Marie Curie and the Discovery of Radium

At the end of 1897 Marie Curie, a young Polish émigré married to the physicist Pierre Curie had produced her first child and was looking for a new scientific challenge. She wanted to complete a doctorate, but she had to decide what area of science to research. Her imagination was caught by the findings of Becquerel.

Becquerel's theory had been that X-rays might be produced by fluorescent material. He was working with a uranium compound which fluoresces when it is exposed to sunlight. But as luck would have it the sun did not shine for several days. Becquerel left his uranium sample and a photographic plate in a drawer and eventually, when the cloudy weather showed no signs of lifting, decided to develop the plate anyway. The plate was heavily fogged - radiation had been emitted by the uranium compound without the stimulation of sunlight.

Becquerel's work on the rays produced by uranium was so new that the field for research was wide open - ideal for work on a doctor's thesis! Where did the energy come from and what was it made of? These were the questions which drove Marie Curie on, even though she had had nowhere to work but a closed-in, damp and crowded storeroom at the bottom of the School of Physics. The humidity and temperature in Marie's workroom changed all the time - making careful scientific measurements almost impossible - but it was all that was available and Marie set out to make the best of it!

Shut away in her damp and gloomy storeroom Marie worked furiously. Her apparatus was not complex but she struggled to keep everything working accurately in the poor conditions of her makeshift 'laboratory'. Yet in spite of all the difficulties, interesting results soon began to emerge.

Radioactivity emerges
Firstly Marie showed that the intensity of the radiation produced by uranium depended only on the amount of uranium present. The relationship between the amount of uranium and the amount of radiation was constant and was not affected by light, temperature or the chemical state of the uranium. These results were of vital importance in showing that the radiation discovered by Becquerel really was a new and unique phenomenon, probably of the very uranium atoms themselves.

Then Marie moved on - was uranium the only element to possess this new property, or did other elements produce these strange rays as well? In a rush of enthusiasm she started to examine all the known chemical elements of the time. The hoped-for result was not long in coming - compounds of the element thorium also emitted rays like those of uranium. This showed that the phenomenon was not unique to uranium and needed its own name. Marie Curie suggested radioactivity.

Not content with this discovery Marie then began to examine different ores and minerals. As she expected, only the ores of uranium and thorium showed radioactivity.
However when she studied these minerals in detail her results showed more radiation being given off than should have resulted from the amount of uranium or thorium present. Her first assumption was that her results were wrong, and with painstaking care she redid them all twenty times - but the results were consistently the same! Marie had already tested all the known elements for radioactivity. This new source or radioactivity, far more powerful than uranium or thorium, could only mean one thing - she had discovered a new element!

Marie and Pierre went over and over the evidence and were convinced. As Marie said to her sister :

…the radiation I couldn't explain comes from a new chemical element. The element is there and I've got to find it. We are sure! …. I am convinced I am not mistaken!

Marie's early discoveries were so exciting that Pierre decided to leave his research on crystals and join his wife in her search for the new element. Two great scientific brains focused on the task made the work progress much faster. The couple shared observations, wrote joint papers and shared the credit for the work they performed. The papers Marie and Pierre wrote at the time shows how completely their work was intertwined. Although they each kept an individual laboratory note book where they recorded the details of their experiments, they always wrote we found or we observed in their published papers and so avoided identifying who actually did what :

Certain minerals ... are very active from the point of view of the emission of Becquerel rays. In a preceding communication, one of us showed that their activity was even greater than that of uranium or thorium.

Was it Marie or Pierre? It was their intention that no-one should know.

The secret of the pitchblende
Bit by bit Marie and Pierre separated pitchblende (uranium ore) into the various elements of which it was made, and then looked for radioactivity in each of these elements. To their astonishment, after eliminating the uranium, they still had two sources of radioactivity. They had discovered not one but two new elements! The first Marie christened polonium after Poland, her beloved birthplace. The second they named radium - and this was the most stable, radioactive and useful of the two.

The final proof
Some of the Curie's fellow scientists at the university did not share their belief in the new elements. To convince them that radium and polonium existed, the Curies needed to isolate the pure elements, yet they could not afford to buy the ore they needed to do this.

Marie realised the waste left after uranium salts used in the glass industry had been extracted from pitchblende would be cheap but still contain radium and polonium. She arranged for tons of the industrial waste to be brought from Bohemia. It then took four years (1898-1902) of hard labour in appalling conditions to melt and treat the pitchblende waste to extract the tiny amounts of radioactive materials within.

Finally, in 1902, the Curies prepared one tenth of a gram of pure radium, and determined its atomic mass as 225. Any doubting fellow-scientists were silenced for good - radium officially existed as a newly-discovered element! In 1903 they were awarded the Nobel prize for Physics, along with Henri Becquerel, for their work on radioactivity.

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Radium and cancer treatment

X-rays, Marie Curie and the Great War