Radioactivity: A History of its discovery

In July of 1898, Marie Sklodoska Curie and her husband Pierre Curie published the discovery of a new substance they called Polonium by declaring that they had found “Une nouvelle substance radio-active” (yeah, I got bad grades in French class, why do you ask) which is translated as “a new radio-active substance[1]”.  This is actually the origin of the term so why did they make up this term?  Well, Polonium was very radiation active (400 times more powerful than uranium) and radio was short for radiation, and was used for other systems that were in the air.  For example, wireless telegraphs were often called radio-grams.  Although some people used that term in the following years, the phrase “radioactivity” only gained popularity when Curie published her Ph.D. thesis titled “radio-active substances” in 1903, and even she dropped the dash by 1904.  But why would the world listen to the words of a shy Polish immigrant to France who was, gasp, a woman?  To answer that question, I need to start with why she discovered so much about radioactivity and how the world reacted to her discoveries.  Ready for the story? Let’s go!

It all started when a scientist named Henri Becquerel found to his frustration that the sun never really came out (it was cloudy) in Paris between February 26th and March 1st of 1896.  Why was he frustrated you ask?  Well, Becquerel had a theory that when certain fluorescent salts absorbed sunlight, they produced x-rays, and how could he validate that without the sunlight?  This wasn’t a completely crazy idea.  The x-ray was newly discovered and no one really knew what x-rays were, just that they were produced when something called a cathode ray hit a solid.  Becquerel also knew that cathode rays were invisible “light” that make glass and fluorescents glow.  Since cathode rays make things glow and cathode rays make x-rays, Becquerel wondered if the production of x-rays was related to fluorescence or phosphorescence (phosphorescent materials glow even after the outside light is removed, fluorescents don’t).  This was an easy thing for him to study as he was the director of the Physics department of the Parisian Museum of Natural History so he had ready access to a huge quantity of luminescent minerals[2].  Anyway, after a few weeks of playing with different glowing materials, Becquerel was happy to report that phosphorescent uranium salts that sat in sunlight would produce invisible rays that would go through the paper and could be seen in the film[3].  After publishing, Becquerel prepared some more uranium salt to do further experiments but he was irritated when his sample was ready but the sun was not, and he just threw his uranium in a drawer with his photographic plates.  The next two days were also cloudy and Becquerel realized that he had missed the window of the “usefulness” of his uranium.  Annoyed, Becquerel decided to just develop the film anyway as a baseline, “expecting to find the images very weak.[4]”  Instead, to his complete surprise, “the silhouettes appeared with great intensity[5].”  He immediately decided that the uranium was making invisible waves on its own without any input of sunlight!  By November, Becquerel found that the uranium didn’t even need to be florescent, all uranium would produce these rays, for that reason, he called these new rays “uranium rays” and he also determined that these rays would make the air slightly conductive.  However, there wasn’t much interest in these new rays.  X-rays were all the rage both for scientists and for the general public and could take a picture of your bones!  “Uranium Rays” (actually gamma rays from uranium), could go through the bones and skin and just leave a vague shadow of the hand.  In addition, uranium isn’t very radioactive, and it took days to take a picture.  Even Becquerel got bored and moved on to study other subjects.

Henri Becquerel
Henri Becquerel

Fast forward 10 months, to September 1897.  Marie Sklodowska Curie was 29 years old and had just given birth to a baby named Irene.  Her husband, Pierre, had just finished his Ph.D., and it was her turn.  Curie recalled, “It became a serious problem how to take care of our little Irene and of our home without giving up my scientific work.  Such a renunciation would have been very painful to me, and my husband would not even think of it.”  Luckily, her father-in-law moved in to help with babysitting and, soon, Marie buckled down with the task of getting her doctorate.  After a search of the latest scientific papers, Marie Curie decided to conduct her Ph.D. on studying these new Uranium rays with electricity.  Years earlier, Pierre Curie had discovered that electricity is related to pressure for certain materials called piezoelectricity, and had used these materials to make the world’s most sensitive electricity meter so it seemed a good plan.  After a bit of electric study on uranium, Marie Curie started to believe that the radiation seemed to be a property of the uranium itself.  It then seemed illogical to her that these magic rays should only come from one material, so she gathered all the substances that she could and exhaustively studied them in the electroscope looking for more radioactive substances.  By April of 1898, Curie had found that a substance called Thorium was about as radioactive as Uranium.  Before she could publish, however, she learned that a German scientist had already discovered that Thorium was radioactive with photographic plates.  However, her method allowed one to determine quantitatively *how* radioactive these materials were.  She then published her first paper on radioactivity on April 12, 1898, where she mentioned how “active” the different materials were[6].

Marie continued to search.  While experimenting with uranium ore (also called pitchblende), which contains many elements including uranium, Marie Curie was astonished to realize that the ore was significantly more radioactive than pure uranium and realized it must contain a new material that was even more radioactive.[7]  Pierre was so intrigued by her results that he dropped his research and joined her studies.  For the next several years they worked side by side sometimes even filling in different sides of the same notebook!  By July of 1898, as I said in the introduction, Marie and Pierre Curie published the discovery of the highly radioactive material, polonium, named after Marie’s home country of Poland.  In this paper, the Curies also named the radiation rays “Becquerel rays” in honor of Henri Becquerel which it was called for many years.  Becquerel was intrigued and even introduced the paper to the Parisian academy (the Curies weren’t members).  This paper also excited a German chemist named Friedrich Giesel, who began doing his own experiments with uranium pitchblende, although as he told the Curies a few years later, “even Rontgen (the man who discovered x-rays) did not believe in the existence of the Becquerel rays [at first].[8]” 

A few months after the discovery of Polonium, in December 1898, the Curies discovered that the uranium ore they were playing with contained an even more powerful substance they called Radium (named after radiation as it was so strongly radioactive).  With even a bit of isolation, Radium exhibited some very strange and amazing properties: it was always hot and would burn the skin would continuously glow without any power source and without any change in size or shape!  In Germany, Giesel realized that the new substance that he had discovered that glowed with a “most splendid” light[9] that he could read by was also Radium!  Now the race was on to isolate it, which the Curies were sure would take a few weeks, instead, it took 4 years to get enough radium to measure its atomic weight, which was particularly daunting as they were working in a literal shed that was described by a visiting Chemist as “a cross between a stable and a potato cellar.[10]

In 1900, in Germany Friedrich Giesel had a friend named Walkoff who was a dentist and interested in studying the effects of x-rays in dentistry. Giesel loaned a coveted 0.2 g piece of partially isolated radium to Walkoff and Walkoff then published his study of the effects of putting it on his skin for a couple of hours, noting how the burn would last for many weeks!  Meanwhile, in France, Becquerel had been given a piece of radium by the Curies, and the glass holding it had broken and given him a burn whereupon he went to the Curies and told them, “I love this radium, but I’ve got a grudge against it![11]”  Pierre read about Walkoff’s experiment in Germany and was inspired, decided to up it and burn his skin for 10 hours and then Pierre Curie and Becquerel published a paper about their trials and tribulations.  Then, Pierre Curie used to make burning his own arm a regular part of his lectures!  After testing on animals, Pierre Curie found that radium could destroy tumors and forms of cancer, although it was too expensive to be a regular part of medical communities and instead used as specialty medicine or as part of scams.  As Pierre and Marie Curie’s daughter Eve wrote years later, “Radium was useful – magnificently useful… [and] the extraction of the new element no longer had a merely experimental interest.  It had become indispensable… a radium industry was about to be born.[12]

piezoelectric balance presented by Pierre Curie
piezoelectric balance presented by Pierre Curie

By the beginning of 1902, Marie and Pierre Curie managed to get 1/10th of a gram of radium-chloride salt that was more than a million times more radioactive than uranium[13] which the Curies refused to patent and donated to universities for free (in 1909, Marie had to do a go fund me to get some radium)!  A few weeks after that, in Germany, Friedrich Giesel managed to isolate radium-chloride in a simpler method, and, sold 30-milligram samples for 240 English pounds each (around $80,000 in today’s dollars[14]).  Even at these exorbitant prices, Giesel had too many people clamoring for some so he turned and upped the price twelve times, making himself a fortune[15].  Giesel was wealthy, but the Curies were not, even worse, their health was suffering. (Giesel was radioactive too, in 1905 it was recorded that even his breath was radioactive for 18 hours after he left the laboratory. However, despite being a walking biohazard, he lived until he was 75 and died of lung cancer).  Anyway, Pierre Currie was very ill but his doctors could not determine the cause (we now know it was radiation poisoning) and they were both covered in burns.  Then, in August of 1903, Marie Curie suffered a miscarriage.  She wrote her sister, “I am absolutely desperate and cannot be consoled… I had wanted [the child] so badly.[16]”  Meanwhile, Marie Curie had to hold herself together to publish her Ph.D. Thesis.  She titled it “Radio-Active Substances” and defended it in June of 1903, where the committee told her she had produced, “the greatest scientific contribution ever made in a doctoral thesis.[17]”  Within months it had been translated into several languages and even went through several editions and her thesis was considered the bible of radioactivity for scientists throughout the world! 

A 1903 illustration by André Castaigne of Pierre and Marie Curie (left) experimenting with radium

Marie, however, was still mourning the miscarriage and Pierre was still pretty ill and they were both suffering from exhaustion from overwork and radiation poisoning.  It was around this time that Pierre heard from a friend that he and Henri Becquerel had been nominated for a Nobel Prize without Marie Curie!  Pierre had no interest, even with a sizeable amount of money, to win an award without his wife for this amazing work.  Luckily, a friend basically pulled some strings and they ended up splitting Pierre’s part in two so that they both got ¼ a Nobel Prize!  Making Marie Curie the first woman to win a Nobel prize in science.  There wasn’t another female science Nobel for anyone not named Curie for another 43 years! (Marie Curie won another in Chemistry in 1911 and her daughter Irene Joliot-Curie won one in 1935).   The Curies were glad for the cash but hated the publicity.  The public, however, was instantly obsessed with Marie Curie.  A tiny female scientist, a romance, a mysterious substance that glowed and could cure disease?  It was all too fascinating.  In fact, their nomination actually increased the popularity of the Nobel Prize with the public.  And everywhere it was said that Marie Curie discovered radioactivity and radium, and we have used the term radioactive and haven’t used the term Becquerel rays ever since.   

So that is how radioactivity got its name.  In Marie Curie’s thesis, as I have explained, she did not come up with the term radio-activity (she came up with it 5 years earlier) but it was her thesis that made the term popular.  However, she did come up with a new scientific term in this thesis, a term that no one gives her credit for.  In 1903, Marie Curie came up with the term “gamma rays” for the third type of radiation from radioactive materials.  Why she did that and why she is never given credit is next time on the Lightning Tamers.       

[In case you want to know, Marie Curie had her second daughter Eve in 1906, Pierre Curie recovered a bit from radiation poisoning but was run over by a carriage in 1909,


[1] “Une nouvelle substance radio-active; le polonium” (July 30, 1898) Revue Générale Des Sciences No. 14 p. 558

[2] Becquerel’s job is according to Malley, M Radioactivity, A History (2011) Oxford Press p. 26

[3] Becquerel, Henri “On the Rays Emitted by Phosphorescence” Feb 24, 1896 Comptes Rendus 122, 420 (1896) p?

[4] Becquerel “On the Invisible Rays Emitted by Phosphorescent Bodies” Comptes Rendus 122, 501 (1896) p??

[5] Becquerel “On the Invisible Rays Emitted by Phosphorescent Bodies” Comptes Rendus 122, 501 (1896) p??

[6] Sklodowska Curie, M “Rayons émis par les composes de l’uranium et du thorium” (April 12, 1898) Comptes Rendus de L’Académie des Sciences No. 14 TOME CXXVI p. 1101

[7] Curie, Marie Pierre Currie p. 97

[8] “even Rontgen…” Friedrich Giesel to the Curies (Dec 22, 1899) quoted in Malley, M Radioactivity, A History (2011) Oxford Press p. 40

[9] According to Malley, M Radioactivity, A History (2011) p. 38

[10] “a cross between…” Wilhelm Ostwald quoted in Malley, M Radioactivity, A History (2011) Oxford Press p. 42

[11] “Radium was useful…” Curie, E Madame Curie (2013) Knopf Doubleday

[12] “Radium was useful…” Curie, E Madame Curie (2013) Knopf Doubleday

[13] Curie, Marie “Radium and Radioactivity” Century Magazine (January 1904) pp. 461-466

[14] HistoricalStatistics.com, using amount of gold 30 UK pounds = 220 g gold = $9,991 today

[15] Rutherford, E (who also added that the price increased by 12x), “Forty Years of Physics” Background to Modern Science 1938 p. 54

[16] “I am absolutely desperate…” Marie Curie to her sister “Bronya” (August 25, 1903) from Curie, E Madame Curie (2013) Knopf Doubleday

[17] quoted in Borzendowski, Janice Marie Curie: Mother of Modern Physics p. 67

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