the Academy of Sciences, following his grandfather Antoine César, his father Alexandre Edmond, and preceding his son Jean. Like his father and grandfather, this Becquerel was an expert on phosphorescent minerals. He is best known for his discovery of radioactivity, first reported just over a century ago in the selection reproduced here.[1]

Neither Becquerel nor anyone else at the time had any idea of the nature of the "radiations" he reported. In fact, they were fragments ejected from uranium atoms as those atoms fell apart. The discovery of radioactivity thus represents a key development in the modern understanding of the atom--in particular in the knowledge that atoms of elements are not indestructible--even though its implications were not immediately recognized. Becquerel shared the 1903 Nobel Prize in physics with Marie and Pierre Curie for their work on radioactivity.

On the rays emitted by phosphorescence

Comptes Rendus 122, 420-421 (1896);[2] translated by Carmen Giunta

In an earlier session, M. Chairman Henry announced that phosphorescent zinc sulfide[3] placed in the path of rays emanating from a Crookes tube[4] augmented the intensity of rays passing through the aluminum.

Elsewhere, M. Niewenglowski recognized that commercial phosphorescent calcium sulfide emits rays which pass through opaque bodies.[5]

This fact extends to various phosphorescent bodies, and in particular to uranium salts whose phosphorescence has a very brief duration.

With potassium uranyl double sulfate[6], of which I have a few crystals forming a thin transparent crust, I was able to perform the following experiment:

One wraps a Lumière photographic plate with a bromide emulsion[7] 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.[8] 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 can repeat the same experiments placing a thin pane of glass between the phosphorescent substance and the paper, which excludes the possibility of chemical action due to vapors which might emanate from the substance when heated by the sun's rays.[9]

One must conclude from these experiments that the phosphorescent substance in question emits rays which pass through the opaque paper and reduces silver salts.[10]

On the invisible rays emitted by phosphorescent bodies

Comptes Rendus 122, 501-503 (1896); [11] translated by Carmen Giunta

In the previous session, I summarized the experiments which I had been led to make in order to detect the invisible rays emitted by certain phosphorescent bodies, rays which pass through various bodies that are opaque to light.

I was able to extend these observations, and although I intend to continue and to elaborate upon the study of these phenomena, their outcome leads me to announce as early as today the first results I obtained.[12]

The experiments which I shall report were done with the rays emitted by crystalline crusts of the double sulfate of uranyl and potassium [SO4(UO)K+H2O] [13], a substance whose phosphorescence is very vivid and persists for less than 1/100th of a second. The characteristics of the luminous rays emitted by this material have been studied previously by my father, and in the meantime I have had occasion to point out some interesting peculiarities which these luminous rays manifest.

One can confirm very simply that the rays emitted by this substance, when it is exposed to sunlight or to diffuse daylight, pass through not only sheets of black paper but also various metals, for example a plate of aluminum and a thin sheet of copper. In particular, I performed the following experiment:

A Lumière plate with a silver bromide emulsion was enclosed in an opaque case of black cloth, bounded on one side by a plate of aluminum; if one exposed the case to full sunlight, even for a whole day, the photographic plate would not become clouded; but, if one came to attach a crust of the uranium salt to the exterior of the aluminum plate, which one could do, for example, by fastening it with strips of paper, one would recognize, after developing the photographic plate in the usual way, that the silhouette of the crystalline crust appears in black on the sensitive plate and that the silver salt facing the phosphorescent crust had been reduced. If the layer of aluminum is a bit thick, then the intensity of the effect is less than that through two sheets of black paper.

If one places between the crust of the uranium salt and the layer of aluminum or black paper a screen formed of a sheet of copper about 0.10 mm thick, in the form of a cross for example, then one sees in the image the silhouette of that cross, a bit fainter yet with a darkness indicative nonetheless that the rays passed through the sheet of copper. In another experiment, a thinner sheet of copper (0.04 mm) attenuated the active rays much less.

Phosphorescence induced no longer by the direct rays of the sun, but by solar radiation reflected in a metallic mirror of a heliostat,[14] then refracted by a prism and a quartz lens, gave rise to the same phenomena.[15]

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, 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.[16] I immediately thought that the action had to continue in darkness, and I arranged the following experiment:

At the bottom of a box of opaque cardboard I placed a photographic plate; then, on the sensitive side I put a crust of the uranium salt, a convex crust which only touched the bromide emulsion at a few points; then, alongside, I placed on the same plate another crust of the same salt but separated from the bromide emulsion by a thin pane of glass; this operation was carried out in the darkroom, then the box was shut, then enclosed in another cardboard box, and finally put in a drawer.

I did the same with the case closed by a plate of aluminum in which I put a photographic plate and then on the outside a crust of the uranium salt. The whole was enclosed in an opaque box, and then in a drawer. After five hours, I developed the plates, and the silhouettes of the crystalline crusts appeared in black as in the previous experiments and as if they had been rendered phosphorescent by light.[17] For the crust placed directly on the emulsion, there was scarcely a difference in effect between the points of contact and the parts of the crust which remained about a millimeter away from the emulsion; the difference can be attributed to the different distance from the source of the active rays. The effect from the crust placed on a pane of glass was very slightly attenuated, but the shape of the crust was very well reproduced. Finally, through the sheet of aluminum, the effect was considerably weaker, but nonetheless very clear.

It is important to observe that it appears this phenomenon must not be attributed to the luminous radiation emitted by phosphorescence, since at the end of 1/100th of a second this radiation becomes so weak that it is hardly perceptible any more.

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[18] and M. Röntgen, are invisible rays emitted by phosphorescence and persisting infinitely longer than the duration of the luminous rays emitted by these bodies.[19] 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.


[1]See Habashi 2001 for a brief account of an earlier description of the action of a uranium compound on photosensitive paper. Claude Niepce de Saint-Victor, a cousin of photographic pioneer Joseph Nicéphore Niepce, described his observations to the French Academy in 1858. [Niepce de Saint-Victor 1858]

[2]Read before the French Academy of Sciences February 24, 1896.

[3]Phosphorescent crystals glow when exposed to light. Cathode ray research in general and the discovery of X-rays in particular (See next footnote.) led to increased interest in the phenomenon of phosphorescence.

[4]William Crookes was a productive researcher and highly original and speculative thinker in many areas of physics and chemistry, including electrical discharges in vacuum tubes. (See chapter 14, note 29.) Indeed, a certain design of cathode ray tube was known as "Crookes tube."

These tubes were also known as cathode ray tubes because of the "rays" emitted from the cathode of such a tube when electricity was passed through it. Cathode ray research was a hot topic in late 19th century physics. (See previous chapter.) In the course of trying to learn the nature of the cathode rays, Wilhelm Röntgen (1845-1923; see photo at Access Excellence) had discovered in late 1895 that the tubes emit a kind of ray which penetrates opaque objects such as flesh or thin sheets of light metals (such as aluminum). [Röntgen 1895] These rays are now known to most of the world as X-rays. At the time, the X-rays seemed to be associated with another phenomenon of cathode rays, namely the glow (phosphorescence) caused when cathode rays struck the glass of a vacuum tube. The apparent association proved to be illusory: phosphorescence and X-rays are essentially unrelated phenomena with quite different causes.

[5]The discovery of X-rays led to increased interest in phosphorescence; for example, researchers wondered if penetrating rays associated with phosphorescence existed in the absence of cathode rays. Several reports to the French Academy of penetrating rays from fluorescent minerals preceded and followed Becquerel's [Henry 1896, Niewenglowsky 1896, Troost 1896]. These reports, involving calcium sulfide and zinc sulfide, could not be reproduced [Curie 1903], for calcium, zinc, and sulfur are not radioactive elements. In the wake of discovery of a new phenomenon (in this case X-rays), scientists frequently look for other occurences of that phenomenon. Sometimes they even think they see it when it is not present. See, for example, chapter 6, note 15.

[6]The formula is K2UO2(SO4)2.2H2O. Uranium (U) was the heaviest element known at that time (discovered in 1789), and it is still the heaviest naturally occurring element.

[7]Photographic technology has come a long way in the last century. The plate Becquerel describes was the photographic film of the time. Instead of today's thin and flexible film--not to mention filmless digital photography!--photographers of the 19th century had to deal with plates of glass onto which an emulsion of light-sensitive chemicals was fixed. With "film" like that, it is no wonder that the photographic apparatus of the time was bulky and cumbersome!

[8]Like some other researchers Becquerel wondered whether penetrating rays associated with phosphorescence existed in the absence of cathode rays. He was looking to see if such penetrating rays were produced at the same time as they glowed with visible light.

[9]Let us summarize the observations up to this point by listing the conditions of various experiments and the observations recorded:

  1. photographic plate (film) wrapped in paper placed in sun for hours --> no image;
  2. crystals on wrapped film in sun for hours --> black image of crystals on photo negative;
  3. crystals on coin or screen on wrapped film in sun for hours --> black image of crystals on negative with white image of coin or screen (Becquerel is not explicit on this point, but the image of the coin or screen must have been white to be seen in contrast to the black image of the crystals.);
  4. crystals on glass sheet on wrapped film in sun for hours --> same as (2) above;
  5. crystals on coin or screen on glass sheet on wrapped film in sun for hours --> same as (3) above.

[10]Becquerel writes of reducing silver salts because that is the chemistry involved in forming a photographic image. The photographic plates at the time contained silver salts (silver bromide, AgBr, for example), which are sensitive to light. When these salts come into contact with light, the silver in the whitish AgBr is changed ("reduced") to darker metallic silver. Thus, the dark spots on the negative are those spots which were exposed to light.

It is worthwhile to take stock of Becquerel's conclusions to this point. The crystals are the source of something ("radiations") which produces images on the film, for the wrapped film without the crystals bears no images. Whatever causes the images can pass through thick black paper and glass, but not through metal. Because it can pass through the paper, it cannot be identical to visible light. Because it can pass through glass, it cannot be a reactive vapor. So far so good. Now consider the evidence for the sun's role. The crystals were exposed to the sun in all the trials so far, because Becquerel knows that the sun is what stimulates phosphorescence in the crystals.

[11]Read before the French Academy of Sciences on March 2, 1896.

[12]Note the date of the paper and the dates given below when the experiments were done. Becquerel found something unusual within the past week, and he reports preliminary results at the next weekly meeting of the French Academy of Sciences. He knew that he had found something new and unusual, and he wasted no time in announcing it.

[13]As noted above, the formula is actually K2UO2(SO4)2.2H2O. Notation for chemical formulas by the end of the 19th century was similar to present conventions, but not identical everywhere. Notice the superscript numbers in this French journal, compared to subscripts in British journals (chapters 13 and 14 for instance).

[14]A heliostat is a mechanical device which turns to point continuously at the sun.

[15]In the previous two paragraphs, Becquerel recapitulates the results he had announced the previous week. This paragraph describes a new experiment, but no surprising results: the light believed to be necessary to cause phosphorescence need not be direct sunlight, but can be reflected or refracted.

[16]Becquerel was surprised by the results of this accidental "experiment." He had set up the crystals, screen, and wrapped film as usual, but since the day was not sunny, he put the whole setup in a drawer for several days. When he developed the film, he expected a weak image if any; instead he got a very intense image. (View an image of exposed photographic plates from Becquerel's experiments, at the ChemTeam site.) To summarize (in terms similar to the summary in note 9): crystals on screen on wrapped film for days --> intense black image of crystals on negative with white image of screen.

[17]Recall that experiments require observations made under carefully controlled conditions. In this experiment, Becquerel took pains to minimize the effects of light and to control the time when the crystals were near the plate. Neither of these conditions was controlled in the accidental "experiment" described above: the apparatus had been exposed to whatever diffuse sunlight could penetrate a cloudy winter sky in Paris, and the crystals were near the plate much longer than in the previous experiments. The controlled experiments confirm the impression suggested by the "accidental" experiment: sunlight is not needed to produce the images.

It is sometimes said that the discovery of radioactivity occurred by accident. This is true insofar as cloudy weather and the development of plates Becquerel expected to be blank provided the first clue that sunlight was not needed to produce these rays. It is false insofar as it implies that the accidental experiment provided definitive proof of the new rays; that required Becquerel's controlled follow-up experiment.

[18]Philipp Eduard Anton Lenard was awarded the 1905 Nobel Prize in physics for his experiments on cathode rays.

[19]Becquerel speculates that the penetrating rays may be caused by light striking the crystals. If so, the penetrating rays persist for much longer than does the emission of visible light. The speculation turns out to be incorrect: the "luminous rays" of visible phosphorescence and the penetrating rays of radioactivity are unrelated. The hypothesis was a reasonable one for a scientist with Becquerel's expertise, for he would have known that some crystals have long-lasting visible phosphorescence; that is, some crystals phosphoresce for minutes after the light which stimulates the phosphorescence is removed. It is natural for a scientist's hypotheses to stem from his or her experience.


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