Frederic Joliot and Irene Curie

from Susan Quinn's book -
F. Joliot , born 1900
I. Curie, b 1897
- F. Joliot joined M. Curie's laboratory in 1924 after his graduation.
- I. Curie had the task of teaching lab techniques to F.
- quotes from Frederic. (p 425)
" I didn't have the slightest idea that we might marry one day... But I watched her... With her cold appearance, her forgetting to say hello, she didn't always create sympathy around her at the laboratory. In observing her, I discovered in this young woman, that others saw as a little brutish, an extraordinary, poetic and sensitive being who, in a number of ways, was a living representation of of her father. I had read a lot about Pierre Curie... I found in his daughter this same purity, this good sense, this tranquility."

Irene and her mother traveled to Brazil once before the wedding, Frederic wrote that he found the laboratory "a fraud" without her.
"I link myself more with human beings than with things; one can work anywhere...what gives interest in life in the lab are the people who animate it."

Discovery of the neutron:

1920: Ernest Rutherford had suggested a particle called the "neutral doublet" which could be used to roughly double the mass of the nucleus without adding any additional charge.

1921: Solvay conference: Rutherford reiterated idea using the word "neutron". Although M. Curie was more interested in discovering radioactive elements her interests in radioactive elements was indirectly responsible for the discovery of the neutron. Before the invention of particle accelerators only alpha rays from radioactive decay could be used to bombard substances. It was necessary to use a source that only emitted alpha rays, and polonium was such a source. This element had been discovered by the Curies.

In 1930 Walther Bothe and Herbert Becker in Germany had discovered that when they bombarded beryllium with polonium alpha rays they produced a very penetrating radiation of greater energy than the  bombarding particles. This was interesting to Irene Curie because she had been working for years to produce a strong polonium source. Marie Curie had for years been collecting "radon seed", e. g. donations from hospitals, and by 1931 the Joliot-Curies had a polonium source ten times stronger than anyone else. In December 1931 Irene reported that beryllium bombarded by gave off an even more penetrating radiation than the Germans had earlier reported. The Joliot-Curies had allowed the radiation from the beryllium to bombard a paraffin screen(hydrogen source) and found that the secondary radiation from beryllium was knocking protons out of the screen.  the explanation they gave of the phenomenon was that gamma rays were responsible for knocking out the protons.

In England Rutherford did not believe their explanation. It is kinematically not possible. He had a young researcher named James Chadwick who repeated the Curie-Joliot experiment. Chadwick , after " a few days of strenuous work " showed that the radiation from the beryllium was the neutron that Rutherford had postulated in 1920.

The discovery of artificial radioactivity:

In the summer of 1932 Carl Anderson in California discovered the positron in cosmic ray showers. He used a Wilson cloud chamber for this. F. Joliot had his own improved version of the cloud chamber and looked through their store of photographs and realized that they too had seen positrons. They began bombarding various elements with alphas from their polonium source. medium weight elements emitted protons, but light elements sometimes produced neutrons and a positron. In October 1933 F. and I. went with M. Curie to the Solvay conference in Brussels. They presented their findings and suggested that the proton was really a compound and breaking up into a neutron and electron. In fact, Lise Meitner, among others, doubted that they had seen neutrons. When they returned to Paris they resumed their experiments and showed that indeed, neutrons were being emitted. They were bombarding aluminum with alpha particles. As they moved the alpha source farther and farther away from the aluminum the neutron emission, correspondingly diminished. However, the positron emission continued! The positron emission continued and decayed in time as would a short lived radioactivity. They next used a Geiger counter to test for the short lived activity, and this confirmed what they had seen with the cloud chamber. This was Jan. 15, 1934. They soon concluded that the radioactivity seen was induced by the alpha particle bombardment. They had converted aluminum into phosphorus with the emission of a neutron. The phosphorus then underwent positron decay.

27Al + 4He -> 30P + neutron

30P -> 30Si + positron + (neutrino)

According to F. Joliot, Marie Curie was overjoyed to find that artificial, i.e. , man-made radioactivity had been produced. The Joliot-Curies won the 1935 Nobel prize in Chemistry.