Fermi Rememberedy


 

"Fermi was a physicist, first and foremost. In the words of his longtime colleague Gilberto Bernardini, a colorful mangler of the English language, 'Fermi was a physicist with a capital F.' That's what he was, all the time—a physics machine…once he got his teeth into a problem, he would not let go."—Marvin L. Goldberger

"Fermi was known by his colleagues as 'the pope.' This made it all very clear that he was the supreme authority on all matters. He held this position in all of our minds as an accepted fact—no big deal, just an accepted realization that he really knew more than the rest of us, or anyone else involved in our scientific work."—Harold Agnew

"What did make Fermi so special? As a physicist, his universality, his versatility (both in experiment and in theory), and his constant disposition and ability to share knowledge. As a person, his modesty, his lack of neuroses, and his man-on-the-street mannerism.…Fermi gave you the illusion that his way of thinking was just like yours-only clearer, sharper, and better oiled. No jumps, no flashes, no strokes of genius. The genius lay in the result. In his presence we all became a little smarter."—Valentine Telegdi

 		With Fermi at Columbia, Chicago, and Los Alamos
Darragh Nagle
An excerpt from Fermi Remembered

The story begins for me back in 1940 at Columbia University when, as a shiny new graduate student, I thought I should pay a courtesy call on the illustrious Professor Enrico Fermi, whose presence at Columbia was one reason for my coming. His office was listed as seventh floor of Pupin physics laboratory. So I took the elevator and got off at that floor. There was a sort of foyer serving two wings. I looked around for the office. I wondered whether he would be very formal, as I had heard some Europeans were, and would require an appointment set up with his secretary. Suddenly… a door burst open, and a dark-haired man ran past me at full tilt, disappearing through the opposite door. He was wearing a lab coat and was carrying a bit of something in a pair of chemical tongs. I stood there flat-footed wondering what to do, when the first door burst open again, and what appeared to be a younger version of the first gentleman, similarly attired and similarly burdened, dashed by me and also disappeared through the second door. Although the full significance of this minidrama was not completely clear to me then, it seemed clear enough that this was not the day for a courtesy call on the professor. The bit of something that was being carried was probably a rhodium foil, which had been activated by neutrons. Because rhodium decays with a half-life of 42 seconds, one had to be quick in carrying it from the lab where it was activated to a place where its activity could be measured. Only later did I find myself caught up in races devised by Fermi and Herbert Anderson.

Soon afterward I was to meet Fermi in the classroom. The course was mechanics, elasticity, hydrodynamics, and a short introduction to quantum mechanics. The course was exceptionally clear, beautifully organized. He wrote everything out on the board: as a consequence I have complete notes of the course. He also gave a course in thermodynamics, using his book. Other faculty members—Maria Mayer, Arnold Nordseick—also gave great lectures. Edward Teller, on the other hand, was interesting and informative, but quite disorganized.

In February 1942 Fermi transferred his activities to the University of Chicago. It was December of that year that he directed the historic startup of the pile in the west stands of Stagg Field. I always thought of the pile as a sleeping, malevolent monster, and the Gothic style entrance to the stands as appropriate for some kind of horror movie. Inside you saw demonic figures, with red eyes glaring in faces black as pitch (although of course it was graphite) scurrying about. Instead of chains there were the control rods to restrain the monster. Outside the students and citizens of Chicago plodded through the snow, unaware.

A few months later the pile was torn down and moved to the Argonne Palos Park site. Later, after the pile had been rebuilt and brought back into operation, a vigorous program of experimentation was instituted, and Fermi would often be at Palos Park. Sometimes he would stay overnight in the dorm. At night there was no food service, so we did our own thing in the kitchen. Fermi would sometimes make a frittata. I remember his sitting up in bed one night, reaching for a little notebook, writing a few lines, and then going back to sleep.

Later, many from the "Met lab" moved to Los Alamos. Anderson and I drove out together. We were met by Fermi, who characteristically took us on a hike: to the Upper Crossing of Frijoles Canyon, in the course of which he told us about some current activities.

Fermi had a little group headed by Herb, which carried out experiments at Omega site. Perc King was in charge of the Water Boiler, the enriched reactor we used for measurements of cross sections, etc. Others in the group were myself, Julius Tabin, Joan Hinton, Bob Carter, and Jim Bridge. We shared the building with a group doing critical assembly work: Fermi was deeply suspicious and ordered us to stay away whenever critical assembly tests were planned. We would go up to the mesa or sometimes take a hike, often with Fermi. Sadly, he was proven right by the tragic, needless deaths of Louis Slotin and Harry Daghlian. I have often wondered why Fermi did not intervene with Robert Oppenheimer.

Following the Trinity test in July 1945, one wanted to collect soil samples from the crater a few hours after the explosion. The samples were to be analyzed by the radiochemists to obtain the ratio of the fission products to the unreacted fissionable material. From this one got a value for the yield of the explosion. The Anderson-Nagle-Tabin method was to use a Sherman tank to go into the crater, to dig up samples thru a hole in the bottom of the tank. The tank bottom and personnel compartments were lined with lead.

Julius Tabin, Anderson, and I took turns going into the crater, and of course there was a driver. It was dangerous because if the tank stalled, there was no escape; we would have cooked. Our luck held, we got our samples, and the tank didn't stall, but we all got significant doses of radiation.

Fermi's method was simple, practical, and safer. He had George Weil fit out a second Sherman tank with rockets that were to be fired into the crater and retrieved with cables attached to the rockets. As it turned out, the second tank stalled some distance away from where it was supposed to be, and the cables got scrambled up. The experiment failed, but nobody got hurt.

I joined the faculty of the University of Chicago in 1952, with the intention of working on the new cyclotron. Herb Anderson told me how the cyclotron happened. After the war, scientists were held in the highest regard in the United States, Fermi in particular. The story is that Urner Liddell, chief of the nuclear physics branch of the Office of Naval Research, came to Chicago and asked, "What instrument would Fermi like to have?" Herb said to Fermi, "What would you like? I'll build it. An accelerator, a big computer,… You name it." Fermi chose a cyclotron. The ONR was as good as its word providing funds (along with the Atomic Energy Commission), and also making available the facilities of the New York Naval Shipyard for much of the construction. The machine followed principles developed at Berkeley. Indeed they provided drawings and invaluable advice. But the success of the Chicago project was due in large measure to the engineering and organizing skills of Herbert Anderson.

In 1952, the cyclotron was at the stage of final assembly and testing in the new building known as John Marshall's Barn. I had come back to Chicago, and one day I had taken time off from hunting leaks to sit in my lab and read some reports. Suddenly, Herb burst through the door in a manner reminiscent of our first encounter years ago. He demanded, rather truculently I thought, to know what I was doing. I said politely that I was thinking of building a liquid hydrogen target in order to study pion-proton scattering. He said, politely this time, that that was a good idea, and to get going. So I got out of my chair and went across the street to the famous west stands to see Earl Long of the Institute for the Study of Metals, who then was very helpful to me in getting a hydrogen target built quickly out of simple materials.

Anderson, Fermi, and I began to measure the total cross sections of hydrogen at several energies, first for negative pions, then for the charge-exchange reaction, followed by deuterium and hydrogen cross sections for several energies of negative and positive pions. Later we were joined by Anderson's students Ron Martin, Maurice Glicksman, and Guarang Yodh, who made important contributions and added to the pleasure of the work.

The results showed the cross section for negative pions rising about linearly with the energy up to about 100 MeV and then appearing to level off at about 60 millibarns, the "geometrical" value, that is, pi times the pion Compton wavelength squared. At the time this type of behavior seemed not unreasonable. The positive pion cross sections for a more limited range of energies seemed to be larger, rising, but not yet "leveling off."

At first we were puzzled: why were the positive pion cross sections larger, considering that only one reaction channel was open for pi plus, whereas for pi minus three channels were open: namely, elastic scattering, charge exchange, and radiative capture?

One day Anderson and Fermi were in the cyclotron control room working on the experiment. Fermi was operating the counters, and Anderson was reading his mail. Herb said, "Here's a letter from Keith Brueckner. He says he can explain our cross sections." Fermi grunted something like "What does he know?" Anderson said, "It says here that the sections should be in ratios 9:2:1, and there should be a peak around 180 MeV." At this point Fermi grabbed the paper, went up to his office, and left Anderson to run the counters. Fermi reappeared soon and explained it all to him.

Briefly, Brueckner introduced a phenomenology based on ideas from strong coupling meson theory, and the charge independence of nuclear forces, including the pions. He predicted a resonance at about 180 MeV for pi plus, and that near 180 MeV the cross sections for the pi plus elastic reaction, the charge exchange reaction, and the pi minus elastic scattering would be 9:2:1. Strong suggestions of this seemed to be in our data.

We then set about measuring the angular dependence of these reactions. The target was modified to accommodate this. Partial wave analysis of the data was done first by Fermi, then by the rest of us, and by many others. In particular, Fermi and Nick Metropolis, working at Los Alamos with the Maniac computer, did analyses requiring energy continuity of the several partial wave amplitudes. The Maniac found several plausible solutions, one of which showed the P3/2, I-spin 3/2 resonance. It took a long time to convince everyone (including Fermi) that this latter one was the correct one. The work of Martin and Glicksman was important in this connection.

Fermi was proud of his strength and stamina, and liked to lead on hikes, swims, and so on. During the west stands times, we would swim in Lake Michigan, Fermi in the lead, and Herb, me, Tabin, and Leona Woods following along. He liked tennis. He liked to ski: in New Mexico we would ski at places like Sawyer's Hill, the bowl above Camp May, and Hyde Park.

I remember a story about Fermi and Emilio Segrè. His old friend was trying to persuade Fermi that trout fishing was a great sport. Fermi was not persuaded.

Segrè: "But you don't understand, Enrico. It requires great technique: you must select just the right fly, cast it, and make it move through the water just so, and then the fish will think it's a real fly.…"

Fermi: "Ah, I see, a battle of wits!"

 

Copyright notice: Excerpt from pages 165-711 of Fermi Remembered edited by James W. Cronin, published by the University of Chicago Press. ©2004 by the University of Chicago. All rights reserved. This text may be used and shared in accordance with the fair-use provisions of U.S. copyright law, and it may be archived and redistributed in electronic form, provided that this entire notice, including copyright information, is carried and provided that the University of Chicago Press is notified and no fee is charged for access. Archiving, redistribution, or republication of this text on other terms, in any medium, requires the consent of the University of Chicago Press.

James W. Cronin, editor
Fermi Remembered
©2004, 298 pages, 34 halftones, 84 line drawings
Cloth $45.00 ISBN: 0-226-12111-9

For information on purchasing the book—from bookstores or here online—please go to the webpage for Fermi Remembered.

See also: