Henri Becquerel was born into a family of scientists. His grandfather had made important contributions in the field of electrochemistry while his father had investigated the phenomena of fluorescence and phosphorescence. Becquerel not only inherited their interest in science, he also inherited the minerals and compounds studied by his father.And so, upon learning how Wilhelm Roentgen discovered X rays from the fluorescence they produced, Becquerel had a ready source of fluorescent materials with which to pursue his own investigations of these mysterious rays.  The material Becquerel chose to work with was potassium uranyl sulfate, K2UO2(SO4)2, which he exposed to sunlight and placed on photographic plates wrapped in black paper. When developed, the plates revealed an image of the uranium crystals. Becquerel concluded "that the phosphorescent substance in question emits radiation which penetrates paper opaque to light." Initially he believed that the sun's energy was being absorbed by the uranium which then emitted X rays.  Further investigation, on the 26th and 27th of February, was delayed because the skies over Paris were overcast and the uranium-covered plates Becquerel intended to expose to the sun were returned to a drawer. On the first of March, he developed the photographic plates expecting only faint images to appear. To his surprise, the images were clear and strong. This meant that the uranium emitted radiation without an external source of energy such as the sun. Becquerel had discovered radioactivity, the spontaneous emission of radiation by a material.  Later, Becquerel demonstrated that the radiation emitted by uranium shared certain characteristics with X rays but, unlike X rays, could be deflected by a magnetic field and therefore must consist of charged particles. For his discovery of radioactivity, Becquerel was awarded the 1903 Nobel Prize for physics.

"The Discovery of Radioactivity" , Lawrence Badash, in Physics Today
February, 1996.

Becquerel, a member of the French Academy of Sciences, heard Henri Poincare' describe the results of Roentgen's x-ray work at a regular weekly meeting of the Academy on January 20, 1896.  Poincare' mused on the possibility that the x-rays emanated from the fluorescing end of the glass vacuum tube where the cathode rays struck. Becquerel's interest was immediately drawn to this possibility. He, along with his father, were and had been leading authorities on luminescence. In particular, he had studied the phosphorescent properties of some uranium compounds.  Becquerel was also quite skilled in photography.

Definitions - Luminescence is the emission of visible radiation from bodies. There are two types:

fluorescence - stimulation of light emission by an energy source which disappears when the energy source is removed.

phosphorescence - persistent emission of light from a body that had been subjected to an external energy source. This emission continues after the source is removed.

Becquerel's father, Edmond, had shown that some uranium compounds were phosporesecent, for example, the uranic series, whereas the uranous series was not.  Becquerel's hypothesis was that a luminescing body also emitted the highly penetrating x-rays of Roentgen. He tested this hypothesis over a period of about five months in Winter and Spring of 1896.

"On the Radiation Emitted in Phosphorescence" , H. Becquerel in
Compte rendus de l'Academie des Science, Paris, 1896 122: 420-421
(24 February)

He reports that a photographic plate is wrapped in two sheets of very black paper so that a whole day's exposure to the sunlight does not fog the plate. If a plate of phosphorescent substance is laid on the plate and the whole is exposed to the sunlight for several hours then the plate after being developed shows the outline of the phosphorescent substance. The image of a coin or thin sheet of metal can also be produced if the coin or metal is interposed between the phosphorescent substance and the plate.

"On the Invisible Radiations Emitted by Phosphorescent Substances"
Compte rendus de l'Academie des Science, Paris, 1896 122: 501-503
(2 March)

He continues with his hypothesis that phosphorescent substances emit invisible radiations. For these experiments he uses a sulfate of potassium and uranium  [ K(UO)SO₄ + H₂O ] which is brilliantly luminescent and has a very short persistence of luminescence, 1/100 sec, as shown by his father. As before he had laid the whole arrangement of covered photographic plate and phosphorescent minerals in bright sunlight for several hours and found exposure of the plates to have occurred.

quote from his paper
" I shall particularly insist on the following fact, which appears to me very important and quite outside the range of phenomena one might expect to observe. The same crystalline lamellas ... shielded from excitation by incident radiation and kept in darkness , still produce the same photographic images."

He came upon this discovery because there were several days that were overcast and he could not expose his minerals and plates to the sunlight. He had stored them inside a light tight drawer, assembled as he would want them for an exposure to sunlight. It is remarkable that he developed the plates even after they had been in the dark because he had no reason, based on his hypothesis, to expect the plates to have been fogged. The images he saw in the developed plates were as strong as those he saw in the plates with minerals exposed to sunlight.

"A hypothesis which presents itself rather naturally to mind would be to suppose that these radiations, whose effects  possess a strong analogy with the effects produced by the radiations studied by Lenard and Roentgen, might be invisible radiations emitted by phophorescence, whose duration of persistence might be infinitely greater than that of luminous radiation emitted by these substances."

" On New Properties of the Invisible Radiation Emitted by Various Phosphorescent Bodies"  Compte rendus de l'Academie des Science, Paris, 1896 122: 501-503 (9 March)

Becquerel announces that the penetrating rays, like x-rays, have the ability to discharge electrified bodies. He also believes that the rays could be reflected and refracted like ordinary light. He had also kept some of his phophorescent crystals in the dark for 160 hours without any loss of their power to produce the penetrating rays. He also has narrowed down the list of substances that can produce rays to compounds of uranium. However, he also reported that two specimens of calcium sulfide crystals also produced penetrating rays.

" On the Invisible radiations Emitted by the Salts of Uranium"
Compte rendus de l'Academie des Science, Paris, 1896 122: 501-503
(23 March)

He reports that he has continued the use of the electroscope to study the penetrating rays. He compared the absorption characteristics of the penetrating rays from the "phophorescent" substances and from the x-rays from a Crookes tube. He also discovered that the non-luminous compounds of uranium also emit the penetrating rays.

" ... although the salts of the sequioxide of uranium are very fluorescent, we know that the green uranous salts ... are neither phophorescent nor fluorescent ... uranous sulfate behaves like uranic sulfate, and emits invisible radiations which are equally intense. "

Becquerel also  destroyed the luminscent property of a sample of uranium nitrate. His belief that the radiation emitted from the uranium compounds was due to energy stored in the compound led him to expect that destroying the compound would also destroy the penetrating radiation.

" I shall report still another interesting experiment. It is known that uranium nitrate ceases to be either phophorescent or fluorescent when in solution or melted in its water of crystallization. I then took a crsytal of this salt and, after having placed it in a little tube closed by a thin plate of glass, I warmed it in darkness, managing to avoid even the radiations from the alcohol lamp that heated it. The salt melted, I let it crytallize in darkness and then I placed it on a photographic plate wrapped in black paper, protecting the salt always from the action of light. One might have expected to observe no effect, since all luminous excitation had been avoided from the moment when the substance ceased to be phosphorescent. Nevertheless the impression was as strong as for salts exposed to the light..."

"Emission of New Radiations by Metallic Uranium"
Compte rendus de l'Academie des Science, Paris, 1896 122: 501-503
(18 May)

By May Becquerel was convinced that the only requirement to produce the penetrating rays was the presence of uranium itself. The compounds, luminescent or not, were not the causes of the emissions. He now began tests with metallic uranium.

" ... I have been led to think that the effect was due to the presence in these salts of the element uranium, and that the metal would give more intense effects than its compounds would."

He performed tests using the photographic technique and the electroscope. At this time there was really no other language to describe the phenomenon, other than as a type of phosphorescence. Nevertheless, the energy source for this phophorescence was not identifiable.

" Although I am continuing the study of these new phenomena, I thought it was not without interest to point out the emission produced by uranium, which, I believe, is the first example of a metal exhibiting a phenomenon of the type of an invisible phophorescence."

Becquerel produced more papers on uranium's penetrating rays, but there was really nothing much else that happened until Marie Curie decided to pursue Becquerel's rays as a thesis topic. Many more scientists were interested in the properties and applications of Roentgen's x-rays.

In 1903 Becquerel shared the Nobel prize with Marie and Pierre Curie for the discovery of radioactivity.