Editing Nuclear fission (section)

===Fission chain reaction realized===
During this period the Hungarian physicist [[Leó Szilárd]] realized that the neutron-driven fission of heavy atoms could be used to create a nuclear chain reaction. Such a reaction using neutrons was an idea he had first formulated in 1933, upon reading Rutherford's disparaging remarks about generating power from neutron collisions. However, Szilárd had not been able to achieve a neutron-driven chain reaction using beryllium. Szilard stated, "...if we could find an element which is split by neutrons and which would emit ''two'' neutrons when it absorbs ''one'' neutron, such an element, if assembled in sufficiently large mass, could sustain a nuclear chain reaction."  On 25 January 1939, after learning of Hahn's discovery from [[Eugene Wigner]], Szilard noted, "...if enough neutrons are emitted...then it should be, of course, possible to sustain a chain reaction. All of the things which [[H. G. Wells]] predicted appeared suddenly real to me." After the Hahn-Strassman paper was published, Szilard noted in a letter to [[Lewis Strauss]], that during the fission of uranium, "the energy released in this new reaction must be very much higher than all previously known cases...," which might lead to "large-scale production of energy and radioactive elements, unfortunately also perhaps to atomic bombs."<ref name="tz">{{cite book |last1=Zoellner |first1=Tom |title=Uranium |date=2009 |publisher=Viking Penguin |isbn=978-0-670-02064-5 |pages=28–30}}</ref><ref name=rr/>{{rp|26–28,203–204,213–214,223–225,267–268}}

Szilard now urged Fermi (in New York) and [[Frédéric Joliot-Curie]] (in Paris) to refrain from publishing on the possibility of a chain reaction, lest the Nazi government become aware of the possibilities on the eve of what would later be known as [[World War II]]. With some hesitation Fermi agreed to self-censor. But Joliot-Curie did not, and in April 1939 his team in Paris, including [[Hans von Halban]] and [[Lew Kowarski]], reported in the journal ''Nature'' that the number of neutrons emitted with nuclear fission of uranium was then reported at 3.5 per fission.<ref>{{cite journal |author1=H. Von Halban |author2=F. Joliot |author3=L. Kowarski  |name-list-style=amp |title=Number of Neutrons Liberated in the Nuclear Fission of Uranium|journal=Nature|volume=143|issue=3625|page=680|year=1939|doi=10.1038/143680a0|bibcode= 1939Natur.143..680V|s2cid=4089039 |doi-access=free}}</ref> Szilard and [[Walter Zinn]] found "...the number of neutrons emitted by fission to be about two." Fermi and Anderson estimated "a yield of about two neutrons per each neutron captured."<ref name=rr/>{{rp|290–291,295–296}}

[[File:Stagg Field reactor.jpg|thumb|Drawing of the first artificial reactor, [[Chicago Pile-1]]]]

With the news of fission neutrons from uranium fission, Szilárd immediately understood the possibility of a nuclear chain reaction using uranium. In the summer, Fermi and Szilard proposed the idea of a nuclear reactor (pile) to mediate this process. The pile would use natural uranium as fuel. Fermi had shown much earlier that neutrons were far more effectively captured by atoms if they were of low energy (so-called "slow" or "thermal" neutrons), because for quantum reasons it made the atoms look like much larger targets to the neutrons. Thus to slow down the secondary neutrons released by the fissioning uranium nuclei, Fermi and Szilard proposed a graphite "moderator", against which the fast, high-energy secondary neutrons would collide, effectively slowing them down. With enough uranium, and with sufficiently pure graphite, their "pile" could theoretically sustain a slow-neutron chain reaction. This would result in the production of heat, as well as the creation of radioactive fission products.<ref name=rr/>{{rp|291,298–302}}

In August 1939, Szilard, [[Edward Teller|Teller]] and [[Eugene Wigner|Wigner]] thought that the [[German nuclear weapons program|Germans might make use of the fission chain reaction]] and were spurred to attempt to attract the attention of the United States government to the issue. Towards this, they persuaded [[Albert Einstein]] to lend his name to a letter directed to President [[Franklin Roosevelt]]. On 11 October, the [[Einstein–Szilárd letter]] was delivered via [[Alexander Sachs]]. Roosevelt quickly understood the implications, stating, "Alex, what you are after is to see that the Nazis don't blow us up." Roosevelt ordered the formation of the [[Advisory Committee on Uranium]].<ref name=rr/>{{rp|303–309,312–317}}

In February 1940, encouraged by Fermi and [[John R. Dunning]], [[Alfred O. C. Nier]] was able to separate U-235 and U-238 from [[uranium tetrachloride]] in a [[mass spectrometry|glass mass spectrometer]]. Subsequently, Dunning, bombarding the U-235 sample with neutrons generated by the Columbia University [[cyclotron]], confirmed "U-235 was responsible for the slow neutron fission of uranium."<ref name=rr/>{{rp|297–298,332}}

At the [[University of Birmingham]], Frisch teamed up with [[Rudolf Peierls|Peierls]], who had been working on a critical mass formula. assuming isotope separation was possible, they considered <sup>235</sup>U, which had a [[nuclear cross section|cross section]] not yet determined, but which was assumed to be much larger than that of natural uranium. They calculated only a pound or two in a volume less than a golf ball, would result in a chain reaction faster than vaporization, and the resultant explosion would generate temperature greater than the interior of the sun, and pressures greater than the center of the earth. Additionally, the costs of isotope separation "would be insignificant compared to the cost of the war." By March 1940, encouraged by [[Mark Oliphant]], they wrote the [[Frisch–Peierls memorandum]] in two parts, "On the construction of a 'super-bomb; based on a nuclear chain reaction in uranium," and "Memorandum on the properties of a radioactive 'super-bomb.' ". On 10 April 1940, the first meeting of the [[MAUD Committee]] was held.<ref name=rr/>{{rp|321–325,330–331,340–341}}

In December 1940, [[Franz Simon]] at Oxford wrote his Estimate of the size of an actual separation plant." Simon proposed [[gaseous diffusion]] as the best method for uranium isotope separation.<ref name=rr/>{{rp|339,343}}

On 28 March 1941, [[Emilio Segré]] and [[Glen Seaborg]] reported on the "strong indications that <sup>239</sup>Pu undergoes fission with slow neutrons." This meant chemical separation was an alternative to uranium isotope separation. Instead, a nuclear reactor fueled with ordinary uranium could produce a plutonium isotope as a nuclear explosive substitute for <sup>235</sup>U. In May, they demonstrated the cross section of plutonium was 1.7 times that of U235. When plutonium's cross section for fast fission was measured to be ten times that of U238, plutonium became a viable option for a bomb.<ref name=rr/>{{rp|346–355,366–368}}

In October 1941, MAUD released its final report to the U.S. Government. The report stated, "We have now reached the conclusion that it will be possible to make an effective uranium bomb...The material for the first bomb could be ready by the end of 1943..."<ref name=rr/>{{rp|368–369}}

In November 1941, John Dunning and [[Eugene T. Booth]] were able to demonstrate the enrichment of uranium through gaseous barrier diffusion. On 27 November, Bush delivered to third [[National Academy of Sciences]] report to Roosevelt. The report, amongst other things, called for parallel development of all isotope-separation systems. On 6 December, Bush and Conant reorganized the Uranium Committee's tasks, with [[Harold Urey]] developing gaseous diffusion, Lawrence developing electromagnetic separation, [[Eger V. Murphree]] developing centrifuges, and [[Arthur Compton]] responsible for theoretical studies and design.<ref name=rr/>{{rp|381,387–388}}

On 23 April 1942, [[Metallurgical Laboratory|Met Lab]] scientists discussed seven possible ways to extract plutonium from irradiated uranium, and decided to pursue investigation of all seven. On 17 June, the first batch of uranium nitrate hexahydrate (UNH) was undergoing neutron bombardment in the [[Washington University in St. Louis]] cyclotron. On 27 July, the irradiated UNH was ready for [[Glenn T. Seaborg]]'s team. On 20 August, using ultramicrochemistry techniques, they successfully extracted plutonium.<ref name=rr/>{{rp|408–415}}

In April 1939, creating a chain reaction in natural uranium became the goal of Fermi and Szilard, as opposed to isotope separation. Their first efforts involved five hundred pounds of uranium oxide from the Eldorado Radium Corporation. Packed into fifty-two cans two inches in diameter and two feet long in a tank of manganese solution, they were able to confirm more neutrons were emitted than absorbed. However, the hydrogen within the water absorbed the slow neutrons necessary for fission. Carbon in the form of graphite, was then considered, because of its smaller capture cross section. In April 1940, Fermi was able to confirm carbon's potential for a slow-neutron chain reaction, after receiving [[National Carbon Company]]'s graphite bricks at their [[Pupin Laboratories]]. In August and September, the Columbia team enlarged upon the cross section measurements by making a series of exponential "piles".  The first piles consisted of a uranium-graphite lattice, consisting of 288 cans, each containing 60 pounds of uranium oxide, surrounded by graphite bricks. Fermi's goal was to determine critical mass necessary to sustain neutron generation. Fermi defined the [[reproduction factor]] k for assessing the chain reaction, with a value of 1.0 denoting a sustained chain reaction.  In September 1941, Fermi's team was only able to achieve a k value of 0.87.  In April 1942, before the project was centralized in Chicago, they had achieved 0.918 by removing moisture from the oxide. In May 1942, Fermi planned a full-scale chain reacting pile, Chicago Pile-1, after one of the exponential piles at [[Stagg Field]] reached a k of 0.995. Between 15 September and 15 November, [[Herbert L. Anderson]] and [[Walter Zinn]] built sixteen exponential piles. Acquisition of purer forms of graphite, without traces of boron and its large cross section, became paramount. Also important was the acquisition of highly purified forms of oxide from [[Mallinckrodt]] Chemical Works. Finally, acquiring pure uranium metal from the [[Ames process]], meant the replacement of oxide pseudospheres with [[Frank Spedding]]'s "eggs". Starting on 16 November 1942, Fermi had Anderson and Zinn working in two twelve-hours shifts, constructing a pile that eventually reached 57 layers by 1 Dec. The final pile consisted of 771,000 pounds of graphite, 80,590 pounds of uranium oxide, and 12,400 pounds of uranium metal, with ten cadmium [[control rod]]s. Neutron intensity was measured with a [[boron trifluoride]] counter, with the control rods removed, after the end of each shift. On 2 Dec. 1942, with k approaching 1.0, Fermi had all but one of the control rod removed, and gradually removed the last one. The neutron counter clicks increased, as did the pen recorder, when Fermi announced "The pile has gone critical." They had achieved a k of 1.0006, which meant neutron intensity doubled every two minutes, in addition to breeding plutonium.<ref name=rr/>{{rp|298–301,333–334,394–397,400–401,428–442}}