The experiments of 1934. The periodic table

On March 27, Fermi bombarded aluminium and fluorine with neutrons, causing the Geiger-Muller counter to record some counts: the target nuclei had become ‘activated’. In the following weeks, he involved the whole group in the work. The plan was to systematically bombard all the periodic table elements with neutrons to explore the characteristics of the affected nuclei. Emilio Segrè was responsible for procuring the various substances to be exposed to neutron radiation. Laura Capon says in “Atoms in the Family”:

“Emilio therefore went to Mr. Troccoli, the main supplier of chemical products [in Rome]. In one of the shops near via della Botteghe Oscure, Troccoli ran an old shop that Segrè found very well stocked: there was everything one could ask for from the most modern suppliers. Very happy, Emilio began to chat with Troccoli, partly in Italian, partly in Latin, to explain to him the work on artificial radioactivity. Meanwhile he threw substances into the shopping bag and gradually crossed their names off the [shopping] list. Thus he arrived at cesium and rubidium, two soft and silvery metals little used in chemistry. Troccoli took them from a dusty shelf and put them in the bag saying: “Rubidium caesiumque tibi donabo gratis et amore dei. I’ve had them in the shop for fifteen years and you’re the first to ask me for them.”

The neutron sources were glass vials containing beryllium power and radon supplied by Giulio Cesare Trabacchi, the Director of the Physics Laboratory of the Institute of Health.

The neutron source was placed in a room at the end of a corridor on the first floor, inside a lead container that shielded its radiation. The counter to measure the activity of the irradiated substances was instead positioned in a room at the opposite end of the building to prevent the radiation emitted by the source from acting directly on it. Amaldi recounts the eventful measurement process that the Corbino boys had adopted:

“Well, we had the source of the neutrons in a room at the end of a corridor, on the first floor of Via Panisperna, inside a small lead castle that we had built so as not to irradiate ourselves too much. The meter for measuring the irradiated substances naturally had to be as far away as possible, to prevent the radiation emitted by the source from acting directly on it: we therefore placed it in a room at the opposite end of the building. So every time we took a measurement we had to take our metal cylinders to be irradiated to the source, leave them there for a while, then remove them and take them at full speed to the meter, running down the entire corridor at full speed.” [AND. Amaldi, Interview on the matter. From the nucleus to the galaxies]

The corridor still runs through this building and is over 25 meters long. Naturally, for the boys from Corbino, it was an opportunity to test themselves in speed races:

“Ah, there was a period in which we took a measurement every two minutes… […] it was 30 runs per hour for eleven hours. We were very well trained.” [AND. Amaldi, Interview on the matter. From the nucleus to the galaxies]

 

In a few months, Fermi’s group irradiated 60 periodic table elements with neutrons, discovering around 40 new radioactive nuclei. The results were welcomed with great interest by the international scientific community.

Lord Rutherford himself, the father of the planetary model of the atom, in April 1934, from Cambridge, sent Fermi a letter in which he welcomed the results obtained and, among other things, wrote to his Italian colleague:

Dear Fermi,

I have to thank you for your kindness in sending me an account of your recent experiments in causing temporary radioactivity in a number of elements by means of neutrons. Your results are of great interest, and no doubt later we shall be able to obtain more information as to the actual mechanism of such transformations. It is by no means clear that in all cases the process is as simple as appears to be the case in the observations of the Joliots.

I congratulate you on your successful escape from the sphere of theoretical physics! You seem to have struck a good line to start with. You may be interested to hear that Professor Dirac also is doing some experiments. This seems to be a good omen for the future of theoretical physics!

Congratulations and best wishes

yours sincerely

Rutherford

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