Marie Curie

How to use this course

Marie Curie is famous for the discovery of polonium and radium, for two Nobel Prizes, and for the cost the work exacted from her body. Behind the fame lies a method. In 1897 she chose to measure, with an electrical instrument built by her husband and his brother, how strongly each substance makes the air conduct. The numbers told her that radioactivity belongs to atoms, that some minerals hide elements more active than uranium, and where in a chemical separation those elements had gone. Chemistry guided by a physical measurement was a new way to find elements, and it became the standard method of radiochemistry.

Part II follows the work in order. Lesson 04 treats her first research, on permanent magnets. Lessons 05 to 07 treat the measurement and the discoveries of 1898. Lessons 08 and 09 treat the long labour of isolating radium, fixing its atomic weight and making it a standard of measurement. Lesson 10 treats the medical and military radiology that occupied her from 1914.

Each technical lesson opens with a paragraph for the general reader, then gives precise statements, derivations and exercises with folded solutions. Numerical work uses SI units, with the calories and grams of her papers converted where needed.

Timeline

A Warsaw childhood, 1867–1883

Maria Salomea Skłodowska was born in Warsaw on 7 November 1867, the fifth and youngest child of Władysław Skłodowski and Bronisława Boguska. Warsaw was the capital of the Kingdom of Poland, ruled by the Russian Empire since the partitions and tightly policed after the failed uprising of 1863. Both parents were teachers. Her father taught mathematics and physics and had been a school director; her mother ran a boarding school for girls and gave it up after Maria’s birth. The family called her Mania.

Russian policy pressed hard on Polish schooling. Teaching in Russian was compulsory, Polish history was suppressed, and teachers of patriotic families were demoted. Władysław Skłodowski lost his post and his official lodging, and the family took in boarders to make ends meet. Two deaths followed. Zofia, the eldest daughter, died of typhus, and in May 1878 their mother died of tuberculosis, when Maria was ten.

Maria attended J. Sikorska’s boarding school and then a state gymnasium for girls, from which she graduated on 12 June 1883 with a gold medal. After a period of exhaustion she spent a year with relatives in the countryside.

Governess and Flying University, 1884–1891

Women could not enrol at the University of Warsaw. Maria and her sister Bronisława joined the Flying University, a clandestine set of courses that moved between private flats and admitted women. Both sisters wanted to study abroad, and the family could not afford it. They agreed that Maria would earn money to support Bronisława’s medical studies in Paris, after which Bronisława would help her in turn.

Maria worked as a home tutor in Warsaw and then for two years as a governess with the Żorawski family at Szczuki, north of Warsaw. She fell in love with the family’s son, Kazimierz Żorawski, later a mathematician, but his parents opposed the match with a penniless governess. She returned to Warsaw, kept studying on her own, and in 1890–1891 received her first practical training in chemistry in the laboratory of the Museum of Industry and Agriculture, run by her cousin Józef Boguski. There she repeated experiments from textbooks of physics and chemistry, sometimes with unexpected results, and the work settled her choice of career.

Bronisława, now a physician married to the doctor Kazimierz Dłuski, invited her to Paris. In the autumn of 1891 Maria made the journey.

Paris, the Sorbonne and Pierre, 1891–1897

She arrived at the Gare du Nord at the end of October 1891 and enrolled at the Faculty of Sciences of the University of Paris on 3 November, as Marie. After some months with her sister she rented a garret in the Latin Quarter, near the faculty. In July 1893 she took the licence in the physical sciences, first of her year. A scholarship of 600 roubles let her continue, and in 1894 she took a second licence, in the mathematical sciences.

In 1894 the Society for the Encouragement of National Industry commissioned her to study the magnetic properties of various steels, work she carried out in the laboratory of Gabriel Lippmann (Lesson 04). Looking for laboratory space, she was introduced that spring by the Polish physicist Józef Wierusz-Kowalski to Pierre Curie, head of practical work at the city’s School of Industrial Physics and Chemistry and an authority on magnetism. In March 1895 she attended his doctoral defence. They married in July 1895 in a civil ceremony at Sceaux.

In 1896 she came first in the examination that qualified women to teach in girls’ secondary schools. Their daughter Irène was born on 12 September 1897. At the end of that year Marie looked for a doctoral subject. Becquerel’s uranium rays had been little studied since 1896, and the literature on them was short. She chose them, and Pierre’s school gave her a small, damp storeroom on the ground floor as a laboratory.

The magnetism of tempered steels

For the general reader. A good permanent magnet must take a strong magnetization and keep it against anything that tends to undo it. Steels differ greatly in both respects, depending on what is alloyed with the iron and how the metal is heated and quenched. Marie Curie’s first research, for French industry, measured these properties across many steels. It trained her in exact magnetic measurement and brought her to Pierre Curie.

The hysteresis loop

In a ferromagnet the magnetization \(\mathbf M\) is not a single-valued function of the applied field \(\mathbf H\). Starting from a demagnetized state and cycling \(H\) between large positive and negative values traces a closed loop in the \((H,B)\) plane, with \(B=\mu_0(H+M)\).

Derivation of the loss. In a long solenoid with \(n\) turns per unit length around a core of section \(A\) and length \(\ell\), \(H=nI\), and a change \(dB\) induces an emf \(n\ell A\,dB/dt\). The source supplies work \(I\,n\ell A\,dB=(A\ell)H\,dB\). Integrating over a cycle gives \(w\) per unit volume.

The demagnetizing field

A magnetized body of finite length carries magnetic poles on its ends, and these produce inside the body a field opposed to \(\mathbf M\). For a uniformly magnetized ellipsoid this field is uniform:

A permanent magnet left to itself works at the point of its loop where \(H=-NM\). If the demagnetizing field exceeds the coercivity, the magnet loses most of its magnetization. With the steels of the 1890s, whose coercivities were small, magnets had to be long and thin, which is why horseshoe and bar magnets of the period are so elongated. Curie measured the remanence and coercivity of steels of different compositions and heat treatments, to find those that gave the most durable magnets. Her memoir appeared in 1898 in the Society’s Bulletin.

Measuring activity: the quartz electrometer

For the general reader. Becquerel had detected uranium rays with photographic plates and gold-leaf electroscopes, which gave only rough comparisons. Marie Curie wanted numbers. She measured the tiny electric current that the rays let pass through air, balancing it against a known charge released by squeezing a quartz crystal. The method, devised by Pierre and Jacques Curie, made it possible to compare activities within a few per cent, and everything else in her work rested on it.

The ionization chamber

In her note of 12 April 1898 Curie described the apparatus. Two parallel metal plates, 8 cm in diameter and 3 cm apart, formed a capacitor. The substance to be tested, finely powdered, was spread in a uniform layer on the lower plate, which was held at 100 volts. The upper plate was connected to an electrometer. The rays ionize the air between the plates; the ions drift to the plates and carry a current, of order \(10^{-11}\) ampere for uranium.

Only the saturation current measures the radiation alone.

The piezoelectric quartz

In 1880 Pierre and Jacques Curie found that some crystals develop electric charge on their faces when compressed (Lesson 04 of the companion course on Pierre Curie). A quartz plate cut with its faces perpendicular to an electrical axis, of length \(L\) and thickness \(e\), and stretched along its length by a force \(F\), releases on its silvered faces a charge

The charge depends only on the load and the geometry, so a hanging weight supplies an exactly known charge.

The method needs no calibration of the electrometer, which only has to show zero. The operator let the weight down by hand while watching the electrometer and a timer.

Radioactivity is atomic

For the general reader. With her electrometer, Curie measured dozens of metals, salts, oxides and minerals. The activity of a uranium compound depended only on how much uranium it held, whatever the other elements and whatever the chemical form. That made the radiation a property of the atom. Thorium turned out to be active too. Then came the surprise: two uranium minerals were more active than pure uranium. Something in them had to be more active still.

The conclusion was that the emission is an atomic property. In their July note the Curies proposed the word radio-active for substances that emit such rays. Searching for other active elements, she found that thorium compounds are active. Gerhard Carl Schmidt in Germany found the same independently, in the same months.

The anomaly

Two minerals broke the rule. Pitchblende, a uranium oxide ore, and chalcolite, a copper uranyl phosphate, were more active than uranium metal itself. To be sure the anomaly was not a matter of mineral texture, she synthesized chalcolite from pure products. The artificial crystals were about as active as their uranium content predicted, several times less than the natural mineral. Her Nobel lecture of 1911 gives round figures: pitchblende containing 75% uranium oxide was about four times as active as that oxide; natural chalcolite about twice as active as uranium, artificial chalcolite about half. The minerals had to contain small amounts of a substance far more active than uranium. This conclusion, stated in the April note, is the turning point of her career.

Why the minerals are more active

The modern answer is the decay chain. Uranium-238 decays through a series of daughters to lead-206. In an ore that has been undisturbed for millions of years, each daughter is in secular equilibrium with uranium (Lesson 09), so every member of the chain has the same activity as the uranium itself. The uranium-238 series contains eight alpha emitters: uranium-238, uranium-234, thorium-230, radium-226, radon-222, polonium-218, polonium-214 and polonium-210. Freshly purified uranium keeps only the two uranium isotopes, which cannot be separated chemically, plus the short-lived beta emitters that grow back within months. The mineral therefore emits about four times as many alpha particles per uranium atom as the purified metal.

Polonium and radium

For the general reader. To find the hidden substance, Marie and Pierre Curie took pitchblende apart by ordinary chemistry and measured the activity of every fraction. The activity concentrated in two places: with bismuth, and with barium. In 1898 they announced two new elements, polonium, named for Marie’s country, and radium. Neither had been seen; both were known only by their radiation. Proving that radium was an element took four more years of heavy work.

Separation guided by activity

The Curies began processing a 100 g sample of pitchblende in April 1898. At each step of a standard analysis they measured the activity of the precipitate and the solution and followed the activity. When hydrogen sulphide was passed through an acid solution, the activity went down with the sulphides of the bismuth group. Bismuth sulphide could then be partly separated from the active substance by heating in a vacuum, since the active sulphide is more volatile. The note of 18 July 1898, presented to the Academy by Becquerel, reported a product about 400 times as active as uranium and proposed the name polonium for the metal it should contain, analytically close to bismuth. The spectroscopist Eugène Demarçay found no new lines.

A second activity followed barium. The note of 26 December 1898, by Pierre Curie, Marie Curie and Gustave Bémont, reported barium chloride preparations about 60 times and then 900 times as active as uranium, and proposed the name radium. This time Demarçay found a new spectral line, which strengthened as the activity rose. The chloride of the new element was less soluble than barium chloride in water and in aqueous alcohol, which provided the means of separation.

Fractional crystallization

Radium and barium are chemically almost identical, so no single reaction separates them. Curie used repeated crystallization of the chlorides, and later the bromides. When a saturated solution of mixed chlorides cools, the crystals that form are richer in radium than the solution they leave.

Proof. By induction. Crystals from \((n-1,j)\) have ratio \(k\cdot k^{n-1-2j}r_0=k^{n-2j}r_0\), and liquor from \((n-1,j-1)\) has ratio \(k^{n-1-2(j-1)}r_0/k=k^{n-2j}r_0\). The two parts combined into \((n,j)\) agree, so combining them does not spoil the enrichment.

Real enrichment factors per step are modest, and the scheme must be carried through many rows. In her Nobel lecture Curie spoke of several thousand crystallizations. From 1899 she moved to tonnes of residue from the uranium works at St Joachimsthal in Bohemia, obtained with the help of the Austrian government, treating batches in an old shed across the courtyard of the school. A tonne of residue yielded a few decigrams of radium. In 1902 she had about a decigram of radium chloride pure enough to measure its atomic weight (Lesson 08).

The atomic weight of radium

For the general reader. Chemists of 1900 would accept a new element only if it had a spectrum and an atomic weight. Curie measured the atomic weight of radium on her enriched barium–radium chlorides as they became purer. The apparent value climbed from that of barium, 137, toward 225 in 1902 and 226.45 in 1907. Radium took its place below barium in the periodic table, and in 1910 she and Debierne isolated the metal.

The chloride method

Curie dissolved a weighed quantity of anhydrous chloride, precipitated all the chloride as silver chloride, and weighed the precipitate. This was the classical method of the atomic-weight chemists.

The formula assumes a divalent metal. Curie’s spectroscopic and chemical evidence placed radium with the alkaline earths, so the assumption was justified, and the result put radium in the empty place below barium in the periodic table.

Mixtures

On a mixture of barium and radium chlorides the method returns an apparent atomic weight, the mole-weighted average:

where \(x\) is the mole fraction of radium. Her Nobel lecture lists successive values 138, 146, 174, 225 and 226.45. The last came in 1907 from 0.4 g of very pure salt; later determinations by others gave values between 226.3 and 226.6. The modern atomic mass of radium-226 is 226.025.

The metal

In 1910 Curie and André-Louis Debierne electrolyzed a solution of pure radium chloride with a mercury cathode, forming a radium amalgam. Heating the amalgam in hydrogen drove off the mercury and left a white metal that melts at about 700 °C and reacts vigorously with water and air. The preparation used a single decigram of salt. It settled any doubt that radium was an element, though the salts serve every practical purpose.

Decay, equilibrium and the curie

For the general reader. Radium gives off a radioactive gas, then called the emanation and now radon, and its activity builds up and dies away on fixed time scales. Polonium, Curie found, loses half its activity in about 140 days. Understanding these laws let her define a unit of radioactivity and prepare, in 1911, the sealed tube of radium chloride against which every radium source in the world was compared.

Derivation. \(dN_2/dt=\lambda_1N_1(0)e^{-\lambda_1t}-\lambda_2N_2\). Multiply by \(e^{\lambda_2t}\) and integrate from 0 to \(t\): \(N_2e^{\lambda_2t}=\lambda_1N_1(0)\,(e^{(\lambda_2-\lambda_1)t}-1)/(\lambda_2-\lambda_1)\). The equilibrium limit follows by dropping \(e^{-\lambda_2t}\) and \(\lambda_1\) against \(\lambda_2\).

Polonium decays

Unlike uranium and radium, whose activity seemed constant, polonium preparations weakened. Curie followed the decrease and found it exponential, with activity halved in about 140 days; the modern value for polonium-210 is 138.4 days. With Debierne she prepared polonium of high activity around 1910, and Debierne showed that it produces helium.

The curie and the radium standard

Medicine and industry were buying radium by the milligram by 1910, and there was no agreed way to say how much a tube held. The International Radiology Congress at Brussels in September 1910 set up a committee, with Curie and Ernest Rutherford its leading members. It named the unit the curie and defined it as the quantity of radon in equilibrium with one gram of radium; it later became the activity \(3.7\times10^{10}\) decays per second. The committee charged Curie with preparing an international standard. In 1911 she sealed 21.99 mg of pure radium chloride in a glass tube. In March 1912 it was compared, through the gamma radiation that penetrates the glass, with standards prepared independently by Otto Hönigschmid in Vienna, and the two agreed. Her tube was deposited in 1913 at the International Bureau of Weights and Measures at Sèvres.

Radiology at war, radium in medicine

For the general reader. When war broke out in August 1914, French army hospitals had almost no X-ray equipment. Curie equipped cars with X-ray tubes and generators, learned to drive, trained operators and went to the front with her daughter Irène. She also collected the radon given off by her radium into sealed tubes for treating wounds and tumours. Both activities rest on the physics of how radiation passes through and acts on the body.

X-ray radiography

An X-ray tube accelerates electrons through a voltage \(U\) onto a metal target. Bremsstrahlung gives a continuous spectrum with a sharp short-wavelength limit.

Shell fragments and bullets, of iron and lead, are even more opaque. Radiography let surgeons find them before cutting. Curie’s radiological cars, nicknamed petites Curies, carried a dynamo driven by the engine to power the tube. About twenty were equipped, along with many fixed radiology posts, and she trained women as radiology operators. She obtained her driving licence in 1916.

Radon sources

From 1915 she pumped the radon accumulating over her radium solution into fine glass tubes, which were sealed and sent to hospitals. The radium stayed safe in Paris and regenerated its radon every few weeks. A radon tube delivers its radiation over days; the gamma rays come from the short-lived descendants lead-214 and bismuth-214.

Alone at the Sorbonne, 1906–1914

The years after the discovery were heavy with labour and recognition. From 1900 Marie Curie taught physics at the École normale supérieure for young women at Sèvres. She defended her thesis on 25 June 1903. In November 1903 the Royal Society awarded the Davy Medal to Pierre and Marie jointly, and in December the Nobel Prize in Physics went half to Becquerel and half to the Curies. Her name was included after the Swedish mathematician Gösta Mittag-Leffler alerted Pierre that the original nomination omitted her. She was the first woman to receive a Nobel Prize. Pierre gave their Nobel lecture in Stockholm only in June 1905. In 1904 Pierre obtained a chair at the Sorbonne with a laboratory, and Marie became its head of work. Their second daughter, Ève, was born in December 1904.

On 19 April 1906 Pierre was killed by a horse-drawn wagon in the rue Dauphine. On 13 May the Faculty of Sciences entrusted his course to Marie, and on 5 November 1906 she gave her first lecture to a crowded hall. She moved with her daughters and her father-in-law to Sceaux. On 16 November 1908 she became titular professor, the first woman to hold a professorship at the University of Paris.

The scientific work continued: the atomic weight of 1907, radium metal and the two-volume Traité de radioactivité in 1910, the radium standard in 1911. In 1911 she stood for election to the Académie des sciences and lost by one or two votes to the physicist Édouard Branly, in a contest marked by hostility to her as a woman and a foreigner. That autumn she attended the first Solvay Conference in Brussels, the only woman among the participants; she remained on the Solvay physics council until 1933. In November 1911 newspapers published letters revealing her relationship with the physicist Paul Langevin, a married former student of Pierre’s. A crowd gathered outside her house, and she took refuge with her daughters at a friend’s home. Svante Arrhenius, for the Nobel committee, suggested she stay away from Stockholm. She went, received the Nobel Prize in Chemistry on 10 December 1911 for the discovery of radium and polonium, the isolation of radium and the study of its compounds, and lectured the next day on radium and the new concepts in chemistry. She was the first person to receive two Nobel Prizes.

The Radium Institute, last years and legacy, 1914–1934

In 1909 the University of Paris and the Pasteur Institute agreed to build a Radium Institute with two laboratories, one for physics and chemistry under Curie and one for biological and medical research. The building was finished in 1914, just before the war. After the war (Lesson 10) she directed the Curie laboratory until her death. It became a leading centre of radiochemistry. Around 45 women worked there under her, among them Marguerite Perey, who discovered francium in 1939 and in 1962 became the first woman elected a corresponding member of the Académie des sciences.

Radium remained scarce and expensive. In 1921 the American journalist Marie Mattingly Meloney organized a tour of the United States, and on 20 May President Warren G. Harding presented her at the White House with one gram of radium, bought with money raised by American women. In 1922 she became the first woman member of the Académie de médecine and joined the League of Nations’ Committee on Intellectual Co-operation, where she served until 1934. In 1923 she published a biography of Pierre. In 1925 she laid the foundation stone of a Radium Institute in Warsaw; a second American tour in 1929 raised money for radium for it, and it opened in 1932 under her sister Bronisława.

In January 1934 Irène Curie and her husband Frédéric Joliot announced artificial radioactivity, produced in her laboratory. Marie Curie’s health had been failing for years, with cataracts and chronic fatigue. She died on 4 July 1934 at the Sancellemoz sanatorium at Passy in Haute-Savoie, of aplastic anaemia attributed to long exposure to radiation, and was buried beside Pierre at Sceaux. In April 1995 their remains were moved to the Panthéon; she was the first woman honoured there for her own achievements.

Legacy

• Science: radioactivity as an atomic property, two new elements, and the method of radiochemical analysis, in which activity guides chemistry. Her measurements and standards turned radium into the reference for a generation of nuclear physics.
• Units and names: the curie, \(3.7\times10^{10}\) Bq; the element curium (96), discovered in 1944 and named for Marie and Pierre; the European Union’s Marie Skłodowska-Curie Actions for researcher mobility.
• Institutions: the Institut Curie in Paris and the Maria Skłodowska-Curie National Research Institute of Oncology in Warsaw grew from her two Radium Institutes.
• Family: Irène Joliot-Curie and Frédéric Joliot-Curie shared the Nobel Prize in Chemistry in 1935; Ève Curie wrote her mother’s best-known biography.

Sources and further reading

Biography

1. Marie Curie, Wikipedia (English), and Marie Curie, Wikipédia (French).
2. Nobel Foundation, Marie Curie: Biographical.
3. American Institute of Physics, Marie Curie and the Science of Radioactivity, online exhibit.
4. PSL University and Musée Curie, Marie Curie (1867–1934), virtual exhibition.
5. M. Curie, Pierre Curie, with autobiographical notes, Macmillan, 1923.
6. È. Curie, Madame Curie, Gallimard, 1938.
7. S. Quinn, Marie Curie: A Life, Simon & Schuster, 1995.

Physics and chemistry

1. M. Skłodowska Curie, Rayons émis par les composés de l’uranium et du thorium, Comptes rendus 126 (1898), 1101–1103. English translation, Classic Chemistry, Le Moyne College.
2. P. Curie and M. Curie, Sur une substance nouvelle radio-active, contenue dans la pechblende, Comptes rendus 127 (1898), 175–178.
3. P. Curie, M. Curie and G. Bémont, Sur une nouvelle substance fortement radio-active, contenue dans la pechblende, Comptes rendus 127 (1898), 1215–1217. English translation.
4. M. Curie, Recherches sur les substances radioactives, thesis, Paris, 1903.
5. M. Curie, Radium and the new concepts in chemistry, Nobel lecture, 11 December 1911.
6. M. Curie, Traité de radioactivité, 2 vols., Gauthier-Villars, 1910.
7. M. Curie, The Discovery of Radium, address at Vassar College, 1921 (Project Gutenberg).
8. NIST, The radium standards, history of the 1911 Curie standard and the Hönigschmid standards.
9. R. Guillaumont, La découverte du polonium et du radium, Académie des sciences.