The Manhattan Project - Personalities and Problems Fromm Institute Fall 2020 bebo.white@gmail.com Lecture 2
submitted questions from lecture 1 1. While all this nuclear research was happening in Europe, what was happening in the United States? - Quite a bit, a number of American Nobel Prizes in Physics during this time, e.g., Paul Anderson for the discovery of the positron 2. Are any scientists who were part of the Manhattan Project still alive? - I don’t think so 3. How big was the atomic test that you worked on? - Project Milk Shake, March 25, 1968 - ~20 kt (Trinity was 22 kt, Hiroshima was ~15 kt) 4. How much information about the Manhattan Project is still classified? - ?? I’m reasonably sure that some of the scientific data and engineering is. 5. Will you talk about quarks as being parts of protons and neutrons? - The physics included in the class is what was known at the time.
“the uranium committee” issued its first report on Nov. 1, 1939 • Recommendations: 1. The U.S. should buy a supply of uranium dioxide for R&D 2. The U.S. should fund research into uranium isotope separation (i.e., how to separate U 235 from U 238 ) 3. The U.S. should fund Szilárd’s and Fermi’s work at Columbia University on neutrons and chain reactions NOTE : Alexander Sachs thought that the report was “too academic”
the u.s. should buy uranium dioxide • Obviously a reaction that the Germans would seize the supply (Congo and Czechoslovakia) and shut down availability • 45 tons of uranium dioxide was purchased • Also purchased was 3.6 tons of graphite - why?
research uranium isotope separation • It had been established that U 235 was the natural occurring isotope that is fissile (capable of fission) - Niels Bohr • But, remember that U 235 is only ~0.7% of natural uranium • 1 kg of uranium only has 7 grams of U 235 • 1 quart of uranium only has ~1.5 teaspoons of U 235 • Since they are the same chemically, the only way to separate them must be based on their weight
a thought experiment • You have a giant jar of 1000 jelly beans - all the same color so they all look the same • 7 of the 1000 weigh 1% less than the other 993 • I will give you $100 apiece for each of those 7 • You cannot take them out of the jar individually and weigh them • What strategy would you use?
harold urey 🥈 • American chemist - but had “his fingers in lots of pies” • Won the 1934 Nobel Prize in Chemistry for the discovery of deuterium (an isotope of hydrogen) • Funded by the Uranium Committee to study uranium isotope separation at Columbia University • Favored (at first) the “jelly bean jar method of separation” (my description) using a centrifuge to separate U 238 and U 235
calutrons in berkeley • Lawrence (of course) thought that separation was a good job for cyclotrons • The “calutron” is a new variation of the familiar mass spectrometer using a cyclotron • This is not totally based on weight, but also on the ion’s magnetic moment • Also another excuse for Lawrence to get more $ to build more and bigger machines • He suggested that he could “crank out U 235 by the ton”
szilárd, fermi, fission, chain reactions at columbia • There were still doubters about • whether it was possible to do chain reactions at scale • whether it was possible to control chain reactions without blowing yourself up • The biggest problem was neutron moderation
enrico fermi 🥈 !! • Italian physicist (later naturalized) • One of the greatest physicists in history - both a theoretician and an experimentalist • Winner of the 1938 Nobel Prize - “demonstrations of the existence of new radioactive elements produced by neutron irradiation” • Maybe discovered fission in 1934 (before Hahn and Strassmann) with Segrè (?) • Emigrated to the U.S. in 1938 because his wife was Jewish • Major contributions to statistical and quantum mechanics and particle physics • Some of you may remember The Fermi Paradox from Fraknoi classes - “where are they?”
neutron moderation (1/2) • Neutrons from fission travel very fast - ~1/15 of the speed of light ~12,400 miles/second • When a neutron traveling at this speed hits a U 238 nucleus, one of two things happen • It bounces off • It is absorbed - this squelches any possible chain reaction • That’s why chain reactions don’t happen in nature
neutron moderation (2/2) • If fast neutrons can be slowed down, something else happens • They are less likely to be captured by U 238 nuclei and just bounce off • So they bounce around until they find a U 235 nucleus that they can do business with • So, how to slow down (moderate) fast neutrons? • Fermi had actually discovered years before that light nuclei slow down neutrons
so, szilárd and fermi began to work their way down the periodic table (1/2) • Hydrogen would be best since it is a single proton but it occasionally captures neutrons • Managing a gaseous moderator would hard especially since hydrogen is explosive (remember the Hindenburg in 1937) - who wants to pump energy into hydrogen? • Dihydrogen oxide (water) where the hydrogen atoms were Urey’s deuterium would be great since the atoms already had a neutron - heavy water (11% heavier than normal) - rare (150 ppm) and expensive (more later)
so, szilárd and fermi began to work their way down the periodic table (2/2) • Helium - does not absorb neutrons (remember alpha particles?) - isn’t practical (suspending uranium in a tank of helium?) • Lithium - 3 protons - strong neutron absorber • Beryllium - 4 protons - not a strong absorber but is very toxic • Boron - 5 protons - absorbs neutrons like crazy • Carbon - 6 protons - it captures neutrons at 1% of the rate of hydrogen
so, carbon it is… • Szilárd to Fermi - “It seems to me now that there is a good chance that carbon might be an excellent element to use [as a moderator]. I personally would be in favor of trying a large-scale experiment with a carbon-uranium-oxide mixture if we can get hold of the material.” • The Uranium Committee gave them the money to do it. • Hence the purchase of 3.6 tons of graphite.
the international research community was jumping on the uranium, fission bandwagon • A flurry of journal articles and conference talks • Edwin McMillan 🥈 used the cyclotron in Berkeley • To create new radioisotopes ( transmutation ) • To discover/create (?) the first transuranic element - neptunium • Won the Nobel Prize in Chemistry (after the war)
digression - discovered or invented?
and the great niels bohr 🥈 !! • Danish physicist and philosopher • Another of the greatest physicists in history • Developer of the Bohr atomic model and one of the pioneers of quantum mechanics • Nobel Prize in Physics in 1922- “for his services in the investigation of the structure of atoms and of the radiation emanating from them” • Remember his connection with Otto Frisch?
meanwhile, “across the pond”
the paris group • Collège de France - included Frédéric Joliot-Curie, Francis Perrin and others • By February 1939 had verified that a uranium chain reaction might be possible - Perrin coined the term “critical mass” • Verified that graphite and heavy water would be good moderators but chose heavy water • Convinced the French Ministry of Armaments to try and buy all the heavy water possible from the fertilizer and hydroelectric plant in Vermonk, Norway but were told that Germany had ordered it all; this verified German efforts • Norway gave the entire stock to France just before the invasion of Norway; France smuggled the entire stock to England just after the invasion of France • The Paris Group (except) for Joliot-Curie moved to Cambridge in England and joined research there
serious research moves to england • The big question (as in the U.S.) was whether fission weapons were do-able and/or practical • At the University of Liverpool, James Chadwick’s work was inconclusive • At Cambridge University, George Paget Thomson and William Lawrence Bragg (both Nobel Prize winners) wanted the government to buy uranium ore before supplies from the Congo got shut down • At the University of Birmingham, Mark Oliphant 🥈 gave the job to two refugee scientists - Rudolf Peierls and Otto Frisch 🥈
the frisch-peierls memorandum (march 1940) • Not only is a uranium fission weapon do-able, but it could be much smaller than originally thought • Key points ( Bebo’s summary ) 1. Such a weapon is irresistible - no material or structure could be expected to survive it; 2. The spread of radioactive substances by the wind would cause many more casualties; 3. There is no information that suggests the same idea has not occurred to other scientists given all the theoretical data that has been published; 4. It is quite conceivable that Germany is working on a weapon given their stockpiling of resources; 5. If Germany acquires a weapon, no shelters in England are sufficient, and no infrastructure is available for detection of and response to radioactive effects.
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