Atomic History 1938-1945

2? Attempts to make the bomb

• n + (92,235)U (56,141)Ba +(36,92)Kr + 3n• U 235.043915 u Ba 140.9139 u• n 1.008665 u Kr 91.8973 u • 3n 3.0260 u• Sum 236.052580 u 235.8372 u• Diff = 0.2154 u = 200.6 MeV• Fr dif = 0.000916• 1 Kg (92,235)U loses 0.000916 Kg if all fission• Mc^2 = 0.000916 x (3e8)^2 = 8.247e13 J = 23 Mkwh.

1600 kwh/mo (my House) • 14,000 homes for 1 month

Fission

• December 1938 - Two German chemists, Otto Hahn and Fritz Strassman, publish that they have detected barium in Uranium which was exposed to neutrons???

• Nuclear Transmutation – Alchemy!!• Lisa Meitner (Germany to Netherlands to

Sweden) and Otto Frisch interpret as uranium fission. Frisch confirms Jan 1939.

World Events

• 1932 Hitler ascends to power (also neutron and positron discovered)

• 1939 Germany invades Poland –”official” start of WWII and on to USSR in 1941

• Invasions of most of rest of Europe follow• 1941 Japan bombs Pearl Harbor• 1944 D-Day invasion of Normandy• 1945 German surrender (May 8) • A-bombs and Japan surrender (August)• What happened in German Nuclear Physics in

the period 1939-45?

Werner Heisenberg

• Heisenberg, born in 1901 in south Germany, was clearly the top German physicist. On left looks like might have been a “Hitler-jugend”, but he was too busy developing quantum mechanics (how atoms work). Nobel Prize 1932. His relationship with the Nazis is ambiguous, even today. One article I looked at claimed he was among the 100 people whose ideas had the greatest impact on civilized thought.

Heisenberg and Nazis

• The deutsche Physik movement was anti-Jewish, anti-theoretical physics, and particularly anti relativity and quamtum theories. Heisenberg used his prestige as a physicist to save some Jews and failed to save others. The deutsche Physik group opposed his appointment as professor at the University of Munich and he was investigated by the SS. Only his mother’s acquaintance with Himmler’s mother saved him, but he did not get the professorship until after WWII.

German bomb effort

• Jan 39 – Fission recognized• Apr 39 – Informed Ministry of War and formation of first

Uranium Club, 3 physicists who were drafted in August• Sept 39 - Bureaucrats shuffled and 2nd Uranium Club

formed. At least a dozen top physicists including Heisenberg, but Diebner was in control.

• Feb and April 43 – Heisenberg reports (to Speer) that a bomb could not be built before 45 and would require significant monetary and manpower resources. Not much done after that and research focussed on deuterium. (Fusion plans????)

• Did Heisenberg discourage the bomb work for moral reasons? Did German experiments give grounds for pessimism? His estimate was dead on for Allied effort.

War end - 1945

• Operation Alsos – Heisenberg and most of the Uranium Club captured and incarcerated in a British “safe house”, the farm house, which was extensively bugged. Despite effort, still do not know much about final status of work toward bomb.

• Heisenberg returned to Berlin and then Munich. Vanderbilt collaborated with people from his institute in Munich to measure Sigma Magnetic moment and there is an agreement signed by Heard and Heisenberg on the magnet used.

• The second Heisenberg shown is the one I remember seeing at the Institute.

US bomb effort – problems and initial concerns

• U235 and Plutonium were candidates for chain reactions but Plutonium could be made only by “breeding” U238 in a U235 reactor.

• Natural Uranium is mostly U238 with only a trace (0.72%) of U235. Need 15% for reactor, 95% for a bomb.

• Chemistry does not separate isotopes.• Mass spectrograph, diffusion, centrifuges• How many neutrons per fusion? Fast or slow? • Leakage – fraction of neutrons which escape

rather than produce another fission

Separation method – based on mass

• Need gaseous form: U-hexafloride or U-hexachloride

• Chlorine 2 isotopes: 35 and 37

• Flourine only one: 19, does not confuse mass of molecule

• Flourine is more corrosive

• Diffusion and centrifuge work on molecules, spectrometer on Ions from gas

Plan: Graphite reactor

• Use U235 to demonstrate large scale fission AND to produce Plutonium

• Fermi and Szilard – Columbia and Chicago (advice to Oak Ridge and to Hanford)

• Chemically separate Plutonium from residue in reactor

• Plan 2 bombs 1) U235 and 2) Plutonium – Oppenheimer, Feynman, Compton, … a massive physicist brain trust at Los Alamos

• Oak Ridge charged with separating a few hundred kilograms of U235 ( 0.72%) from the ore – to supply Hanford and for first bomb.

• Electromagnetic separation looked most promising – Lawrence at Berkeley and Wilson (later director of Fermilab) at Cornell – the Calutron – overgrown mass spectrometer.

• The Calutron story is well told in the DVD.• The centrifuge (reference to Duke in DVD) was

not thought practical because metal for the tubes was not strong enough to stand the stress of the required rapid spin. Metals research has made stronger tubes and is now the preferred method (Iran news).

• Diffusion second choice. Big plant built at Oak ridge but turned out not to be the major player. At least one material for the diffuser material is rumored (still classified) to have been developed by Francis Slack and Ernie Jones of VU.