The discovery of radioactivity


Almost everyone has heard about the discovery of radioactivity, which is a phenomenon by which atomic nuclei undergo transformations and emit radiation, and in the process may form new chemical elements. It is often said that this phenomenon was accidentally discovered by Henri Becquerel in 1896. It all happened because Becquerel kept a uranium compound with a photographic plate in a drawer, then revealed the plate and noticed the signs of it. radiation.

The story is not quite like that. It could hardly be said that Becquerel discovered radioactivity; and what he actually discovered was not the result of chance.

This chapter will show what Becquerel's work was, the long and tortuous path that led to the discovery of radioactivity and discuss the difficulties of understanding the facts that were observed. This episode is very instructive because it clearly shows how theoretical expectations can influence one's own observations, leading the researcher to see things that do not exist.

The radiation of luminescent bodies

The discovery of x-rays almost instantly attracted a great deal of work at the Paris Academy of Sciences, and was the main motivation for Becquerel's early work. In this regard, the hypothesis raised by Poincaré that there was a relationship between the emission of x-rays and the fluorescence of the glass of which the x-ray tube was made stands out. In his own words:

"It is therefore the glass that emits the Roentgen rays, and it emits them becoming fluorescent. We may wonder if any bodies whose fluorescence is sufficiently intense would not emit, besides light rays, Roentgen X-rays, whatever they might be." be the cause of their fluorescence. The phenomena would not then be associated with an electrical cause. This is not very likely, but it is possible and undoubtedly easy to verify. "

It is the pursuit of this relationship between fluorescence and X-rays that will lead to Becquerel's studies. In fact, according to our current knowledge, there is no direct relationship between X-ray emission and luminescence. But it is thanks to this false lead that many discoveries will be made.

Several works related to Roentgen's discovery were presented at the Academy in the early sessions of 1896. At the session of 02/03/1896, Nodon reports that an arc flash does not produce X-rays, but Moreau reports that they are emitted by the high voltage discharge. of an induction coil without the use of a vacuum tube and therefore without cathode rays. Benoist and Hurmuzescu note that X-rays are capable of discharging an electroscope. The other week (10/02/1896) appears the first work intended to test Poincaré's suggestion.

In this session, Poincaré presents to the Academy a work of Charles Henry. He initially tests whether phosphorescent zinc sulfide is capable of enhancing the effect of x-rays and concludes that if a metallic object is partially coated with a layer of zinc sulfide, the radiograph of that object becomes stronger and clearer in the coated region. than in the region without zinc sulfide. Even more: Using the light produced by burning a magnesium tape in the laboratory, Henry claims to have achieved the same effects as radiography by simply covering the object with a layer of zinc sulfide. Poincare's hypothesis seemed to be confirmed.

The following week (17/02/1896), amidst the usual profusion of X-ray studies, a work by Niewenglowski emerges that confirms and broadens Henry's results. It uses another phosphorescent material - calcium sulfide. Here's your description:

"Having wrapped a sheet of ordinary sensitive paper (photo paper) with several layers of black or red needle paper, I placed two coins above it and covered one of the halves (of the sheet) with a glass plate with phosphorescent powder (calcium sulfide). After four or five hours of exposure to the sun, half of the sensitive paper that had directly received the solar radiation had remained intact and showed no sign of the coin placed above it, thus indicating that the black or red paper had not been crossed by the sun. light. The half that only received the sun's rays through the phosphorescent plate was completely black, except for the portion corresponding to one of the coins, which produced a white silhouette on a black background.

Placing only one layer of thin red paper, allowing the sun's rays to pass through, I found that the portion of the sensitive paper that received solar radiation only after it passed through the phosphorescent layer blackened much faster than the other. "

Niewenglowski's observations corroborated those of Charles Henry: the phosphorescent materials seemed to emit X-rays when illuminated. Even more: Niewenglowski studies the effect of phosphorescence of calcium sulfide placed in a dark place after receiving sunlight, concluding that in this case too the material continued to emit radiation capable of passing through black paper:

"I also observed that the light emitted by the phosphorescent powder, previously illuminated by the sun in the dark, was able to pass through several layers of red paper and obscure a sensitive paper that was separated from them by those layers of paper. ".

Another week goes by. At the session of 24/02/1896, Piltchikof announced that using a strongly fluorescent substance inside the Crookes tube, where cathode rays hit the glass wall, observed a large increase in x-ray intensity, allowing x-rays within 30 seconds (previously several minutes were required). Poincaré's suggestion was therefore already resulting in important technical applications. All of these results will amaze any modern physicist. No effect similar to that described by such authors is currently known. Experiments should not have provided the results observed. What happened? It is not known.

In this same session of the Academy, Henri Becquerel's first work on the subject appears.