Who was Marie Curie? How radioactivity was discovered and two Nobel Prizes were won

Picture a shed in 1890s Paris, once used by medical students for dissecting bodies, leaking when it rains and freezing cold in winter. Inside, a woman stirs a dark, boiling mixture in a cauldron with an iron rod almost as tall as she is. By processing tonnes of mining residue, she will eventually obtain a substance amounting to a tiny fraction of a gram that glows faintly in the dark. This woman is Marie Curie, and the work she did in this shed would become one of the discoveries that let us look inside the atom for the first time.

From Warsaw to Paris

Marie Curie was born Maria Skłodowska on 7 November 1867 in Warsaw, which was then under the rule of the Russian Empire. Her mother and father were teachers. In those years women could not enter university in Poland. Maria and her older sister attended classes that were organised in secret, because they were forbidden, and that became known as the “Flying University”.

The two sisters made a deal: Maria would work and support her sister’s medical studies in Paris, and then her sister would help her. Maria worked as a governess for years. In 1891 she finally went to Paris and enrolled at the Sorbonne. She earned degrees in physics in 1893 and in mathematics in 1894. In 1894 she met the physicist Pierre Curie, and they married in 1895.

Mysterious rays

The scientific story begins in 1896. The French physicist Henri Becquerel discovered that uranium salts darkened a photographic plate even when no light fell on them. Something invisible was coming out of uranium. The effect was interesting, but few people paid much attention to it.

Marie Curie chose these “uranium rays” as the topic of her doctoral thesis. Using a sensitive electrical measuring instrument developed by Pierre and his brother, she measured how much the rays made the air conduct electricity. This let her express the strength of the radiation in numbers. Her finding was very important: the amount of radiation depended only on the amount of uranium in the sample. It made no difference whether the uranium was wet or dry, powdered or solid, pure or combined with other substances.

This showed that the radiation came not from the chemical arrangement of molecules but from the atom itself. At the time the atom was thought to be the smallest, indivisible particle. Curie’s finding suggested that something was going on inside the atom. It was Marie Curie who gave this phenomenon the name radioactivity.

Polonium and radium

When Curie measured uraninite (pitchblende), an ore containing uranium, she noticed something surprising: the ore gave off much stronger radiation than the amount of uranium in it could explain. The only explanation was that the ore contained an as yet unknown element that gave off far stronger radiation.

Pierre set aside his own research and joined Marie. In 1898 they announced that they had found two new elements:

• Polonium (July 1898): Marie named it after her homeland, Poland, which did not exist on the map as an independent country.
• Radium (December 1898): derived from the Latin word radius, meaning “ray”.

But saying you have “found” an element is different from obtaining it. To be convinced, chemists wanted to see a pure sample and a measured atomic weight. So for years the Curies processed tonnes of ore residue in that cold shed. In 1902 they finally obtained a tenth of a gram of radium chloride. This tiny amount glowed with a bluish light in the dark.

Two Nobel Prizes

In 1903 the Nobel Prize in Physics was awarded jointly to Henri Becquerel, Pierre Curie and Marie Curie for their work on radioactivity. Marie Curie became the first woman to win a Nobel Prize.

In 1906 tragedy struck: Pierre Curie was killed in Paris when he fell under a horse-drawn cart. Marie was appointed to Pierre’s chair at the Sorbonne, becoming the first woman professor to teach at the Sorbonne.

In 1911, this time on her own, she received the Nobel Prize in Chemistry for the discovery of radium and polonium and for studying radium in its pure state. This made her the first person to win two Nobel Prizes, and still the only person to have won Nobel Prizes in two different sciences.

During the First World War, Curie led the setting up of mobile X-ray units so that wounded soldiers could be X-rayed near the front. She learnt to use these units herself, worked in the field and trained other women as X-ray technicians.

What is radioactivity? A simple explanation

Today we can explain radioactivity in a language that was unknown in Curie’s time. At the centre of every atom is a nucleus made of protons and neutrons. Some nuclei are unstable: over time they give off particles or energy and change into a more stable form. What is given off during this change is the radiation that darkened Becquerel’s photographic plate and that Curie measured.

In 1899 the New Zealand physicist Ernest Rutherford showed that there were at least two types of this radiation and named them alpha and beta. Soon afterwards the far more penetrating gamma radiation was also identified.

An important property of radioactive substances is the half-life: the time it takes for half of the atoms in a sample to change. This time is different for each element. The half-life of polonium-210, the best-known form of polonium, is about 138 days; the half-life of radium-226 is about 1600 years. So it takes about 1600 years for half of a piece of radium to change, and another 1600 years for half of what remains to change:

This is why radium is still present in the items Curie used in her laboratory.

Myth or fact?

• “Marie Curie discovered radioactivity.” Partly true. The first person to observe the effect was Becquerel. Curie, by measuring it, showed that it is a property of the atom, gave it its name and found new elements.
• “Radium is good for your health.” In the early 20th century radium was believed to be a miracle substance; products such as radium water, cream and toothpaste were sold. These products were harmful. Workers who painted watch dials with radium paint fell seriously ill, which revealed the danger of radiation in a painful way.
• “Curie’s notebooks are still radioactive.” This is true. Because her laboratory notebooks still contain radioactive material today, they are stored and examined with special precautions.

Curie worked with radioactive substances for years without fully knowing the harm radiation could do. She died on 4 July 1934 of a blood disease very probably linked to long-term exposure to radiation.

Connection to today

Radioactivity can be dangerous, but used properly it saves lives. Radiotherapy for cancer, medical imaging, sterilising medical equipment and the carbon-14 method used in archaeology to date objects are all possible thanks to our understanding of radioactivity. The path Curie opened also led to the discovery of the atomic nucleus and the birth of modern physics.

Summary

• Marie Curie (1867-1934) was born in Warsaw and worked in Paris.
• She showed that uranium radiation comes from the atom itself and gave the name radioactivity to the phenomenon.
• Together with Pierre Curie she discovered the elements polonium and radium in 1898.
• She won the Nobel Prize in Physics in 1903 and in Chemistry in 1911; she is the only person to have won Nobel Prizes in two different sciences.
• The harmful effects of radiation were not well understood at the time; Curie’s story shows both the power of science and the importance of safety.