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In this installment on the history of the atom theory, physics professor (and my dad) Dean Zollman tells the surprising story of how radioactivity was discovered. After reading this post, I wondered “What if the weather had been good in Paris?” – Kim

By Dean Zollman

Dean Zollman

During the short time between 1895 and 1897, three important discoveries helped define physics of the 20th century. We have looked at two of them –the electron and x-rays – in the past two posts. This month, we will discuss the third – radioactivity.

As I mentioned last time, news of Wilhelm Roentgen’s discovery of x-rays at the end of 1895 spread rapidly through both the scientific and general communities. This new form of electromagnetic radiation was the subject of discussion at the meeting of the Academy of Sciences in Paris on January 20, 1896. Henri Poincaré (1854-1912) noted that the x-rays seemed to be emitted at a point in the Crooke’s Tube where some light was being emitted by fluorescence – a process similar to the way fluorescent tubes work today. Poincaré wondered if other substances that emit light might also emit x-rays. In particular he mentioned types of minerals which glowed in the dark after being exposed to sunlight – a process called phosphorescence. (Many of the glow-in-the-dark objects that we have today emit light by phosphorescence. They store energy in their atoms when exposed to light and then reemit that light gradually over a long time.) A. Henri Becquerel (1852-1908) decided to see if Poincaré’s conjecture might be correct.

Henri Becquerel, Library of Congress

Becquerel was in a good position to conduct the necessary experiments. He was professor of applied physics at the Museum of Natural History in Paris. The two previous professors to hold this position were his father, Alexandre Edmond Becquerel (1820-1891), and his grandfather, Antoine César Becquerel (1788-1878). (Later, Henri’s son would also hold the professorship. So it was a four-generation dynasty.) Both of the senior Becquerels conducted research on phosphorescent materials. So, Henri had a laboratory with a large collection of minerals that had the glow-in-the-dark property. In addition, Henri’s father had experimented with the relatively new process of photography. So, Henri understood this process which, as we shall see, is very important to the discovery.

X-rays were known to penetrate materials that ordinary light could not pass through. Henri Becquerel realized that he could take advantage of this property. A photographic plate could be wrapped in heavy black paper so that no light could reach it. He could then place a material that he thought might emit x-rays next to the wrapped plate. X-rays would pass through the black paper and expose the plate. Then when Becquerel developed the plate, he would see a black area exposed to x-rays.

From his own work as well as that of the previous generations, Becquerel understood that the only pure phosphorescent materials known at that time were salts of uranium. So, he used potassium uranyl sulfate for his experiments. He exposed the material to sunlight, placed it on a photographic plate for a while and then developed the plate. His experiment, he thought, was a success; the photographic plate had a black area on it even though the plate had never been exposed to light. Becquerel’s conclusion was that the uranium salt had emitted x-rays.

He could not explain the mechanism for this process, but it seemed as if exposure to sunlight created some type of change in the salts and that change resulted in the emission of x-rays. On February 24, 1896, Becquerel reported his results to the weekly meeting of the Academy of Sciences in Paris. He promised to conduct more experiments and report more results the following week.

Then the French winter weather interfered in an incredibly positive way. Most of the next week in Paris was very cloudy. Becquerel could not expose his uranium salts to sunlight. He put the uranium salts and the photographic plates in a drawer where they sat and had no exposure to any type of light.

So, these plates should have shown nothing remarkable. In fact, there should have been no reason to even develop them. But on Sunday, March 1, 1896 (119 years ago this week), Henri Becquerel developed these never exposed plates. One of the results is shown below.

Photographic plate by Henri Becquerel

The exposure was just as strong as that of the plate from the previous week. The radiation emitted by the uranium salts did not depend on exposure to the sun and was different from x-rays.

One of the mysteries in the history of science is why Becquerel developed these plates. Apparently, he did not reveal his reasons, so he left something for historians of science to speculate about. Some think he was just being frugal; no sense in wasting the plate. The plates get old, but the chemicals can be reused. Others have thought that he wanted to make sure that his developing chemicals had not become too old.

The most plausible idea (at least to me) is that he had promised new results for the Academy of Sciences meeting on Monday, March 2. He was hoping that he would see at least a weak exposure so he could report something, even though it would not be very interesting. Instead, he was able to report an extremely interesting result.

Becquerel conducted some additional experiments and during 1896 reported on the properties of this radiation, which later came to be called radioactivity. A few of his observations turned out to be wrong, but others helped people understand how this radioactivity was different from x-rays.

Unlike x-rays, radioactivity generated no great excitement in the scientific community. Even Becquerel seemed to tire of them quickly. He published seven papers on the topic in 1896, one in 1897, and none in 1898. He was off investigating other things. (In 1896 alone, more than 1,000 about x-rays papers were published.)

This situation would change when Marie Skłodowska Curie (1867-1934) and Pierre Curie (1859–1906) began a systematic study of radioactivity. I will save that story for next time. (If you want to get ahead of my story, I recommend the 1943 film Madame Curie starring Geer Garson and Walter Pidgeon. It is history according to Hollywood but still somewhat fun.)

All images are public domain, via Wikimedia Commons.

Dean Zollman is university distinguished professor of physics at Kansas State University where he has been a faculty member for more than 40 years. During his career he has received four major awards — the American Association of Physics Teachers’ Oersted Medal (2014), the National Science Foundation Director’s Award for Distinguished Teacher Scholars (2004), the Carnegie Foundation for the Advancement of Teaching Doctoral University Professor of the Year (1996), and AAPT’s Robert A. Millikan Medal (1995). His present research concentrates on the teaching and learning of physics and on science teacher preparation.


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