A radioactive family

Madame Marie Curie and Pierre Curie created history with their exceptional hard work and phenomenal discoveries in the field of physics. She became the first-ever woman to be ever awarded a Nobel prize. Irène Joliot-Curie, her daughter and her husband, Frédéric Joliot-Curie, were applauded for the discovery of artificially created radioactive atoms.

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Photograph of Irène Curie and Frédéric Joliot discussing.
Frédéric Joliot (1900-1958) and Iréne Joliot-Curie (1897-1956) had been jointly awarded the Nobel Prize for Chemistry in 1935. Photograph 1940s.

Scientific research can proceed extremely slowly, but the end of the nineteenth century was an extremely profitable period for chemistry, with fundamental discoveries that followed one another in a handful of years. Between them, radium—a scarce chemical element—was discovered on December 21, 1898, by Marie and Pierre Curie.

Three years earlier, in the days following Christmas 1895, the discovery of X-rays had been made known by the German physicist Wilhelm Conrad Röntgen. Many scientists then engaged in X-ray research, fascinated by their ability to make the invisible, visible. Upstream, Marie Curie pointed her attention to another new and mysterious type of radiation, identified in 1896 by the French physicist Henri Becquerel while studying the phosphorescence of uranium salts.

Her analysis focused on pitchblende, another radioactive element. It was a crude mineral containing uranium and thorium that was extracted from the Ore Mountain Mines of today’s Czech Republic. The surprise was enormous when she realized that pitchblende was much more radioactive than the amount of uranium and thorium it contained. After the first discovery, an astonished Marie repeated the experiment several times. Eventually, in July 1898, Marie and Pierre identified a substance 300 times more active than uranium: polonium, named in honor of the scientist’s origins.

Marie is operating the apparatus. With her right hand she is adding/subtracting known weights from a pan hanging from a strip of piezo-electric material which generates a very small elecrical charge (in the region of pico-amps) according to the weight hung on it. This is nulled against the charge accumulated on an ion chamber due to radioactivity. In her left hand she has a stopwatch to measure the rate of change of charge using a quadrant electrometer. When the weight is changed, the time elapsed for the charge to be nulled is measured by the stopwatch. The charge is indicated by a light spot on the scale in front of her projected by the quadrant electrometer, which is off the left of the picture.
Pierre and Marie Curie in the laboratory, demonstrating the experimental apparatus used to detect the ionsation of air, and hence the radioactivity, of samples of purified ore which enabled their discovery of radium, 1904

At the same time, by systematically analyzing uranium in different compounds and forms with an instrument developed by her husband Pierre—the piezoelectric quartz balance—for measuring weak electric currents, Marie Curie realized that radiation was an atomic property of uranium. She called the phenomenon radioactivity.

A few months later, with the discovery of polonium, the couple informed the Academy of Sciences that they identified in the samples the spectral line of a new substance, unknown until then and 900 times more radioactive than uranium: radium.

Radium is not to enrich anyone. It is an element; it is for all people.

― Marie Curie

It took another four years of work and 10 tons of pitchblende to determine the new substances’ atomic weight in the quantity of a few grains of sand.

Radioactivity measurement units

The first unit of radioactivity chosen was the Curie. It was the amount of emanation (radon) in radioactive equilibrium with one gram of radium. Radium decays to radon, which in turn decays into atoms of polonium, lead, and bismuth.

However, radon has a low half-life compared to radium (4 days versus 1600 years), so, at a given moment, the number of radon atoms that decay balances the number of radon atoms that are formed. Measuring the corresponding amount of radon required a radium salt sample dissolved in a small liquid volume and placed in a “bubble trap” – a device to remove bubble air from a liquid source.

A pipe expelled the initial radioactive emanation, and then the bottle was closed. An emanation accumulated again in the bottle and reached radioactive equilibrium. It was then forced out of the sample by air bubbles via another pipe that gets to the liquid, then by air supply, to an ionization chamber connected to an electrometer.

Marie’s method, explained in her thesis, made it possible to measure the quantity of radioactive emanation at the time of saturation.

In 1950, the International Union redefined the Curie as the number of disintegrations per second in one gram of radium: approximately 3,7*1010.

Later, another more straightforward unit was defined, the Becquerel (Bq). One Bq is equal to one decay per second. A radioactivity measurement must account for all α, β, and γ decays of nuclei studied to express this unit.

Radioactivity, the internal heat source of our planet

Shortly after discovering the radioactivity, researchers observed that many rocks forming the ground were a little radioactive. They usually contained a low but measurable amount of uranium or thorium, their offspring, and potassium, which is naturally slightly radioactive.

In 1903, two German physicists, J. Elster and H. Geitel filled a dish with their garden soil and measured its radioactivity. They obtained a tiny concentration of radioelements. This when multiplied by the entire Earth’s mass or simply by the abundance of all rocks forming the Earth’s crust, gave an enormous quantity of radioactive material.

Lord Kelvin, calculated how long the Earth would take to cool in space, had obtained a probable Earth age of 100 million years. In 1895, he figured a period of only 24 million years. However, he did not consider, in its balance, the source of heat that constitutes radioactivity.

R. Strutt and E. Rutherford showed that the Earth is much older. American physicist and mining engineer B. Boltwood determined its minimum age in 1905-1907 by measuring the content of helium and lead—the end products of radioactive families—in minerals containing uranium. It turned out that the Earth was billions of years old—4.6 billion years exactly.

Radioactivity maintains our planet’s internal heat; it is thus at the origin of tectonic movements, continental drift, and volcanism.

Artificial radioactivity

A few months before her death, Marie Curie witnessed the research of her daughter Irène and her husband Frédéric Joliot. As he remembered: “I will never forget the expression of intense joy that gripped her when Irène and I showed her the first human-made radionuclide in a small glass tube […] It was undoubtedly the last great satisfaction of her life.”

Marie Curie is observing her daughter Irene as handles the equipment in the laboratory.
Marie Curie and her daughter, Irene, 1925

On Thursday, January 11, 1934, Frédéric was alone in his laboratory. On that day, he began the new measurements planned to determine the threshold for positive electrons’ appearance—the minimum energy of ⍺ particles of polonium at which positrons start to appear. It used a small chamber, which could be filled or emptied with carbon dioxide. He fixed an intense polonium source against the interior bottom; on the other side, he covered the chamber window with aluminum foil to be irradiated, capable of completely stopping ⍺ particles. Behind the aluminum window was a Geiger-Müller counter.

This counter could count fast electrons one by one. The meter was connected to an amplifier, made from an old radio station, followed by a mechanical numerator. This device did not allow more than two hundred pulses per minute to be counted. Each impulse—each “hit—manifested itself by a “top” of the numerator.

Initially, the chamber was filled with carbon dioxide, then Frédéric began to pump to reduce the pressure. The result was that the energy of the air that arrived on the aluminum sheet increased.

Positive electrons appeared at the same energy threshold as neutrons. It completely emptied the chamber: the power of ⍺ that hit the leaf was maximum. He then reintroduced carbon dioxide into the chamber to bring the energy ⍺ below the threshold. There were no more neutrons, but, to his surprise, the numerator continued to count: the emission of positive electrons continued! What was more interesting was that their number decreased exponentially over time!

The conclusion was clear: neutron and positron emitted in two successive stages. The nuclear reaction first formed in the aluminum nucleus of a hitherto unknown species, phosphorus 30, a radioactive isotope of the stable nucleus phosphorus 31. This phosphorus 30, produced artificially, transformed spontaneously, by the emission of positrons, into stable silicon 30, according to a new type of radioactivity symmetrical to the radioactivity of natural radioelements.

However, Frédéric suddenly seized with doubts, called the Geiger-Müller counter expert, W. Gentner. He asked Gentner to check the counter. The young German physicist experimented again and left a written note saying that “everything is in order.”

The farther the experiment is from theory, the closer it is to the Nobel Prize.

– Irène Curie

After careful validation with Irène, on January 15, 1934, they announced the discovery of artificial radioactivity in a note to the Proceedings of the Academy of Sciences, presented by Jean Perrin. It made them win a Nobel Chemistry Prize in 1935.

The discovery of a new world

The phenomenon was rare. Three Nobel Prize winners in the same family: the case was unique. The two Curie couples created and developed a new research field: radioactivity, a discipline that deeply marked the end of the 19th and 20th centuries. Two couples were working with equal and mutual respect, each bringing their personality and originality.

“That one must do some work seriously and must be independent and not merely amuse oneself in life—this our mother has told us always, but never that science was the only career worth following.”

– Irène Joliot-Curie

The joint scientific work between Pierre and Marie Curie and Frédéric and Irène Joliot-Curie was particularly fruitful; the former were pioneers of the new science of radioactivity, the latter brought an unexpected extension and development to this new discipline.

This breakthrough demonstrated the universality of the natural process of radioactivity. About 2500 species of radioactive nuclei are known today. There is no scientific discipline that the transformation of atomic nuclei has not renewed.

Equipped with this tool, scientists explored the intimate structure of matter. They also explained the life and death of stars, reviewed the terrestrial dynamics maintained by the release of heat resulting from radioactive decays; chemists and biologists used radioactive tracers to analyze reactions. Medicine has been able to draw new treatments from them. Radioactivity opened a new world to us.

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