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Before 1934, radioactivity was something you found in nature. As Irène Joliot-Curie put it in her Nobel lecture, it "remained a property exclusively associated with some thirty substances existing naturally". Irène Joliot-Curie and Frédéric Joliot showed that it could be created in an ordinary element in a laboratory. This post explains the experiment, writes out the nuclear equations as you would in a JEE or board answer, works through the half-life arithmetic, and ends with what a student can learn from how they worked.
Who was Irène Joliot-Curie?
Her Nobel Prize biography says she was born in Paris on 12 September 1897, the daughter of Pierre and Marie Curie. During the First World War she served as a nurse radiographer. She earned her doctorate in 1925 with a thesis on the alpha rays of polonium, and in 1926 married Frédéric Joliot, with whom she did most of her well-known work.
The same biography records that her 1938 research on neutrons acting on heavy elements was "an important step in the discovery of uranium fission". She became a professor in Paris in 1937 and Director of the Radium Institute in 1946, served as Undersecretary of State for Scientific Research in 1936, and helped build France's first atomic pile in 1948. She died in Paris on 17 March 1956. Her mother's story is in our post on Marie Curie's two Nobel Prizes.
What did the Joliot-Curies discover?
They were bombarding light elements with alpha particles from polonium. Their Nature letter of 10 February 1934 reports the "very striking fact": when an aluminium foil was irradiated and the polonium source was then removed, the emission of positrons "does not cease immediately". The foil stayed radioactive, and its activity decayed exponentially "as for an ordinary radio-element". They saw the same with boron and magnesium.
In her Nobel lecture of 12 December 1935, Irène Joliot-Curie described their interpretation in two stages. First, the aluminium nucleus captures an alpha particle and instantly emits a neutron, forming "an isotope of phosphorus of atomic weight 30". Then this new radio-element, which they called radio-phosphorus, decays with a half-life of three minutes. The Nature letter gives 3 min 15 s for aluminium.
How did they prove a new element had formed?
Physics alone could not show which element the activity belonged to, because the amounts were far too small to weigh. According to the Nobel presentation speech, they dissolved the irradiated aluminium in acid and found that all the radioactivity went off with the gas produced. That is what you would expect if phosphorus had formed, since it would leave as gaseous phosphorus hydride. A second chemical test confirmed it. The Nobel Prize in Chemistry 1935 went to the two of them jointly "in recognition of their synthesis of new radioactive elements".
Worked example: writing the nuclear equations
Write each nucleus as (mass number, atomic number). Check that both totals match on each side.
Step 1, the bombardment. Aluminium-27 (27, 13) + alpha particle (4, 2) gives phosphorus-30 (30, 15) + neutron (1, 0).
Mass numbers: 27 + 4 = 31 on the left; 30 + 1 = 31 on the right.
Atomic numbers: 13 + 2 = 15 on the left; 15 + 0 = 15 on the right.
Step 2, the decay. Phosphorus-30 (30, 15) gives silicon-30 (30, 14) + positron (0, +1) + neutrino.
Mass numbers: 30 = 30 + 0. Atomic numbers: 15 = 14 + 1.
A positron emission lowers the atomic number by one and leaves the mass number unchanged.
Step 3, the half-life arithmetic. Take the three-minute half-life from the Nobel lecture. Suppose a detector reads 3,200 counts per minute just after bombardment stops.
After 15 minutes, 15 / 3 = 5 half-lives have passed.
Fraction left = (1/2) raised to 5 = 1/32.
Count rate = 3,200 / 32 = 100 counts per minute.
For more problems of this type, see our guide to radioactive decay and half-life.
Common mistakes
Writing the positron as (0, -1). That is the electron of beta-minus decay. A positron is (0, +1).
Forgetting the neutron in step 1, so the mass numbers do not balance.
Treating half-lives as linear: after two half-lives, a quarter remains, not zero.
If you want to check each line of your own working, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer.
What can a student learn from how she worked?
Notice what does not stop. The key observation was that emission continued after the source was removed. Many people would have ignored it.
Use a second method. A physical measurement suggested a new isotope; a chemical separation confirmed it. Checking one result by an independent route is a habit worth building.
Build on what you know well. Her doctoral thesis was on polonium's alpha rays, and polonium was the source in the 1934 experiment.
Practice routine: balance the boron reaction yourself (boron-10 plus an alpha particle gives nitrogen-13 plus a neutron), then write the positron decay of nitrogen-13 and check both totals.
Next step: write out the aluminium equations from memory tomorrow and check each mass and atomic number against this post.
