After Pierre's death, Irène seemed appointed by Marie to fill the void left by him and become, over the years, his confidante and collaborator. At the age of eleven she was already studying advanced mathematics and at thirteen she traveled alone and spent long periods at the home of Marie's close friends while she gave lectures at different universities or isolated herself in her laboratory. At the Sevigné teaching institute, she excelled so much in mathematics and physics that she was allowed to teach these subjects to her classmates. At fourteen she passed the first stage of high school with a year and a half in advance and with honors. Three years later, at the start of the First World War, she entered the Sorbonne to study mathematics and physics, while also enrolling in a nursing course. By this time, Marie was already referring to her as her “her partner and her friend” and brought her to her front where she had deployed a fleet of sixty portable X-ray units, known as “the little Curies”. Within a few months, she left her alone in charge of a field radiological facility in Hoogstade, where she, single-handedly, radiographed the wounded and carried out a geometric calculation to indicate to the surgeon the location of the bullets and the shrapnel She came of age by training nurses to take her place when she moved to another battlefield position. The next destination was Amiens. She there she learned on her own how to repair X-ray equipment, obtaining remarkable technical experience. She returned to Paris in 1916 to teach a course in X-rays at the new Eith Cavell Hospital and re-enrolled at the Sorbonne graduating with honors in mathematics and physics. In 1920 she began to work as an assistant in the Curie laboratory of the Radium Institute of the University of Paris, dedicated to research and teaching of radioactivity. She focused her early research on atomic phenomena and based her doctoral thesis on the study of alpha particles (helium-4 nuclei) emitted by a polonium source. She presented it in 1925 under the title: “Recherches sur les rayons alfa du polonium, oscillation de parcours, vitesse d’émission, pouvoir ionisant.”
That same year, a young and restless Frédéric Joliot, who had not yet finished his military service, presented himself to his childhood heroine to ask her for a job at the Curie laboratory. He came highly recommended by Paul Langevin and Marie suggested that he start the next day. To do this, she got the colonel to let her finish the militias first. Frédéric, after his father's death, had enrolled at the École de physique et de chimie industrielles in Paris, where Pierre Curie had taught before handing over his post to Paul Langevin. Outgoing and handsome, Frédéric was used to making friends easily. However, he found it difficult to adjust to the serious and silent environment of the Curie laboratory. He felt lonely and avoided Irène, the boss's favorite assistant who didn't even say good morning to him. However, as time passed he began to feel more and more attracted to his intelligence. They both enjoyed talking and taking long walks. They fell in love and decided to get engaged. When Irène told Marie, she did not take the news well. She feared that it was a marriage of convenience, that Joliot wanted to take advantage of the Curie name. Like other people, she could not imagine the taciturn and austere Irène with the cheerful and elegant Frédéric. Her daughter was three years older than him, she had declared that she would never marry and did not give any importance to her appearance. When she wasn't wearing a lab coat with her nursing shoes, she was dressed in simple dark colored dresses. Frédéric, on the other hand, was quite the ladies' man and a heavy smoker. Such was Marie's mistrust of her future partner that she tried to dissuade her daughter and insisted that they come to an agreement that would overturn the French law under which husbands controlled their wives' property. He needed to make sure that Irène would be the only one to inherit the radioactive substances from the Curie Institute.
But her daughter ignored her mother's advice and married Frédéric on October 26, 1926. This did not prevent Marie, for years, from continuing to present him as "the man who married Irène". Frédéric, however, had a great admiration for her mother-in-law and did not hesitate to accept her request when she insisted that he continue with her studies. While continuing his work at the Institute, he graduated from the Sorbonne with a doctorate in 1930 with a thesis on the electrochemistry of radioelements: “Électrochimique des radioelements. Applications diverse”. Langevin had been completely right, he saw in him the great scientist he was going to become.
In 1927, three years before Frédéric read his thesis, they had Hélène and shortly after Irène contracted tuberculosis. The doctor warned her not to have another child and to slow down her work rate. But this was superior to her. Tuberculosis would not lead her to give up what she made her feel complete, to be a researcher and a mother. The following week, battling a disease that she would suffer throughout her life, she was back in the lab and five years later her other wish came true, she gave birth to Pierre. .
The scientific collaboration between the two focused on the study of radioactive emissions. They were attracted by the investigations that Rutherford's group was developing in the Cavendish laboratory and they had 200 millicuries of Polonium, the most powerful source of alpha rays, to carry them out. The first step was to analyze the highly penetrating, neutral radiation that Walther Bothe and Herbert Becker had detected when they bombarded beryllium with alpha particles from a sample of polonium. The Joliot-Curies repeated the experiments and published their findings on January 18, 1932. They had observed that Bothe's powerful radiation was capable of causing the emission of protons from a layer of paraffin. They postulated that it was high-frequency electromagnetic radiation but were unable to interpret the results.
James Chadwick, assistant director of research at Cavendish, reported the paper to Rutherford and the two researchers' conclusions. Rutherford just uttered a terse "I don't think so," an expression that puzzled Chadwick, who had never seen him react in this way. Seeing that the topic had promise and that very soon other physicists would investigate it, he got down to work and discovered the presence of neutrally charged particles, neutrons, in the nucleus. In 1932 he transmitted to Nature a brief note entitled: "Possible existence of a neutron" which was to be followed by the longer article in the Proceedings of the Royal Society entitled "The existence of neutron".
The Joliot-Curies had had the neutron "in their hands" and failed to recognize it. And, unfortunately, it was not the only Nobel Prize that they saw pass in front of their noses. In their work with a cloud chamber that, by means of a magnetic field created by an electromagnet, curves the trajectory of charged particles, they observed that some of the supposed electrons produced in the experiment were deflected in the opposite direction. They did not realize that it was a new type of particle like the electron but with a positive charge that had already been proposed in 1931, the positron, and that it was discovered that same year by Carl David Anderson.
Fortunately, in his case the expression “third time lucky” could not be truer. The year was 1933 and success was about to knock on his door. At that time, the marriage was focused on the study of the disintegrations of polonium. They knew that it was an emitter of alpha particles and wondered if, like other radioactive atoms, it also emitted beta radiation (electrons). To check this, they placed an aluminum sheet to stop the alpha particles before they reached the detector. The latter consisted of a cloud chamber that, by means of a magnetic field created by an electromagnet, would curve the trajectory of the beta particles, making their identification possible. The first experiment gave surprising results: not only did they detect electrons, but also protons and positrons appeared. The presence of protons could easily be explained by a known reaction, the transmutation of aluminum into silicon. The alpha particle absorbed by aluminum-27 produces silicon-30 plus one proton. What they didn't know was what the positrons were doing there, and to find out, they started by substituting the material used as the alpha particle absorber. They observed that by interposing a layer of paraffin, silver or lithium, they did not detect positrons, while in the case of boron they did. Therefore, the origin of the positrons was not found in polonium, they were facing a phenomenon that only occurs in certain absorbers. The first hypothesis was to think that the transmutation of aluminum into silicon, apart from the aforementioned positron, could also result in the emission of a neutron and a positron. In both cases the electric charge was conserved. To verify the second possibility, they modified the device so that it allowed the simultaneous detection of the neutron and the positron.
The first test seemed to confirm his approach but the second provided new information. They realized that when the energy of the alpha particles decreased, neutrons were no longer detected, leaving only positrons. They were wrong and, after reflection, they proposed the new hypothesis: perhaps the absorbent became radioactive by interacting with the alpha particles emitted by the source. To check if they were right, they placed a Geiger counter next to the absorbent material, after removing the polonium source. The Geiger "sang", the material had become radioactive and the emission of radiation decayed exponentially as in the case of ordinary radioactivity. They announced their find in two articles, one written in French, with Irène as the first signatory and submitted on January 15, 1933, and the other in English, with Frédéric heading the list of authors.
"Artificial" radioactivity was born and the Joliot-Curie couple was awarded the Nobel Prize in Chemistry in 1935. The economic endowment of the prize allowed them to settle in Sceaux, where they received their friends on Sunday afternoons. Irène, unlike Marie, she always put her obligations as a mother before everything else, she believed that motherhood was the most incredible experience she had ever lived. In 1936, as a result of the award, she Irène was appointed Undersecretary of State for Scientific Research and, the following year, she accepted a professorship at the Sorbonne. Frédéric, for his part, was elected as a professor at the Collège de France in 1937 and left the laboratory of the Radium Institute to form his own laboratory, where he built the first cyclotron in Western Europe.
The discovery that artificial radioactivity could be produced by man was a fundamental advance in the medical applications of ionizing radiation. The Joliot-Curies, as can be deduced from their Nobel Prize reception speech, already ventured the possibilities of their discovery in the field of Medicine:
"The diversity of the chemical natures, the diversity of the half-lives of these synthetic radioelements, will undoubtedly allow new investigations in biology and physical chemistry."
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