Alamos
Construction Los Secret City in the Sangre de Christo Mountains in New Mexico. The Los Alamos location was picked by Groves and Oppenheimer. On the site of a small boy's school. The sole purpose was to design and build the bombs. Remote area, (Groves, Oppenheimer) Easily to close off (Groves) Stimulating environment (Oppenheimer)
Purchased for ~$ 400,000
Construction started in 1943 on top of Mesa
In January 1943, the population of Los Alamos had risen to 1,500. By January 1944, it reached 3,500, and a year later it reached 5,700.
Accelerator test facilities
Cyclotron building The largest items were the accelerators. Oppenheimer decided that electrostatic generators (Van de Graaff accelerators), a CockcroftWalton machine and a good cyclotron would be required to carry on the experimental measurements that would be transferred to Los Alamos. The Harvard cyclotron was selected as the best. Also selected Electrostatic accelerators were the University of Illinois' Cockcroft-Walton accelerator and two Van de Graaff accelerators at the University of Wisconsin: the "long tank," a 22ft long machine and the "short tank," a 17ft long machine.
Initial plan of "military laboratory" with scientists in uniform failed on refusal of academics which were to be recruited. Final agreement Between Oppenheimer and Groves was academic environment with Oppenheimer as scientific director (Robert Bacher from Cornell (& MIT) would head the experimental section, Hans Bethe (Cornell) the theory (T) division. The ultimate authority over the laboratory, however, would be the Military Policy Committee under General Groves.
Military or Academic Environment?
Tickling the Tail of the Dragon
The determination of the critical mass for explosion The exact size of the critical mass was determined by Otto Frisch at Los Alamos. The important input parameters were the fission cross Sections which were measured in accelerator based experiments. Other parameters were the average neutron yield upon fission, and the mass density which depended on lattice structure of metal and the purity of the available material (absorption of neutrons).
The critical mass?
Depended of neutron flux and neutron release time versus explosion time scale
20 ktons TNT = 8.4 1013 J 1 fission 3.2 10 -11 J 8.4 1013 J number of fissions : N = = 2.6 10 24 3.2 10 -11 J 235 g = 6.02 10 23 atoms m 235U 235 g 2.6 10 24 = = 1.015 kg 23 6.02 10
with 10% efficiency 10 kg was estimated.
Neutron production for chain reaction
N n = N 0 e ( 1- k ) n N 0 = 1; k = 1.693 Nn = N0 e
Neutron/fission number 1E+40 1E+30 1E+20 1E+10 1 0
0.693 n
2
n
20
40
60
80
100
Neutron production factor: k=1.63 is due to neutron absorption in material impurities (238U)! In n80 fission generations 1024 neutrons for fissions have been produced.
fission generations
Timing of explosion
Typical neutron velocity: v=107 m/s Timing for one generation is travel time of neutrons across Uranium mass with density: =1.87104 kg/m3.
M = V
4 V = r3 3 3M r = 4
1/ 3
3M = 4 r 3
= 0.05m
2r t= = 10 -8 s v
For ~80 generations ~1s Mass must remain confined for ~1s since most of the energy production takes place during last 10 generations
Gun Design
This design worked with uranium. A 2000 lb TNT Blockbuster bomb was used as the "trigger" to bring critical mass together.
Plutonium device
Without a neutron reflecting shield, pure Pu-239 metal has a critical mass of 10 kg, for a "reactor grade" isotopic mixture this would be 18 kg. Using a 15 cm U-238 shield, the Pu-239 critical mass is only some 4 kg, while for LWR-produced plutonium this is some 7 kg.
Implosion Design
This design was required for plutonium. 239 240 Impurities of 240Pu (produced by secondary Pu(n, ) Pu reactions) would release too many neutrons and cause premature detonation in the gun design before optimum critical conditions are reached. This would lower the yield. This design needed testing Trinity test
Trinity Site
The chosen site was in the Jornado del Muerto Valley near Alamogordo New Mexico and code named Trinity.
The set-up of the Trinity test
The loading of the Plutonium into the "Gadget"
Gadget
The Dawn of the Nuclear Age
The first nuclear explosion occurred at 5:29:45 am on July 16, 1945 at Trinity site.
Trinity
At 5:29:45 a.m., the gadget exploded with a force of 21,000 tons of TNT, evaporating the tower on which it stood. Groves' deputy, General Farrell, wrote that the "whole country was lighted by a searing light with the intensity many times that of the midday sun. It was golden, purple, violet, gray and blue. It lighted every peak, crevasse and ridge of the nearby mountain range with a clarity and beauty that cannot be described but must be seen to be imagined. Seconds after the explosion came first the air blast pressing hard against the people, to be followed almost immediately by the strong, sustained awesome roar that warned of doomsday and made us feel we puny things were blasphemous to dare tamper with the forces heretofore reserved for the Almighty."
Edward Teller described wearing double welders glasses and was not impressed until he removed his hands from around the glasses. Fermi was holding pieces of paper in his hand and waited for the shock wave to estimate the output. He later commented on missing both fission and the first nuclear explosion.
National Success Story?
International effort,
Hans Bethe, Niels Bohr Enrico Fermi, Otto Frisch Klaus Fuchs, Georg Gamow Leo Szilard, Edgar Teller Georg Uhlenbeck Stansilav Ulam Victor Weisskopf Eugene Wigner
Trinity Summary
How were the Germans doing?
Meeting between Bohr & Heisenberg in Copenhagen in September 1941 Looking for help? Passing information? on Boasting of progress?
Heisenberg maintained claim that he was working on reactor not bomb design! He was officially head of the German Uranium club. 1942 first reactor design in Leipzig 1943 cancellation of Uranium bomb project 1944 reactor in Haigerloch remained too small!
The critical mass from German view
The best evidence we have suggests that Heisenberg had no interest in building an atomic bomb. In mid-1942, Albert Speer, the weapons minister, asked Heisenberg whether he could produce a weapon in nine months. With a clear conscience he could answer "No." And he did not know how much fissionable material he would need. When, on occasion, friends asked him, his replies were varied and vague, ranging from 10 kilograms to a few tons. Why didn't he know? Why hadn't pure intellectual curiosity led him to investigate the properties of uranium-235 with fast neutrons? He could have made a small amount using the cyclotrons available in both Paris and Copenhagen. But he never asked that these properties be measured. The best proof of his lack of interest came at the end of the war. Heisenberg and about ten other German nuclear scientists were interned at Farm Hall, a country estate in England. ... His first attempt at explanation (of the Hiroshima bomb) was totally wrong! He hypothesized something like a nuclear reactor, with the neutrons slowed by many collisions with a moderator. That he was capable of doing the work was shown about a week later when, in another lecture, he corrected himself and presented a theory similar to that worked out by Rudolf Peierls and Otto Frisch in 1940. He estimated the necessary amount of uranium-235 at about 20 kg, which is nearly correct. Hans Bethe in Physics Today Vol 53 (2001)
Indications for activity
Used French cyclotron accelerator of Joliot-Curie Maintained studies of moderator systems (Walter Bothe) Continued reactor research
The German Reactor B8 in Haigerloch
664 U cubes
Uranium blocks immersed in heavy water Too small on neutron enhancement factor 7
The Alsos Mission in Haigerloch
The end of the German nuclear program
Lucky guys!
German Surrender May 7. 1945
Was there a nuclear race?
Did German scientists fail to recognize opportunity? Did Bothe stop program by mistake in moderator calculation? Did Heisenberg fail (initially) in calculating critical mass? Did allied air raids successfully stop German nuclear attempts? Did lack of Hitler support stop nuclear weapons program? or combination of all these points?
New Evidence? A German Bomb after all?
A German historian has claimed in a new book presented on Monday that Nazi scientists successfully tested a tactical nuclear weapon in the last months of World War II.
"It was about 9:30 when I suddenly saw something ... it was as bright as hundreds of bolts of lightning, red on the inside and yellow on the outside, so bright you could've read the newspaper. It all happened so quickly, and then we couldn't see anything at all. We just noticed there was a powerful wind..." The woman complained of "nose bleeds, headaches and pressure in the ears." The next day Heinz Wachsmut, a man who worked for a local excavating company, was ordered to help the SS build wooden platforms on which the corpses of prisoners were cremated. The bodies, according to Wachsmut, were covered with horrific burn wounds. Like Werner, Wachsmut reports that local residents complained of headaches, some...