work with Otto Stern in Hamburg, Amaldi to Leipzig to the laboratory of the physical chemist Peter Debye, Rasetti to Lise Meitner at the KWI. By 1933, with a departmental budget above $2,000 a year, ten times the budget of most Italian physics departments, with a well-made cloud chamber and a nearby radium source and KWI training in the vagaries of Geiger counters, the group was ready to begin.
In the meantime, two months after the Solvay Conference, Fermi completed the major theoretical work of his life, a fundamental paper on beta decay. Beta decay, the creation and expulsion by the nucleus of high-energy electrons in the course of radioactive change, had needed a detailed, quantitative theory, and Fermi supplied it entire. He introduced a new type of force, the “weak interaction,” completing the four basic forces known in nature: gravity and electromagnetism, which operate at long range, and the strong force and Fermi's weak force, which operate within nuclear dimensions. He introduced a new fundamental constant, now called the Fermi constant, determining it from existing experimental data. “A fantastic paper,” Victor Weisskopf later praised it, “… a monument to Fermi's intuition.” In London the editor of
The
Since the neutron carries no charge, there is no strong electrical repulsion to prevent its entry into nuclei. In fact, the forces of attraction which hold nuclei together may pull the neutron into the nucleus. When a neutron enters a nucleus, the effects are about as catastrophic as if the moon struck the earth. The nucleus is violently shaken up by the blow, especially if the collision results in the capture of the neutron. A large increase in energy occurs and must be dissipated, and this may happen in a variety of ways, all of them interesting.
When Fermi began his neutron-bombardment experiments he was thirty-three years old, short, muscular, dark, with thick black hair, a narrow nose and surprising gray-blue eyes. His voice was deep and he grinned easily. Marriage to the petitely beautiful Laura Capon, the daughter of a Jewish officer in the Italian Navy, had encouraged him in methodical habits: he worked for several hours privately at home, arrived at the physics institute at nine, worked until twelve-thirty, lunched at home, returned to the institute at four and continued work until eight in the evening, returning home then for dinner. With marriage he had also gained weight.
He and his team of young colleagues occupied the south section of the second floor of the institute, sharing the space with Corbino and with the chief physicist of Rome's Sanita Pubblica — its health department — a generous soul named G. C. Trabacchi who lent Corbino's boys some of the instruments and supplies they needed for their experiments (in return they cherished him, nicknaming him “Divine Providence”). Antonino Lo Sordo, a frustrated old-guard physicist, fended off the encroaching horde from an office at the north end of the floor. Corbino and his family lived above, the residence overlooking a private garden in back with a goldfish pond at its focus. The first floor served students; the basement held electrical generators and a lead-lined safe for the Sanita's gram of radium, worth 670,000 lire — about $34,000 — in that year of its most historic use. Glass pipes passed through a wall of the special safe to carry radon, formed in the decay of radium, to a compact extraction plant, a modest refinery of glass-pipe towers that purified and dried the radioactive gas. The residential upper story of the institute, contracted above the longer lower floors to make room at one end for the dome of a small rotunda, was roofed with tile. “The location of the building in a small park on a hill near the central part of Rome was convenient and beautiful at the same time,” Segre recalls. “The garden, landscaped with palm trees and bamboo thickets, with its prevailing silence (except at dusk, when gatherings of sparrows populated the greenery), made the institute a most peaceful and attractive center of study.” A gravel path that shone white in the golden Roman sun led down to the Via Panisperna.
As usual, Fermi hewed the neutron experiments by hand. In February and early March he personally assembled crude Geiger counters from aluminum cylinders acquired by cutting the bottoms off tubes of medicinal tablets. Wired, filled with gas, their ends sealed and leads attached, the counters were slightly smaller than rolls of breath mints and a hundred times less efficient than modern commercial units, but with Fermi to operate them they served. While he built Geiger counters he asked Rasetti to prepare a neutron source in the form of polonium evaporated onto beryllium. Since polonium emits relatively low-energy alpha particles, the resulting source emitted relatively few neutrons per second, and Fermi and Rasetti irradiated several samples without success.
At that point Rasetti, showing a surprising lack of eagerness for historic experiment, went off to Morocco for Easter vacation. Fermi cast about for some way to acquire a stronger neutron source. The rationale for using polonium in the first place, in Paris and Cambridge and Berlin as well as in Rome, had been that a stronger alpha emitter like radon also emitted strong beta and gamma radiation, which disturbed the instruments and interfered with measurements. Fermi realized suddenly that since he was trying to observe a
To the basement of the physics institute on the Via Panisperna, in his gray lab coat, in mid-March, Fermi thus carried a snippet of glass tubing no larger than the first joint of his little finger. It was flame-sealed at one end and partly filled with powdered beryllium. He set the sealed end of this capsule into a container of liquid air. The radon, directed from the outlet of the extraction plant into the capsule, condensed on its walls in the –200 °C cold. Fermi then had to attempt quickly to heat and draw closed the other end of the capsule, without cracking the glass, before the radon evaporated and escaped. When he succeeded, he finished preparing the neutron source by dropping it into a two-foot length of glass tubing of larger diameter and sealing it into the far end so that it could be handled at a distance safe from dangerous exposure to its gamma rays. For all the tedious preparation its useful life was brief.
In the beginning Fermi worked alone. He intended eventually to irradiate most of the elements in the periodic table and he started methodically with the lightest. His source, he calculated, supplied him with more than 100,000 neutrons per second. “Small cylindrical containers filled with the substances tested,” he would explain in his first report, “were subjected to the action of the radiation from this source during intervals of time varying from several minutes to several hours.” Fermi first irradiated water — testing hydrogen and oxygen at the same time — then Uthium, berylhum, boron and carbon without inducing them to radioactivity. Laura Fermi says he wavered then, discouraged by the lack of results, but Fermi seldom talked shop at home and doubt seems unlikely: he knew from the Joliot-Curie work that aluminum, a little farther along, reacted with alphas, and neutrons should prove even more effective.
In any case he succeeded on his next attempt, with fluorine: “Calcium fluoride, irradiated for a few minutes and rapidly brought into the vicinity of the counter, causes in the first few moments an increase of pulses; the effect decreases rapidly, reaching the half-value in about 10 seconds.”
Soon he found a radioactivity in aluminum with a half-life of twelve minutes, different from the Joliot-Curies' discovery. Putting aluminum first to link his work with theirs, he reported his findings in a letter to the
A Roman numeral
We organized our activities this way: Fermi would do a good part of the experiments and calculations. Amaldi would take care of what we would now call the electronics, and I would secure the substances to be irradiated, the sources, etc. Now, of course, this division of labor was by no means rigid, and we all participated in all phases of the work, but we had a certain division of responsibility along these lines, and we proceeded at great speed. We needed all the help we could get, and we even enlisted the help of a younger brother of one of the
