Contributor Post Created with Sketch. Saturday Night Science: The Pope of Physics

 

“The Pope of Physics” by Gino Segrè and Bettina HoerlinBy the start of the 20th century, the field of physics had bifurcated into theoretical and experimental specialties. While theorists and experimenters were acquainted with the same fundamentals and collaborated, with theorists suggesting phenomena to be explored in experiments and experimenters providing hard data upon which theorists could build their models, rarely did one individual do breakthrough work in both theory and experiment. One outstanding exception was Enrico Fermi, whose numerous achievements seemed to jump effortlessly between theory and experiment.

Fermi was born in 1901 to a middle class family in Rome, the youngest of three children born in consecutive years. As was common at the time, Enrico and his brother Giulio were sent to be wet-nursed and raised by a farm family outside Rome and only returned to live with their parents when two and a half years old. His father was a division head in the state railway and his mother taught elementary school. Neither parent had attended university, but hoped all of their children would have the opportunity. All were enrolled in schools which concentrated on the traditional curriculum of Latin, Greek, and literature in those languages and Italian. Fermi was attracted to mathematics and science, but little instruction was available to him in those fields.

At age thirteen, the young Fermi made the acquaintance of Adolfo Amidei, an engineer who worked with his father. Amidei began to loan the lad mathematics and science books, which Fermi devoured—often working out solutions to problems which Amidei was unable to solve. Within a year, studying entirely on his own, he had mastered geometry and calculus. In 1915, Fermi bought a used book, Elementorum Physicæ Mathematica, at a flea market in Rome. Published in 1830 and written entirely in Latin, it was a 900 page compendium covering mathematical physics of that era. By that time, he was completely fluent in the language and the mathematics used in the abundant equations, and worked his way through the entire text. As the authors note, “Not only was Fermi the only twentieth-century physics genius to be entirely self-taught, he surely must be the only one whose first acquaintance with the subject was through a book in Latin.”

At sixteen, Fermi skipped the final year of high school, concluding it had nothing more to teach him, and with Amidei’s encouragement, sat for a competitive examination for a place at the elite Sculoa Normale Superiore, which provided a complete scholarship including room and board to the winners. He ranked first in all of the examinations and left home to study in Pisa. Despite his talent for and knowledge of mathematics, he chose physics as his major—he had always been fascinated by mechanisms and experiments, and looked forward to working with them in his career. Italy, at the time a leader in mathematics, was a backwater in physics. The university in Pisa had only one physics professor who, besides having already retired from research, had knowledge in the field not much greater than Fermi’s own. Once again, this time within the walls of a university, Fermi would teach himself, taking advantage of the university’s well-equipped library. He taught himself German and English in addition to Italian and French (in which he was already fluent) in order to read scientific publications. The library subscribed to the German journal Zeitschrift für Physik, one of the most prestigious sources for contemporary research, and Fermi was probably the only person to read it there. In 1922, after completing a thesis on X-rays and already having published three scientific papers, two on X-rays and one on general relativity (introducing what are now called Fermi coordinates, the first of many topics in physics which would bear his name), he received his doctorate in physics, magna cum laude. Just twenty-one, he had his academic credential, published work to his name, and the attention of prominent researchers aware of his talent. What he lacked was the prospect of a job in his chosen field.

Returning to Rome, Fermi came to the attention of Orso Mario Corbino, a physics professor and politician who had become a Senator of the Kingdom and appointed minister of public education. Corbino’s ambition was to see Italy enter the top rank of physics research, and saw in Fermi the kind of talent needed to achieve this goal. He arranged a scholarship so Fermi could study physics in one the centres of research in northern Europe. Fermi chose Göttingen, Germany, a hotbed of work in the emerging field of quantum mechanics. Fermi was neither particularly happy nor notably productive during his eight months there, but he was impressed with the German style of research and the intellectual ferment of the large community of German physicists. Henceforth, he published almost all of his research in either German or English, with a parallel paper submitted to an Italian journal. A second fellowship allowed him to spend 1924 in the Netherlands, working with Paul Ehrenfest’s group at Leiden, deepening his knowledge of statistical and quantum mechanics.

Finally, upon returning to Italy, Corbino and his colleague Antonio Garbasso found Fermi a post as a lecturer in physics in Florence. The position paid poorly and had little prestige, but at least it was a step onto the academic ladder, and Fermi was happy to accept it. There, Fermi and his colleague Franco Rasetti did experimental work measuring the spectra of atoms under the influence of radio frequency fields. Their work was published in prestigious journals such as Nature and Zeitschrift für Physik.

In 1925, Fermi took up the problem of reconciling the field of statistical mechanics with the discovery by Wolfgang Pauli of the exclusion principle, a purely quantum mechanical phenomenon which restricts certain kinds of identical particles from occupying the same state at the same time. Fermi’s paper, published in 1926, resolved the problem, creating what is now called Fermi-Dirac statistics (British physicist Paul Dirac independently discovered the phenomenon, but Fermi published first) for the particles now called fermions, which include all of the fundamental particles that make up matter. (Forces are carried by other particles called bosons, which go beyond the scope of this discussion.)

This paper immediately elevated the twenty-five year old Fermi to the top tier of theoretical physicists. It provided the foundation for understanding of the behaviour of electrons in solids, and thus the semiconductor technology upon which all our modern computing and communications equipment is based. Finally, Fermi won what he had aspired to: a physics professorship in Rome. In 1928, he married Laura Capon, whom he had first met in 1924. The daughter of an admiral in the World War I Italian navy, she was a member of one of the many secular and assimilated Jewish families in Rome. She was less than impressed on first encountering Fermi:

He shook hands and gave me a friendly grin. You could call it nothing but a grin, for his lips were exceedingly thin and fleshless, and among his upper teeth a baby tooth too lingered on, conspicuous in its incongruity. But his eyes were cheerful and amused.

Both Laura and Enrico shared the ability to see things precisely as they were, then see beyond that to what they could become.

In Rome, Fermi became head of the mathematical physics department at the Sapienza University of Rome, which his mentor, Corbino, saw as Italy’s best hope to become a world leader in the field. He helped Fermi recruit promising physicists, all young and ambitious. They gave each other nicknames: ecclesiastical in nature, befitting their location in Rome. Fermi was dubbed Il Papa (The Pope), not only due to his leadership and seniority, but because he had already developed a reputation for infallibility: when he made a calculation or expressed his opinion on a technical topic, he was rarely if ever wrong. Meanwhile, Mussolini was increasing his grip on the country. In 1929, he announced the appointment of the first thirty members of the Royal Italian Academy, with Fermi among the laureates. In return for a lifetime stipend which would put an end to his financial worries, he would have to join the Fascist party. He joined. He did not take the Academy seriously and thought its comic opera uniforms absurd, but appreciated the money.

By the 1930s, one of the major mysteries in physics was beta decay. When a radioactive nucleus decayed, it could emit one or more kinds of radiation: alpha, beta, or gamma. Alpha particles had been identified as the nuclei of helium, beta particles as electrons, and gamma rays as photons: like light, but with a much shorter wavelength and correspondingly higher energy. When a given nucleus decayed by alpha or gamma, the emission always had the same energy: you could calculate the energy carried off by the particle emitted and compare it to the nucleus before and after, and everything added up according to Einstein’s equation of E=mc². But something appeared to be seriously wrong with beta (electron) decay. Given a large collection of identical nuclei, the electrons emitted flew out with energies all over the map: from very low to an upper limit. This appeared to violate one of the most fundamental principles of physics: the conservation of energy. If the nucleus after plus the electron (including its kinetic energy) didn’t add up to the energy of the nucleus before, where did the energy go? Few physicists were ready to abandon conservation of energy, but, after all, theory must ultimately conform to experiment, and if a multitude of precision measurements said that energy wasn’t conserved in beta decay, maybe it really wasn’t.

Fermi thought otherwise. In 1933, he proposed a theory of beta decay in which the emission of a beta particle (electron) from a nucleus was accompanied by emission of a particle he called a neutrino, which had been proposed earlier by Pauli. In one leap, Fermi introduced a third force, alongside gravity and electromagnetism, which could transform one particle into another, plus a new particle: without mass or charge, and hence extraordinarily difficult to detect, which nonetheless was responsible for carrying away the missing energy in beta decay. But Fermi did not just propose this mechanism in words: he presented a detailed mathematical theory of beta decay which made predictions for experiments which had yet to be performed. He submitted the theory in a paper to Nature in 1934. The editors rejected it, saying “it contained abstract speculations too remote from physical reality to be of interest to the reader.” This was quickly recognised and is now acknowledged as one of the most epic face-plants of peer review in theoretical physics. Fermi’s theory rapidly became accepted as the correct model for beta decay. In 1956, the neutrino (actually, antineutrino) was detected with precisely the properties predicted by Fermi. This theory remained the standard explanation for beta decay until it was extended in the 1970s by the theory of the electroweak interaction, which is valid at higher energies than were available to experimenters in Fermi’s lifetime.

Perhaps soured on theoretical work by the initial rejection of his paper on beta decay, Fermi turned to experimental exploration of the nucleus, using the newly-discovered particle, the neutron. Unlike alpha particles emitted by the decay of heavy elements like uranium and radium, neutrons had no electrical charge and could penetrate the nucleus of an atom without being repelled. Fermi saw this as the ideal probe to examine the nucleus, and began to use neutron sources to bombard a variety of elements to observe the results. One experiment directed neutrons at a target of silver and observed the creation of isotopes of silver when the neutrons were absorbed by the silver nuclei. But something very odd was happening: the results of the experiment seemed to differ when it was run on a laboratory bench with a marble top compared to one of wood. What was going on? Many people might have dismissed the anomaly, but Fermi had to know. He hypothesised that the probability a neutron would interact with a nucleus depended upon its speed (or, equivalently, energy): a slower neutron would effectively have more time to interact than one which whizzed through more rapidly. Neutrons which were reflected by the wood table top were “moderated” and had a greater probability of interacting with the silver target.

Fermi quickly tested this supposition by using paraffin wax and water as neutron moderators and measuring the dramatically increased probability of interaction (or as we would say today, neutron capture cross section) when neutrons were slowed down. This is fundamental to the design of nuclear reactors today. It was for this work that Fermi won the Nobel Prize in Physics for 1938.

By 1938, conditions for Italy’s Jewish population had seriously deteriorated. Laura Fermi, despite her father’s distinguished service as an admiral in the Italian navy, was now classified as a Jew, and therefore subject to travel restrictions, as were their two children. The Fermis went to their local Catholic parish, where they were (re-)married in a Catholic ceremony and their children baptised. With that paperwork done, the Fermi family could apply for passports and permits to travel to Stockholm to receive the Nobel prize. The Fermis locked their apartment, took a taxi, and boarded the train. Unbeknownst to the fascist authorities, they had no intention of returning.

Fermi had arranged an appointment at Columbia University in New York. His Nobel Prize award was US$45,000 (US$789,000 today). If he returned to Italy with the sum, he would have been forced to convert it to lire and then only be able to take the equivalent of US$50 out of the country on subsequent trips. Professor Fermi may not have been much interested in politics, but he could do arithmetic. The family went from Stockholm to Southampton, and then on an ocean liner to New York, with nothing other than their luggage, prize money, and, most importantly, freedom.

In his neutron experiments back in Rome, there had been curious results he and his colleagues never explained. When bombarding nuclei of uranium, the heaviest element then known, with neutrons moderated by paraffin wax, they had observed radioactive results which didn’t make any sense. They expected to create new elements, heavier than uranium, but what they saw didn’t agree with the expectations for such elements. Another mystery…in those heady days of nuclear physics, there was one wherever you looked. At just about the time Fermi’s ship was arriving in New York, news arrived from Germany about what his group had observed, but not understood, four years before. Slow neutrons, which Fermi’s group had pioneered, were able to split, or fission the nucleus of uranium into two lighter elements, releasing not only a large amount of energy, but additional neutrons which might be able to propagate the process into a “chain reaction”, producing either a large amount of energy or, perhaps, an enormous explosion.

As one of the foremost researchers in neutron physics, it was immediately apparent to Fermi that his new life in America was about to take a direction he’d never anticipated. By 1941, he was conducting experiments at Columbia with the goal of evaluating the feasibility of creating a self-sustaining nuclear reaction with natural uranium, using graphite as a moderator. In 1942, he was leading a project at the University of Chicago to build the first nuclear reactor. On December 2nd, 1942, Chicago Pile-1 went critical, producing all of half a watt of power. But the experiment proved that a nuclear chain reaction could be initiated and controlled, and it paved the way for both civil nuclear power and plutonium production for nuclear weapons. At the time he achieved one of the first major milestones of the Manhattan Project, Fermi’s classification as an “enemy alien” had been removed only two months before. He and Laura Fermi did not become naturalised U.S. citizens until July of 1944.

Such was the breakneck pace of the Manhattan Project that even before the critical test of the Chicago pile, the DuPont company was already at work planning for the industrial scale production of plutonium at a facility which would eventually be built at the Hanford site near Richland, Washington. Fermi played a part in the design and commissioning of the X-10 Graphite Reactor in Oak Ridge, Tennessee, which served as a pathfinder and began operation in November, 1943, operating at a power level which was increased over time to 4 megawatts. This reactor produced the first substantial quantities of plutonium for experimental use, revealing the plutonium-240 contamination problem which necessitated the use of implosion for the plutonium bomb. Concurrently, he contributed to the design of the B Reactor at Hanford, which went critical in September 1944, running at 250 megawatts, that produced the plutonium for the Trinity test and the Fat Man bomb dropped on Nagasaki.

During the war years, Fermi divided his time among the Chicago research group, Oak Ridge, Hanford, and the bomb design and production group at Los Alamos. As General Leslie Groves, head of Manhattan Project, had forbidden the top atomic scientists from travelling by air, “Henry Farmer”, his wartime alias, spent much of his time riding the rails, accompanied by a bodyguard. As plutonium production ramped up, he increasingly spent his time with the weapon designers at Los Alamos, where Oppenheimer appointed him associate director and put him in charge of “Division F” (for Fermi), which acted as a consultant to all of the other divisions of the laboratory.

Fermi believed that while scientists could make major contributions to the war effort, how their work and the weapons they created were used were decisions which should be made by statesmen and military leaders. When appointed in May 1945 to the Interim Committee charged with determining how the fission bomb was to be employed, he largely confined his contributions to technical issues such as weapons effects. He joined Oppenheimer, Compton, and Lawrence in the final recommendation that “we can propose no technical demonstration likely to bring an end to the war; we see no acceptable alternative to direct military use.”

On July 16, 1945, Fermi witnessed the Trinity test explosion in New Mexico at a distance of ten miles from the shot tower. A few seconds after the blast, he began to tear little pieces of paper from fron a sheet and drop them toward the ground. When the shock wave arrived, he paced out the distance it had blown them and rapidly computed the yield of the bomb as around ten kilotons of TNT. Nobody familiar with Fermi’s reputation for making off-the-cuff estimates of physical phenomena was surprised that his calculation, done within a minute of the explosion, agreed within the margin of error with the actual yield of 20 kilotons, determined much later.

After the war, Fermi wanted nothing more than to return to his research. He opposed the continuation of wartime secrecy to postwar nuclear research, but, unlike some other prominent atomic scientists, did not involve himself in public debates over nuclear weapons and energy policy. When he returned to Chicago, he was asked by a funding agency simply how much money he needed. From his experience at Los Alamos he wanted both a particle accelerator and a big computer. By 1952, he had both, and began to produce results in scattering experiments which hinted at the new physics which would be uncovered throughout the 1950s and ’60s. He continued to spend time at Los Alamos, and between 1951 and 1953 worked two months a year there, contributing to the hydrogen bomb project and analysis of Soviet atomic tests.

Everybody who encountered Fermi remarked upon his talents as an explainer and teacher. Seven of his students: six from Chicago and one from Rome, would go on to win Nobel Prizes in physics, in both theory and experiment. He became famous for posing “Fermi problems”, often at lunch, exercising the ability to make and justify order of magnitude estimates of difficult questions. When Freeman Dyson met with Fermi to present a theory he and his graduate students had developed to explain the scattering results Fermi had published, Fermi asked him how many free parameters Dyson had used in his model. Upon being told the number was four, he said, “I remember my old friend Johnny von Neumann used to say, with four parameters I can fit an elephant, and with five I can make him wiggle his trunk.” Chastened, Dyson soon concluded his model was a blind alley.

After returning from a trip to Europe in the fall of 1954, Fermi, who had enjoyed robust good health all his life, began to suffer from problems with digestion. Exploratory surgery found metastatic stomach cancer, for which no treatment was possible at the time. He died at home on November 28, 1954, two months past his fifty-third birthday. He had made a Fermi calculation of how long to rent the hospital bed in which he died: the rental expired two days after he did.

There was speculation that Fermi’s life may have been shortened by his work with radiation, but there is no evidence of this. He was never exposed to unusual amounts of radiation in his work, and none of his colleagues, who did the same work at his side, experienced any medical problems.

This is a masterful biography of one of the singular figures in twentieth century science. The breadth of his interests and achievements is reflected in the list of things named after Enrico Fermi. Given the hyper-specialisation of modern science, it is improbable we will ever again see his like.

Segrè, Gino and Bettina Hoerlin. The Pope of Physics. New York: Henry Holt, 2016. ISBN 978-1-6277-9005-5.

Saturday Night Science will take its customary summer break for les vacances and return on 2017-09-02.

The World of Enrico Fermi is a 1970 documentary with reminiscences about Fermi by his students, colleagues, and Laura Fermi. The original film was in two parts, which are presented together here.

To Fermi with Love is a two-part audio program totaling 90 minutes, including recordings of Fermi and many of those who worked with him. Part 1:

Part 2:

This is a short film about the construction of Chicago Pile-1, the first artificial nuclear fission reactor.

Here is the last remaining artefact of the Chicago Pile: a graphite moderator block with uranium fuel elements.

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There are 31 comments.

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  1. Judge Mental, Secret Chimp Member

    John Walker: revealing the plutonium-240 contamination problem which necessitated the use of implosion for the plutonium bomb.

    John, could you flesh this out a bit? I can’t recall hearing of such a thing.

    • #1
    • July 1, 2017, at 1:09 PM PDT
    • 3 likes
  2. John Walker Contributor
    John Walker

    Judge Mental (View Comment):

    John Walker: revealing the plutonium-240 contamination problem which necessitated the use of implosion for the plutonium bomb.

    John, could you flesh this out a bit? I can’t recall hearing of such a thing.

    This is discussed in more detail in Saturday Night Science for 2015-04-11, “Plutonium Production”. Plutonium is made by irradiating U-238 with neutrons in a nuclear reactor. This causes U-238 to absorb a neutron and become neptunium-239, which then decays into plutonium-239. But, as the Pu-239 remains in the reactor core, it can capture another neutron and be transformed into Pu-240. Pu-240 has a much shorter half-life than Pu-239, but, more importantly, undergoes spontaneous fission 41,500 times faster than Pu-239. The consequence is that plutonium with a substantial contamination of Pu-240 (which all reactor-produced plutonium will have), has a neutron background emission sufficiently high that if used in a gun assembly bomb, will result in predetonation and a fizzle yield. The only way around this is to use implosion, which assembles the critical mass quickly enough to avoid predetonation. U-235 has a very low neutron background, so there’s no problem using it in a gun bomb.

    The first plutonium was produced in a cyclotron, which does not produce the intense neutron flux of a reactor, so the microscopic samples tested at Los Alamos were almost pure Pu-239, so the problem was never detected until reactor-bred plutonium from the Oak Ridge reactor arrived for testing. That is what moved implosion from a back-burner project to the critical path.

    When making plutonium for weapons use, fuel is irradiated for a relatively short time, which reduces Pu-240 contamination. Plutonium with less than 7% Pu-240 is considered weapons grade. Plutonium from reprocessing power reactor fuel rods usually has 18% or more Pu-240. This is called reactor grade, and works fine in power reactors, but is more difficult to use in weapons. That’s “difficult”, not impossible—in 1962 the U.S. tested a bomb built from reactor grade plutonium, but the Pu-240 fraction of the core remains classified to this day.

    • #2
    • July 1, 2017, at 1:31 PM PDT
    • 4 likes
  3. Gary McVey Contributor
    Gary McVey Joined in the first year of Ricochet Ricochet Charter Member

    I wish I could track down a quote to the effect that “Los Alamos wasn’t about Hiroshima; it was about Nagasaki”, meaning the classic gun-style uranium bomb that exploded over Hiroshima didn’t require a vast laboratory to figure out how to detonate it, but the plutonium bomb set off over Nagasaki did.

    • #3
    • July 1, 2017, at 1:34 PM PDT
    • 4 likes
  4. Judge Mental, Secret Chimp Member

    John Walker (View Comment):

    Judge Mental (View Comment):

    John Walker: revealing the plutonium-240 contamination problem which necessitated the use of implosion for the plutonium bomb.

    John, could you flesh this out a bit? I can’t recall hearing of such a thing.

    This is discussed in more detail in Saturday Night Science for 2015-04-11, “Plutonium Production”. Plutonium is made by irradiating U-238 with neutrons in a nuclear reactor. This causes U-238 to absorb a neutron and become neptunium-239, which then decays into plutonium-239. But, as the Pu-239 remains in the reactor core, it can capture another neutron and be transformed into Pu-240. Pu-240 has a much shorter half-life than Pu-239, but, more importantly, undergoes spontaneous fission 41,500 times faster than Pu-239. The consequence is that plutonium with a substantial contamination of Pu-240 (which all reactor-produced plutonium will have), has a neutron background emission sufficiently high that if used in a gun assembly bomb, will result in predetonation and a fizzle yield. The only way around this is to use implosion, which assembles the critical mass quickly enough to avoid predetonation. U-235 has a very low neutron background, so there’s no problem using it in a gun bomb.

    The first plutonium was produced in a cyclotron, which does not produce the intense neutron flux of a reactor, so the microscopic samples tested at Los Alamos were almost pure Pu-239, so the problem was never detected until reactor-bred plutonium from the Oak Ridge reactor arrived for testing. That is what moved implosion from a back-burner project to the critical path.

    When making plutonium for weapons use, fuel is irradiated for a relatively short time, which reduces Pu-240 contamination. Plutonium with less than 7% Pu-240 is considered weapons grade. Plutonium from reprocessing power reactor fuel rods usually has 18% or more Pu-240. This is called reactor grade, and works fine in power reactors, but is more difficult to use in weapons. That’s “difficult”, not impossible—in 1962 the U.S. tested a bomb built from reactor grade plutonium, but the Pu-240 fraction of the core remains classified to this day.

    So it’s the classic problem in explosives that it will blow itself apart and scatter before it can explode with efficiency?

    I’m reminded of the 1986 movie The Manhattan Project, where John Lithgow is producing ultra-pure plutonium, 99.997% pure. Would that be an actual scientific basis for why it was particularly dangerous? Near absolute efficiency?

    • #4
    • July 1, 2017, at 1:43 PM PDT
    • 2 likes
  5. John Walker Contributor
    John Walker

    Gary McVey (View Comment):
    I wish I could track down a quote to the effect that “Los Alamos wasn’t about Hiroshima; it was about Nagasaki”, meaning the classic gun-style uranium bomb that exploded over Hiroshima didn’t require a vast laboratory to figure out how to detonate it, but the plutonium bomb set off over Nagasaki did.

    I don’t know the source of the quote but, historically, Los Alamos was in operation well before it was discovered that gun assembly would not work for plutonium. The initial design for both bombs were gun designs, with a more extreme, higher-velocity gun for the plutonium bomb which was called “Thin Man”. Work on Thin Man continued into 1944, when results from testing the reactor plutonium showed it would not work. Thin Man was cancelled at a meeting on July 17th, 1944, and afterward all work on the plutonium bomb was on the implosion design.

    There was a great deal of work for Los Alamos to do apart from implosion research. At the inception of the project, nobody knew the critical masses of either uranium or plutonium, and these had to be determined experimentally by “tickling the dragon’s tail”. Experiments also had to be done on neutron reflectors and initiators (neutron generators) which were used in both the gun and implosion bombs.

    • #5
    • July 1, 2017, at 1:51 PM PDT
    • 5 likes
  6. John Walker Contributor
    John Walker

    Judge Mental (View Comment):
    So it’s the classic problem in explosives that it will blow itself apart and scatter before it can explode with efficiency?

    Precisely—the core has to stay assembled long enough for sufficient generations of fission (each of which roughly doubles the yield) to occur before it explosively disassembles. A gun design using plutonium will always predetonate before it is completely assembled due to a stray neutron from the Pu-240 contamination, resulting in a fizzle yield.

    I’m reminded of the 1986 movie The Manhattan Project, where John Lithgow is producing ultra-pure plutonium, 99.997% pure. Would that be an actual scientific basis for why it was particularly dangerous? Near absolute efficiency?

    Plutonium used in bombs is essentially that pure in terms of elements. The question is isotopic purity. If you could get Pu-239 with 99.997% purity, you’d be able to build a gun bomb with it, but once you’d figured out implosion that wouldn’t make any sense since implosion is so much more efficient you can make four implosion bombs with the same quantity of fissile material it would take to make one gun bomb. But the only way to get plutonium with that isotopic purity would be to first produce it in a reactor, then perform isotope separation as you do with uranium. But the whole point of the plutonium path is that you don’t need to do isotope separation! If you’re building an enrichment plant, there’s no reason to bother with plutonium. Also, since the difference in atomic weight for plutonium is just 240/239 compared to 238/235, a separation plant would be even less efficient and correspondingly larger than one for uranium.

    • #6
    • July 1, 2017, at 2:02 PM PDT
    • 4 likes
  7. Percival Thatcher
    Percival Joined in the first year of Ricochet Ricochet Charter Member

    The second pile was in Palos Park Woods? That is about 2-3 miles from Argonne National Laboratory. I had a job interview there once.

    Stagg Field was named after famed football coach Amos Alonzo Stagg. The land for the field had been donated to the University of Chicago by Marshall Field, founder of Marshall Field and Company, and was therefore known as “Marshall Field,” or maybe “Marshall Field Field,” or if you wanted to “Marshall Field Marshall Field.” So they renamed it after Stagg to cut down on the jabbering.

    • #7
    • July 1, 2017, at 2:10 PM PDT
    • 8 likes
  8. John Walker Contributor
    John Walker

    May I speak of something about Fermi that didn’t make the cut for the main article and doesn’t have anything to do with bombs?

    By my count, he had four achievements in physics each of which would individually have merited a Nobel prize:

    1. Theory of Fermi-Dirac statistics
    2. Theory of beta decay and the weak interaction
    3. Discovery of neutron moderation and thermal neutron cross-section (won prize)
    4. Production of the first self-sustaining fission chain reaction

    Of these four, the first two were theoretical and the second two experimental.

    The Nobel committee has been reluctant to award the prize twice to one person. The only person to win two Nobel prizes in physics is John Bardeen (1956 for the invention of the transistor, and 1972 for the BCS theory of superconductivity). The only person to win a Nobel in two different sciences was Marie Curie (Physics, 1903, for radioactivity, Chemistry, 1911, for the discovery of radium and polonium).

    • #8
    • July 1, 2017, at 2:16 PM PDT
    • 5 likes
  9. John Walker Contributor
    John Walker

    Percival (View Comment):
    The land for the field had been donated to the University of Chicago by Marshall Field, founder of Marshall Field and Company, and was therefore known as “Marshall Field,” or maybe “Marshall Field Field,” or if you wanted to “Marshall Field Marshall Field.”

    Just imagine if he had previously held five-star rank in the British Army: Field Marshal.

    The “Marshall Field Field Marshal Marshall Field Field”.

    • #9
    • July 1, 2017, at 2:22 PM PDT
    • 12 likes
  10. Judge Mental, Secret Chimp Member

    Thanks, Teach!

    • #10
    • July 1, 2017, at 2:22 PM PDT
    • 2 likes
  11. cqness Member

    Fermi became my physics hero when I studied the subject in post-graduate school. Probably it was due to his being top drawer both in theoretical and experimental physics as well as his personality. I always felt he did not receive the public acclaim that was his due like some of the other giants of his time.

    One of my favorite films on the Manhattan Project is “Fat Man and Little Boy” (1989) with Paul Newman as General Groves and Dwight Schultz (Murdock of The A-Team) as Oppenheimer. The film is fairly accurate historically and technically and also entertaining (at least to me).

    There is a portrayal of an actual accident that occurred during the “tickling the tail of the dragon” experiments referred to above, in which a malfunction in the equipment allowed two sub-critical masses to get close to at least a near-critical stage. An experimenter named Michael Merriman, played by John Cusack in the film, swept the experiment off the table with his arm exposing himself to a lethal dose of radiation. He died within days. There were other people in the room and, again as I had heard the story before the film came out and as shown in the film, Merriman calmly strode over to a blackboard and calculated that he was the only person in the room to have received a lethal dose.

    • #11
    • July 1, 2017, at 3:49 PM PDT
    • 4 likes
  12. ctlaw Coolidge

    John Walker (View Comment):
    The Nobel committee has been reluctant to award the prize twice to one person. The only person to win two Nobel prizes in physics is John Bardeen (1956 for the invention of the transistor, and 1972 for the BCS theory of superconductivity).

    It seems two factors may have been relevant.

    First, the second was a shared prize (so was the first, but that’s not relevant). Denying it to Bardeen would deny it to the other two worthy recipients.

    Second, the 1973 prize seemed to build on the BCS work. Contrary to Nobel’s intent there is a long lag between Nobel work and the award. It likely would have been anticipated in 1972 that the 1973 winners would be up for it soon. Granting it to them without having granted one to BCS would seem an omission.

    • #12
    • July 1, 2017, at 3:51 PM PDT
    • 2 likes
  13. John Walker Contributor
    John Walker

    cqness (View Comment):
    An experimenter named Michael Merriman, played by John Cusack in the film, swept the experiment off the table with his arm exposing himself to a lethal dose of radiation. He died within days. There were other people in the room and, again as I had heard the story before the film came out and as shown in the film, Merriman calmly strode over to a blackboard and calculated that he was the only person in the room to have received a lethal dose.

    The Michael Merriman character in the film was based upon Louis Slotin, a Canadian physicist and chemist who received a lethal dose of radiation in a criticality accident on 1946-05-21. He was the second person to die from a criticality accident from this same core: Harry Daghlian was the first, irradiated on 1945-08-21 and died 25 days later.

    This so-called “demon core”, involved in both accidents, was originally intended to be used in the Operation Crossroads tests at Bikini Atoll, but was eventually recycled for use in other weapons.

    The “demon core” was not responsible for either accident: both were the result of operator error. As a result of these accidents, procedures were put in place to prevent further criticality accidents. Note that both of these accidents happened after the end of World War II, so they cannot be attributed to wartime crash project carelessness.

    • #13
    • July 1, 2017, at 4:03 PM PDT
    • 6 likes
  14. The Reticulator Member

    John Walker: Upon being told the number was four, he said, “I remember my old friend Johnny von Neumann used to say, with four parameters I can fit an elephant, and with five I can make him wiggle his trunk.” Chastened, Dyson soon concluded his model was a blind alley.

    I will remember this one. I wonder how many it takes to fit climate change.

    BTW, here is an example of the elephant: https://www.johndcook.com/blog/2011/06/21/how-to-fit-an-elephant/

    • #14
    • July 1, 2017, at 6:13 PM PDT
    • 3 likes
  15. JimGoneWild Coolidge

    John, well done.

    • #15
    • July 2, 2017, at 9:42 AM PDT
    • 1 like
  16. barbara lydick Coolidge

    The Reticulator (View Comment):
    I wonder how many it takes to fit climate change.

    As your reference notes, “with enough parameters, you can fit any data set” !!!

    It’s interesting to note that “on the ship back home from Brazil, [Ottorino] Respighi* met by chance with Italian physicist Enrico Fermi. During their long conversation, Fermi tried to get Respighi to explain music in terms of physics, which Respighi was unable to do. They remained close friends until Respighi’s death in 1936.”

    *Two pretty well-known pieces: Fountains of Rome (1916); Ancient Airs and Dances Suite No. 1 (1917)

    • #16
    • July 2, 2017, at 3:47 PM PDT
    • 3 likes
  17. JimGoneWild Coolidge

    barbara lydick (View Comment):

    The Reticulator (View Comment):
    I wonder how many it takes to fit climate change.

    As your reference notes, “with enough parameters, you can fit any data set” !!!

    It’s interesting to note that “on the ship back home from Brazil, [Ottorino] Respighi* met by chance with Italian physicist Enrico Fermi. During their long conversation, Fermi tried to get Respighi to explain music in terms of physics, which Respighi was unable to do. They remained close friends until Respighi’s death in 1936.”

    *Two pretty well-known pieces: Fountains of Rome (1916); Ancient Airs and Dances Suite No. 1 (1917)

    Cool. To see that dialog would be awesome and educational.

    • #17
    • July 2, 2017, at 10:41 PM PDT
    • 2 likes
  18. Instugator Thatcher
    Instugator Joined in the first year of Ricochet Ricochet Charter Member

    John Walker (View Comment):
    Precisely—the core has to stay assembled long enough for sufficient generations of fission (each of which roughly doubles the yield) to occur before it explosively disassembles.

    Interesting tidbit – each of these generations is called a “shake” – as in “two shakes of a lamb’s tail.”

    • #18
    • July 2, 2017, at 11:35 PM PDT
    • 1 like
  19. Instugator Thatcher
    Instugator Joined in the first year of Ricochet Ricochet Charter Member

    John Walker (View Comment):
    The “demon core” was not responsible for either accident: both were the result of operator error. As a result of these accidents, procedures were put in place to prevent further criticality accidents.

    Laxity in those procedures has resulted in Los Alamos being unable to perform its work on warheads since 2013.

    • #19
    • July 2, 2017, at 11:39 PM PDT
    • 1 like
  20. Bombgineer Inactive

    Instugator (View Comment):

    John Walker (View Comment):
    The “demon core” was not responsible for either accident: both were the result of operator error. As a result of these accidents, procedures were put in place to prevent further criticality accidents.

    Laxity in those procedures has resulted in Los Alamos being unable to perform its work on warheads since 2013.

    There is no comparison between procedures in the 1940s and those of today. Also, your definition of “shake” is not quite right — a shake is simply 10 nanoseconds.

    @johnwalker, great article on a great man. Fermi deserves far more recognition for his contributions to national (and international) security.

    • #20
    • July 3, 2017, at 12:40 AM PDT
    • 1 like
  21. Gary McVey Contributor
    Gary McVey Joined in the first year of Ricochet Ricochet Charter Member

    John Walker (View Comment):

    Gary McVey (View Comment):
    I wish I could track down a quote to the effect that “Los Alamos wasn’t about Hiroshima; it was about Nagasaki”, meaning the classic gun-style uranium bomb that exploded over Hiroshima didn’t require a vast laboratory to figure out how to detonate it, but the plutonium bomb set off over Nagasaki did.

    I don’t know the source of the quote but, historically, Los Alamos was in operation well before it was discovered that implosion would not work for plutonium. The initial design for both bombs were gun designs, with a more extreme, higher-velocity gun for the plutonium bomb which was called “Thin Man”. Work on Thin Man continued into 1944, when results from testing the reactor plutonium showed it would not work. Thin Man was cancelled at a meeting on July 17th, 1944, and afterward all work on the plutonium bomb was on the implosion design.

    There was a great deal of work for Los Alamos to do apart from implosion research. At the inception of the project, nobody knew the critical masses of either uranium or plutonium, and these had to be determined experimentally by “tickling the dragon’s tail”. Experiments also had to be done on neutron reflectors and initiators (neutron generators) which were used in both the gun and implosion bombs.

    To be sure, I don’t think the quote I recalled (which was, I think, from a British or Canadian participant) was meant to be taken literally, but to make the point that the 1941 UK conception of how to make a bomb wasn’t, at least as its original goal was concerned, unimaginably off the mark. The scale and the expense of the industrial-level U-238/U-235 separation techniques was certainly greater than the British anticipated, justifying their painful, if tacit, yielding of real control of the bomb project to the US. Certainly in any case a real world calculation of things that John lists, like critical mass and the design of a neutron initiator, would have required a lab compound run within a military base or reservation of some kind.

    The remark should probably be taken to mean simply, “It turned out that implosion was required and damn, even with Von Neumann’s help, it was tough. If we hadn’t had to deal with implosion, it would have been a lot simpler”.

    If I may paraphrase a John Walker quote (I think accurately), “No nation that’s ever mastered implosion has ever looked back”.

    • #21
    • July 3, 2017, at 1:00 AM PDT
    • 1 like
  22. Instugator Thatcher
    Instugator Joined in the first year of Ricochet Ricochet Charter Member

    Bombgineer (View Comment):
    There is no comparison between procedures in the 1940s and those of today. Also, your definition of “shake” is not quite right — a shake is simply 10 nanoseconds.

    Guess again – the term was developed as part of the weapons program because I guess 10 nanoseconds is somehow magical enough to deserve its own term?

    The term is used to define the generation.

    Bombgineer (View Comment):
    There is no comparison between procedures in the 1940s and those of today.

    Other than they are both designed to prevent criticality.

    And Los Alamos is unable to do their job because they lack the expertise to either determine or follow whichever procedures are necessary conduct their evaluations.

    • #22
    • July 3, 2017, at 1:02 AM PDT
    • Like
  23. John Walker Contributor
    John Walker

    Instugator (View Comment):

    John Walker (View Comment):
    Precisely—the core has to stay assembled long enough for sufficient generations of fission (each of which roughly doubles the yield) to occur before it explosively disassembles.

    Interesting tidbit – each of these generations is called a “shake” – as in “two shakes of a lamb’s tail.”

    Another curious unit coined during the Manhattan Project was the “barn”, a unit of area used to measure cross-section in scattering processes. In SI units, a barn is equal to 10^−28 m². One barn is roughly the cross-section of a nucleus of uranium. It comes from the idiom “couldn’t hit the side of a barn”—a uranium nucleus is a large target for a particle accelerator.

    The inverse femtobarn (fb^−1) is the conventional unit for integrated luminosity of particle accelerators and colliders. It allows estimating the number of events for a process with a cross-section given in femtobarns you can expect to find in a run with a given luminosity and time. Hence the announcement that at the conclusion of the Large Hadron Collider’s 2016 run, it had accumulated 40 inverse femtobarns of integrated luminosity.

    • #23
    • July 3, 2017, at 4:52 AM PDT
    • 2 likes
  24. Instugator Thatcher
    Instugator Joined in the first year of Ricochet Ricochet Charter Member

    John Walker (View Comment):
    Another curious unit coined during the Manhattan Project was the “barn”, a unit of area used to measure cross-section in scattering processes.

    John – I don’t say this as frequently as I should – I am really thankful that you are a contributor here.

    I appreciate the effort and the outcome of your work.

    Thank you.

    • #24
    • July 3, 2017, at 7:55 AM PDT
    • 2 likes
  25. John Walker Contributor
    John Walker

    Bombgineer (View Comment):
    There is no comparison between procedures in the 1940s and those of today.

    True, but the 2011 plutonium photo-op incident at Los Alamos demonstrates how subtle avoiding criticality accidents can be and the extent to which attention to detail is essential.

    Near criticality configuration of plutonium rods at Los Alamos, 2011.

    Had only a few more rods been included in this picture, this could have ended badly. Further more, once you’ve created a potentially dangerous configuration, it’s necessary to step back and let the criticality experts decide how to proceed. Removing something with your hand or a with a plastic tool might provoke a criticality accident because the flesh of the hand and the plastic tool contain plenty of hydrogen atoms which can moderate the neutrons emitted by the Pu-240 in the rods and increase the fission cross-section as those neutrons encounter other nearby plutonium.

    • #25
    • July 3, 2017, at 4:11 PM PDT
    • 3 likes
  26. ctlaw Coolidge

    John Walker (View Comment):

    Bombgineer (View Comment):
    There is no comparison between procedures in the 1940s and those of today.

    True, but the 2011 plutonium photo-op incident at Los Alamos demonstrates how subtle avoiding criticality accidents can be and the extent to which attention to detail is essential.

    Near criticality configuration of plutonium rods at Los Alamos, 2011.

    Had only a few more rods been included in this picture, this could have ended badly. Further more, once you’ve created a potentially dangerous configuration, it’s necessary to step back and let the criticality experts decide how to proceed. Removing something with your hand or a with a plastic tool might provoke a criticality accident because the flesh of the hand and the plastic tool contain plenty of hydrogen atoms which can moderate the neutrons emitted by the Pu-240 in the rods and increase the fission cross-section as those neutrons encounter other nearby plutonium.

    If memory serves, the Chernobyl disaster occurred because they put the reactor in a condition where the correct response turned out to be counterintuitive. The intuitive response (i.e., inserting the withdrawn control rods) sped up the reaction.

    • #26
    • July 3, 2017, at 4:20 PM PDT
    • 2 likes
  27. Hank Rhody, Badgeless Bandito Contributor

    John Walker: Just twenty-one, he had his academic credential, published work to his name, and the attention of prominent researchers aware of his talent. What he lacked was the prospect of a job in his chosen field.

    Makes me feel a lot better about not turning my physics degree into a paying gig.

    • #27
    • July 3, 2017, at 7:12 PM PDT
    • 1 like
  28. Tim H. Member

    John Walker (View Comment):

    Percival (View Comment):
    The land for the field had been donated to the University of Chicago by Marshall Field, founder of Marshall Field and Company, and was therefore known as “Marshall Field,” or maybe “Marshall Field Field,” or if you wanted to “Marshall Field Marshall Field.”

    Just imagine if he had previously held five-star rank in the British Army: Field Marshal.

    The “Marshall Field Field Marshal Marshall Field Field”.

    Major Major Major Major?

    • #28
    • July 14, 2017, at 7:56 AM PDT
    • 1 like
  29. Tim H. Member

    John Walker (View Comment):

    True, but the 2011 plutonium photo-op incident at Los Alamos demonstrates how subtle avoiding criticality accidents can be and the extent to which attention to detail is essential.

    Near criticality configuration of plutonium rods at Los Alamos, 2011.

    Had only a few more rods been included in this picture, this could have ended badly.

    I had not heard of this before. My goodness! I just read the Science article you linked, and it’s surprising how the safety rules for avoiding criticality were treated so cavalierly.

    • #29
    • July 14, 2017, at 8:01 AM PDT
    • 2 likes
  30. Percival Thatcher
    Percival Joined in the first year of Ricochet Ricochet Charter Member

    Tim H. (View Comment):

    John Walker (View Comment):

    True, but the 2011 plutonium photo-op incident at Los Alamos demonstrates how subtle avoiding criticality accidents can be and the extent to which attention to detail is essential.

    Near criticality configuration of plutonium rods at Los Alamos, 2011.

    Had only a few more rods been included in this picture, this could have ended badly.

    I had not heard of this before. My goodness! I just read the Science article you linked, and it’s surprising how the safety rules for avoiding criticality were treated so cavalierly.

    Richard Feynman was sent to Oak Ridge during the war to do a safety assessment. He found huge barrels of uranium oxide being stored way too close together because while the uranium itself was nowhere near enough for criticality, being in solution slows down the neutrons sufficiently to pose a real danger with less mass.

    When Feynman told it, it was funnier.

    • #30
    • July 14, 2017, at 9:54 AM PDT
    • 2 likes

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