Collected by Alfred 6. Garreft
Ohio State University Columbus
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The Flash of Genius, 8
Radioactivity:
This discovery was made by a third generation of Becquerel researchers in the field of fluorescence. Henri Beequerel had assisted his father in his study of the pbosphorescence of uranium and continued the work after his father's death. His discovery of radioactivity in 1896 was the result of a prepared mind which carefully planned a research; he was the victim of an unplanned set of circumstmces, but was equal to the challenge of the unexpected. In 1895 Roentgen observed that cathode rays (in addition to causing fluorescence) either have X-rays associated with them or are transformed into X-rays, when they strike the target in a Crookes tube. The X-rays also cauee materials to fluoresce. Becquerel reasoned that. perhaps light rays could be transferred into deeply penetrating radiations, einee he believed that cathode rays were transferred into X-rays in the Crookes tubes. To.test this theory he planned the followingexperiment:' A photographic plate w a s wrapped in paper to protect it from light; on it was placed a uranium mineral that would fluoresce when exposed to light. After exposure of this assembly of fluorescing mineral and carefully covered photographic plate, the plate was then developed and found to he affected over the spot where the uranium salt had been placed. This result seemed to confirm the anticipated results of the experiment. But here is where an unanticipated event gave unexpected information-the sun did not shine for several days and Becquerel had to postpone further experiments for a short time. So he he had bpen exDerimentine wranned the uranium ore with n-nd laid it away in a drawer on s. wrapped packaie of photographic plates, not expecting the plates to be affected. When he developed theplates, to his amazement he found them streaked in the same manner as in the other experiment in which the sun's and cause it to flur a p bad heen d o w d to strike the oreere. What did this mean? He describes his experiments and conclusions as f~llorvs:~-~
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"With the double sulfate of uranium and potassium, of which I possess crystals in the form of thin transparent crust, I have made the following experiment: "I wrapped a Lnmiere photographic plate with bromized emulsion with two sheets of thick black paper, so thick that the plate did not become clouded by exposure to the sun for a whole day. I placed on the paper a plate of phosphorescent substance, and exposed the whole thing to the sun for several hours. When I developed the photographic plate I saw the silhouette
' BECQUERE~ HENRI, "Sur 18s radiations emises per phosphorescence" in Compt. red,., 122, 420, (1896). An English transl&m is found in "A Source Book in Physics" edited by ~ New York and London, McGrawMacm, W n ~ 1 . 4FRANCIS, Hill Book Company, 1935. MOULTON, FOREST R., Editor, "The Autobiography of Science," Doubleday and Co., 1951,Znd ed. This book contains in edited form the material from Magie's book, "A Source Book in Physice." A. HENRI, "On the Radio-Activity of Matter," ~BECQUEREL, Smilhsonian Institute Repcrt 1902, Smithsonian Institute, Wmhington. "ROCA. ANDRE. "The Work of Henri Becauerel." Smith-
Henri Becquerel
of the phosphorescent substance in hlack on the nega tive. If I placed between the phosphorescent substance and the paper a coin or a metallic screen pierced with an open work design, the image of these objects appeared on the negative. "The same experiments can be tried with a thin sheet of glass placed between the phosphorescent snbstance and the paper, which excludes the possibility for chemical action resulting from the vapors which might emanate from the substance when heated by the sun's rays. "We may therefore conclude from these experiments that the phosphorescent substance in question emits radiations which penetrate paper that is opaqne to light, and reduce silver salts. . . . "I particularly insist on the following fact, which appears to me exceedingly important and not in accord with the phenomena which one might expect to observe: the same encrnsted crystals placed with respect to the conditionsi n d acting photographic plates in the through the same screens, but protected from the excitation of incident rays and kept in the dark, still produce the same photographic effects. I may relate how I was led to make this observation: among the preceding experiments some had been made ready on Wednesday the twenty-sixth and on Thursday the twenty-seventh of February and as on those days the sun had only shown itself intermittently I kept my arrangements all prepared and put back the holders in the dark in the drawer of the case, and left in place the crusts of the uranium salt. Since the sun did not show itself again for several days I developed the photographic plates on the first of March, expecting to find the images very feeble. The silhouettes appeared on the contrary with great intensity. I a t once thought that the action might be able to go on in the dark, and I arranged the following experiment. "At the bottom of a box made of opaque cardboard I placed a photographic plate, and then on the sensitive face I laid a crust of uranium salt which was convex, so that it touched the emulsion a t a few points only; then alongside of it I placed on the same plate another crust of the same salt, separated from the emulsion by a thin plate of glass; this operation was carried out in the darkroom, the box was shut, was then enclosed in another cardboard box, and put away in a drawer. "I did the same thing with a holder closed by an ahminum plate, in which I put a photographic plate and then laid on it a crust of uranium salt. The whole was enclosed in an opaque box and put in a drawer. After five hours I developed the plates, and the silhouettes of the encrusted crystals showed black, as in the former experiment, and as if they had been rendered phosphorescent by light. . . . Volume 39, Number 10, October 1962
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"It is important that this phenomenon seems not to luminous radiation emitted by phosphorescence, since at the end of one hundredth of a second these radiations become so feeble that they are scarecely perceptible."
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Journal o f Chemical Education
He concluded that the radiations must be coming from an element hitherto undiecovered. The phenomenon was called radioactivity. At this point he offered Madam Curie as her doctor's thesis problem the search for the element that possessed this peculiar property. Her discovery is still another story.
