Irène Joliot-Curie

How to use this course

Irène Curie grew up inside the science of radioactivity. Her parents discovered polonium and radium the year after her birth. She learned physics from her mother and her mother’s friends, worked beside her mother in the First World War, and joined her laboratory in 1918. Her thesis on the alpha rays of polonium made her an expert in the particle that became the main probe of the nucleus. With her husband Frédéric Joliot she found the evidence that led to the neutron and to positron pair production, and in 1934 the first artificial radioactivity. On her own, with Pavle Savić, she found in 1938 a radioactive substance in uranium that behaved like lanthanum. It was a fission product, and it set Otto Hahn and Fritz Strassmann on the path that led to fission.

This course pairs with the Frédéric Joliot-Curie course, which covers the cloud chamber, pair production, artificial radioactivity, the chemistry of transmutation and the chain reaction. Here the technical lessons follow her own specialities: polonium, the physics of alpha particles, counting statistics, the theory of alpha decay and alpha reactions, and the radiochemistry of the heavy elements. A final technical lesson treats radiation dose, which shaped her life and her death.

Each technical lesson opens with a paragraph for the general reader, then gives precise statements, derivations and exercises with folded solutions. Lesson 09 is the centre of the course. Masses are atomic masses in unified units u, with \(1\,\mathrm u\,c^2=931.5\) MeV, and \(e^2/4\pi\varepsilon_0=1.44\) MeV fm. Irène signed her papers “Irène Curie” throughout her career.

Timeline

A Curie childhood, 1897–1914

Irène Curie was born in Paris on 12 September 1897. Her mother, Marie Skłodowska-Curie, was then beginning the doctoral research on uranium rays that led, with her father Pierre Curie, to the discovery of polonium and radium in 1898. Her sister Ève was born in 1904. Pierre Curie was killed on 19 April 1906 when he was struck by a horse-drawn wagon in a Paris street. His father, the physician Eugène Curie, lived with the family and helped raise the two girls. He was a strong influence on Irène.

The Coopérative

Marie Curie thought French schools gave children too many hours at a desk and too little science. With a group of friends from the Sorbonne she set up a teaching cooperative for their own children, about ten pupils in all. Each parent taught a subject; the teachers included Marie Curie, Paul Langevin and Jean Perrin. The lessons were practical and informal, with much time in the laboratory. Irène later finished her secondary education at the Collège Sévigné, a private school in Paris, and in 1914 began studying at the Faculty of Science.

In December 1911 she went with her mother to Stockholm, where Marie Curie received her second Nobel Prize, in chemistry. Irène was fourteen.

X-rays at the front, 1914–1918

When war broke out in August 1914, Marie Curie saw that army surgeons had almost no X-ray equipment for locating bullets and shell fragments in the wounded. Through the Red Cross she organised radiology cars, soon nicknamed “petites Curies”, and fixed radiology posts in hospitals near the front. Irène trained as a nurse and from 1916 worked as a radiographer, first with her mother and then alone, at hospitals in Belgium and northern France, including Furnes, Ypres and Amiens. She was eighteen, and she received the Médaille militaire in 1918.

Her work combined physics and medicine. She set up X-ray tubes, generators and screens, located fragments by triangulating from two exposures, and taught surgeons to use the images. From 1917 she trained nurses in radiology at the Edith Cavell hospital in Paris while resuming her own studies. Protection against X-rays was rudimentary. The doses she received in these years were the first of many.

Locating a fragment

A standard method was to take two exposures with the tube moved sideways by a known distance \(d\). A fragment at height \(h\) above the plate, with the tube focus at height \(H\), shows a shift \(s\) of its shadow. By similar triangles

A few millimetres of shift fix the depth of the fragment to about a centimetre, enough to guide the surgeon. Lesson 10 returns to the doses involved.

The Radium Institute and the thesis, 1918–1926

In 1918 Irène became an assistant (préparatrice) in her mother’s laboratory at the Radium Institute, and in 1920 her assistant with degrees in physics and mathematics. Her first research measured the atomic weight of chlorine from different minerals, a test of whether isotopic composition varied in nature. In 1924, with Catherine Chamié, she measured the decay constant of radon.

Her doctoral thesis, defended in March 1925, was titled Recherches sur les rayons α du polonium. Oscillation de parcours, vitesse d’émission, pouvoir ionisant. It studied three things: the fluctuations in the range of individual alpha particles from polonium (range straggling), their speed of emission, and their ionising power along the track. The thesis is the subject of Lesson 05. She also studied the ionisation curve of alpha particles in air with the Czech physicist František Běhounek.

In 1925 a young engineer, Frédéric Joliot, arrived as her mother’s assistant, and Irène taught him radioactive techniques. She was reserved, direct and indifferent to social convention; he was outgoing and charming. They married in October 1926. Their daughter Hélène was born in 1927 and their son Pierre in 1932. From 1929 they published together. The story of their joint work of 1932–1934, the neutron and positron they did not identify and the artificial radioactivity they did, is told in the Frédéric Joliot-Curie course, Lessons 03 and 05–08.

Polonium: making the sources

For the general reader. The Joliot-Curies’ discoveries depended on one material: polonium, the element her parents had found in 1898. Polonium emits alpha particles and almost no gamma rays, so it bombards a target cleanly. It cannot be bought in a jar. It must be extracted from a long chain of radioactive decays, and its supply depends on how long ago its ancestors were made. Irène Curie prepared the strongest polonium sources in the world from the residues of old radon tubes.

The chain

Radon-222 (3.82 days) decays through short-lived products to lead-210, historically called radium D, which has a half-life of 22.2 years. Radium D decays to bismuth-210 (radium E, 5.01 days), which decays to polonium-210 (138.4 days), which decays to stable lead-206. Hospitals used sealed radon tubes for cancer therapy. After the radon had decayed, the tubes held radium D, which kept producing polonium for decades.

Derivation. The equations are \(\dot N_1=-\lambda_1N_1\), \(\dot N_2=\lambda_1N_1-\lambda_2N_2\), \(\dot N_3=\lambda_2N_2-\lambda_3N_3\). Solve in turn with an integrating factor: \(N_2=\frac{\lambda_1N_1(0)}{\lambda_2-\lambda_1}(e^{-\lambda_1t}-e^{-\lambda_2t})\), then substitute into the third equation. Each term \(e^{-\lambda_it}\) is driven with coefficient fixed by the others, and the constant multiplying \(e^{-\lambda_3t}\) is set by \(N_3(0)=0\). Laplace transforms give the same result in one line: \(\tilde N_3(s)=\lambda_1\lambda_2N_1(0)/[(s+\lambda_1)(s+\lambda_2)(s+\lambda_3)]\), whose partial fractions are the three terms.

A source of radium D therefore produces as many polonium atoms per second as it holds radium D decays per second, once a few polonium half-lives have passed. Polonium can be separated chemically, for example by spontaneous deposition on silver or nickel from acid solution, and the radium D left behind regenerates it.

Alpha particles in matter

For the general reader. An alpha particle ploughs through air, knocking electrons off molecules as it goes. It slows down, and the slower it goes the more it ionises, until it stops abruptly after a few centimetres. All alpha particles from polonium start with the same energy, yet they do not all stop at exactly the same place. Irène Curie’s thesis measured how much their ranges vary, how fast they leave the nucleus, and how their ionising power changes along the path.

Stopping power

Sketch. An electron at impact parameter \(b\) receives the transverse impulse \(\Delta p=2ze^2/(4\pi\varepsilon_0bv)\), hence energy \(\Delta p^2/2m_e\). Integrating over a cylinder of electrons, \(2\pi n_e b\,db\,dx\), gives \(\frac{4\pi n_ez^2}{m_ev^2}(e^2/4\pi\varepsilon_0)^2\ln(b_{\max}/b_{\min})\). The cut-offs come from the maximum energy transfer \(2m_ev^2\) and the binding of the electrons, measured by \(I\); the quantum calculation by Hans Bethe in 1930 fixes the logarithm. The loss depends on the particle only through \(z^2/v^2\), so slower particles lose energy faster. This produces the Bragg peak in the figure at the top of the page.

Range

The mean range is \(R(E_0)=\int_0^{E_0}dE/(-dE/dx)\). Neglecting the slow variation of the logarithm, \(-dE/dx\propto1/E\), so \(R\propto E^2\propto v^4\). The logarithm makes the dependence weaker. Hans Geiger found empirically that for alpha particles in air

Polonium-210 alpha particles have 5.30 MeV and a mean range of about 3.9 cm in air at 15 °C.

Straggling

Energy loss happens in many independent collisions, each random. The total number of collisions along a path fluctuates, so identical particles stop at slightly different depths. By the central limit theorem the stopping points are close to normally distributed about the mean range. For polonium alphas in air the standard deviation is about 1% of the range. Measuring this distribution precisely, with a cloud chamber and with ionisation methods, was the first part of Irène Curie’s thesis. The same theory, extended by Niels Bohr in 1915, tests the statistical model of the energy loss.

The statistics of decay

For the general reader. Each radioactive nucleus decays at a random moment, without memory of how long it has lived. A counter clicking near a source therefore never clicks at a perfectly steady rate. To measure a half-life, or to decide whether a weak new activity is real, one has to understand these fluctuations. Irène Curie’s measurement of the decay constant of radon in 1924 and the Joliot-Curies’ decay curves of 1934 rested on this statistics.

Derivation. The count is binomial, \(\binom Nk p^k(1-p)^{N-k}\). For \(N\to\infty\) with \(Np=\mu\) fixed, \(\binom Nk p^k\to\mu^k/k!\) and \((1-p)^{N-k}\to e^{-\mu}\). The mean and variance of the binomial are \(Np\) and \(Np(1-p)\to\mu\). A count of \(k\) thus has standard error about \(\sqrt k\), and its relative error is \(1/\sqrt k\).

Measuring a half-life

With counts \(C_1\) and \(C_2\) in equal intervals centred at times \(t_1\) and \(t_2\), the estimate is \(\hat\lambda=\ln(C_1/C_2)/(t_2-t_1)\). Propagating the Poisson errors,

In practice one fits a straight line to \(\ln C\) against \(t\) for many intervals, weighting each point by its count, and subtracts the background measured with no source. Two activities with different half-lives give a curve whose tail is the longer-lived component; subtracting its extrapolation reveals the shorter one. Irène Curie relied on such analyses in all her work on neutron-induced activities.

Dead time

After each count a Geiger–Müller counter is insensitive for a dead time \(\tau\), typically \(10^{-4}\) s. If the recorded rate is \(m\), the counter is dead for a fraction \(m\tau\) of the time, so the true rate is

Barriers, tunnelling and alpha reactions

For the general reader. Why does polonium-210 live for months while polonium-212 lives for less than a millionth of a second, though their alpha particles differ in energy by less than a factor of two? And why did the alpha particles of polonium produce artificial radioactivity in light elements but not in heavy ones? The same quantum effect answers both: an alpha particle must tunnel through the electrical wall around the nucleus, and the probability depends exponentially on its energy.

Derivation. Substitute \(r=b\cos^2\theta\). Then \(V-Q=Q\tan^2\theta\) and \(dr=-2b\cos\theta\sin\theta\,d\theta\), so the integral becomes \(2b\sqrt Q\int_0^{\theta_0}\sin^2\theta\,d\theta\) with \(\cos^2\theta_0=x\). Evaluate and use \(\sqrt{2\mu Q}\,b/\hbar=\eta\). For \(x\ll1\), \(2G\approx2\pi\eta-8\eta\sqrt x\), and since \(\eta\propto Z/\sqrt Q\), \(\ln T\approx-c_1Z/\sqrt Q+c_2\sqrt{ZR}\).

Alpha-induced reactions

The same factor controls the reverse process. An alpha particle approaching a nucleus of charge \(Z_X\) below the barrier height \(B=2Z_Xe^2/(4\pi\varepsilon_0R)\) enters only by tunnelling. Polonium alphas, at 5.3 MeV, are above the barrier for beryllium (about 2.6 MeV) and boron (about 3.2 MeV), somewhat below it for aluminium (about 6.8 MeV) and hopelessly below it for heavy elements. This is why the Joliot-Curies found artificial radioactivity only in light elements, and why neutrons, which feel no barrier, became the tool for the heavy elements.

Neutrons on heavy elements, 1934–1938

For the general reader. Within weeks of artificial radioactivity, Enrico Fermi in Rome bombarded every element he could find with neutrons instead of alpha particles. Neutrons have no charge and enter even the heaviest nuclei. In uranium Fermi found several new activities and thought he had made elements beyond uranium. For four years laboratories in Rome, Berlin and Paris tried to sort out what the activities in irradiated uranium and thorium were. Irène Curie led the Paris effort.

Capture and the 1/v law

A slow neutron absorbed by a nucleus \({}^A Z\) forms \({}^{A+1}Z\) in an excited state, which usually emits a gamma ray: the \((n,\gamma)\) reaction. The product often has too many neutrons and decays by \(\beta^-\) emission to element \(Z+1\).

A neutron spends time proportional to \(1/v\) within range of the nucleus, and at low energy the quantum amplitude for capture is roughly constant, so the probability goes as \(1/v\). This is why Fermi found in October 1934 that neutrons slowed in paraffin were far more effective.

Uranium and thorium

Fermi’s expectation for uranium was \({}^{238}\mathrm U(n,\gamma){}^{239}\mathrm U\xrightarrow{\beta^-}{}^{239}93\xrightarrow{\beta^-}{}^{239}94\). Ida Noddack objected in 1934 that the nucleus might break into large fragments, but no one tested her suggestion. Hahn, Meitner and Strassmann in Berlin assigned the activities to a series of “transuranic” elements with chemistry like rhenium, osmium and iridium. From 1935 Irène Curie worked on the thorium activities with Hans von Halban and Peter Preiswerk, and from 1937 on uranium with Pavle Savić, a young Yugoslav physicist working at the Radium Institute.

The assignments were hard for three reasons. Many activities were produced at once, with similar half-lives. The amounts were invisible, so every identification relied on carriers (Frédéric Joliot-Curie course, Lesson 08). And the chemistry of the elements beyond uranium was unknown; chemists assumed they would continue the transition-metal groups of the periodic table. In fact neptunium and plutonium, discovered at Berkeley in 1940 and 1941, behave more like uranium.

The 3.5-hour substance and fission

For the general reader. In 1937 Irène Curie and Pavle Savić found in irradiated uranium an activity with a half-life of about 3.5 hours. It did not behave like any element near uranium. In 1938 they showed that it followed lanthanum, element 57, through chemical separations. Lanthanum sits less than two-thirds of the way up the periodic table from uranium. They did not take the final step of concluding that the uranium nucleus had split. Their results led Otto Hahn and Fritz Strassmann to repeat their own experiments, and in December 1938 these found barium. The split nucleus, fission, had been found.

Separating unweighable elements

An invisible radioactive species is identified by the element it follows. When a carrier is precipitated or crystallised, a trace element that is chemically identical to the carrier is incorporated uniformly. A trace element that is merely similar is incorporated in a different proportion. Repeated fractional crystallisation magnifies the difference.

Derivation. If each newly deposited layer takes up trace and carrier in the ratio \(\lambda_{\mathrm{DH}}\) times their ratio in solution, then \(dT/T=\lambda_{\mathrm{DH}}\,dC/C\), where \(T\) and \(C\) are the amounts left in solution. Integrating gives the law.

The 3.5-hour body

Irène Curie and Savić first proposed that the activity was an isotope of thorium. In Berlin, Strassmann looked for thorium and found none. In the spring of 1938 Curie and Savić reported that the substance had properties like those of the rare earths, and in September 1938 that it followed lanthanum, from which they could separate it only partially by fractional crystallisation. They did not conclude that the uranium nucleus had split. In modern terms the activity was a mixture of fission products, mainly isotopes of lanthanum and yttrium with half-lives near 3.5 hours, which explains the partial separation.

Barium

Hahn and Strassmann now repeated their own experiments, looking for products resembling radium, the heavier homologue of barium. They found activities that followed a barium carrier and assigned them to radium. In December 1938 they tried to separate the “radium” from barium by fractional crystallisation of barium salts and failed. Mixed with genuine radium isotopes, the genuine radium separated and the new activities did not. They concluded, against their training as nuclear chemists, that the activities were barium. Their paper appeared on 6 January 1939. Lise Meitner and Otto Frisch interpreted the result as the splitting of the nucleus into two roughly equal parts. Part of the Paris lanthanum activity came from the decay of fission barium.

Radiation dose

For the general reader. Irène Curie was exposed to radiation from the age of eighteen: X-rays in wartime hospitals, then radium, radon and polonium in the laboratory. In 1946 a sealed capsule of polonium burst on her bench. She developed leukaemia and died at 58, as her mother had died of an illness linked to radiation at 66. This lesson introduces the quantities used to measure radiation dose and applies them to the sources she handled.

Alpha particles deposit all their energy within about 40 µm of tissue, so an alpha emitter outside the body is almost harmless while the same emitter inside it is extremely dangerous. Gamma rays penetrate. For a point gamma source the fluence falls as \(1/r^2\) and is attenuated by shielding as \(e^{-\mu x}\). The radiation weighting factors are modern conventions, set by the International Commission on Radiological Protection; in the 1920s and 1930s protection was based on rules of thumb and on avoiding reddening of the skin.

Nobel, government and war, 1935–1945

Marie Curie died on 4 July 1934, six months after seeing the first artificial radioelements. In 1935 Irène Joliot-Curie and Frédéric Joliot shared the Nobel Prize in Chemistry for their synthesis of new radioactive elements. Irène and Marie Curie remain the only mother and daughter to have won Nobel Prizes.

In government

In June 1936 Léon Blum formed the Popular Front government and appointed three women as under-secretaries of state, although French women did not yet have the vote. Irène Joliot-Curie became Under-Secretary of State for Scientific Research. She had agreed to serve only for a short time, and she resigned in September after about three months, handing over to Jean Perrin. In that time she helped prepare the organisation of state-funded research that led to the CNRS. She remained committed to women’s rights and to education throughout her life, and was active in the Union des femmes françaises after the war.

In 1937 she became professor at the Faculty of Science in Paris. From then on she and Frédéric worked largely in separate laboratories, she at the Radium Institute and he at the Collège de France and Ivry. The work with Savić described in Lesson 09 belongs to these years. She was made an officer of the Legion of Honour in 1939.

Occupation

Tuberculosis, first contracted in the war years, forced her into sanatoria repeatedly from the 1930s. During the Occupation Frédéric led the Resistance movement Front national. In June 1944, with Frédéric in hiding, she crossed into Switzerland through the Jura with their two children, and returned after the Liberation.

The CEA, Orsay and legacy, 1945–1956

In 1946 she became director of the Radium Institute’s Curie laboratory, her mother’s former post. She was one of the commissioners of the new Commissariat à l’énergie atomique for six years and took part in the construction of ZOE, the first French reactor, which went critical in December 1948. Her term ended in 1951, the year after Frédéric’s dismissal from the commission. She was not a member of the Communist Party, but she was active in the peace movement and a member of the World Peace Council.

In 1951 she stood for election to the Académie des sciences, which had rejected her mother in 1911. She was turned down, and said she would stand again at every opportunity in order to make the point that women should be admitted. The Académie elected its first woman, as corresponding member, in 1962: Marguerite Perey, who had discovered francium as Marie Curie’s assistant.

In her last years she planned a large new centre for nuclear physics at Orsay, south of Paris, with a synchrocyclotron of 160 MeV. Frédéric completed it after her death. She was diagnosed with leukaemia and died in Paris on 17 March 1956, aged 58. The government gave her a state funeral. She is buried at Sceaux, beside Frédéric, who died two years later. Her daughter Hélène Langevin-Joliot became a nuclear physicist and her son Pierre Joliot a biologist who studied photosynthesis.

Legacy

• Artificial radioactivity: with Frédéric Joliot, the discovery that radioactive atoms can be made at will, the basis of radioisotope production, tracers and nuclear medicine.
• Polonium and alpha physics: her sources and measurements underpinned the experiments that revealed the neutron and positron pair production.
• The road to fission: the 3.5-hour lanthanum-like activity of 1938 was the first chemical sign that neutrons produce mid-weight elements from uranium.
• Women in science and public life: one of the first three women in a French government, and a persistent public challenge to the exclusion of women from the Académie des sciences.

Connections

People linked to Irène Joliot-Curie across this collection. A name without a link has no page here yet. See Irène Joliot-Curie in the network of all 337 lives →

Teachers and advisors

• Marie Curie 1918-1925

Influenced by

• Enrico Fermi
• Jean Perrin
• Paul Langevin

Influenced

• Enrico Fermi
• James Chadwick 1932

Colleagues and collaborators

• Jean Perrin
• Hans von Halban 1935-1937
• Marguerite Perey
• Pavle Savić 1937-1939

Rivals

• James Chadwick
• Lise Meitner 1933-1938
• Otto Hahn 1935-1938

Family

• Frédéric Joliot-Curie 1926-1956
• Marie Curie
• Pierre Curie
• Hélène Langevin-Joliot
• Pierre Joliot
• Ève Curie

Also mentioned in this life

• Hans Bethe
• Niels Bohr
• Charles-Augustin de Coulomb
• George Gamow
• Hans Geiger
• Pierre-Simon Laplace
• Siméon Denis Poisson
• Arnold Sommerfeld

Fields

• radioactivity 10 lives
• nuclear physics 13 lives
• nuclear chemistry 3 lives
• radiochemistry 6 lives

On CoursesHub

• Nuclear Physics

Sources and further reading

Biography

1. Irène Joliot-Curie: Biographical, NobelPrize.org.
2. Irène Joliot-Curie, Wikipedia; and the French article Irène Joliot-Curie.
3. Irène Joliot-Curie, research profile, Lindau Nobel Laureate Meetings mediatheque.
4. Irène Joliot-Curie, biography, Science Story Telling project (Flensburg).
5. È. Curie, Madame Curie, Gallimard, 1938 (for the family background).

Physics and chemistry

1. I. Curie, Recherches sur les rayons α du polonium. Oscillation de parcours, vitesse d’émission, pouvoir ionisant, Annales de Physique (1925).
2. I. Curie, F. Joliot, Un nouveau type de radioactivité, C. R. Acad. Sci. 198, 254 (1934); Artificial production of a new kind of radio-element, Nature 133, 201 (1934).
3. I. Joliot-Curie, Artificial production of radioactive elements, Nobel lecture, 12 December 1935.
4. I. Curie, P. Savitch, Sur le radioélément de période 3,5 heures formé dans l’uranium irradié par les neutrons, and Sur la nature du radioélément de période 3,5 heures formé dans l’uranium irradié par les neutrons, C. R. Acad. Sci. 206 (1938).
5. F. Guerra, M. Leone, N. Robotti, Neutron physics in the early 1930s, arXiv:physics/0510044.
6. Discovery of nuclear fission, Wikipedia (with references to the Curie–Savić and Hahn–Strassmann papers).
7. G. Gamow, Zur Quantentheorie des Atomkernes, Z. Physik 51, 204 (1928).
8. K. S. Krane, Introductory Nuclear Physics, Wiley, 1988; W. R. Leo, Techniques for Nuclear and Particle Physics Experiments, Springer, 1994.