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Fission

After Otto Hahn and Fritz Strassmann conducted experiments that involved bombarding uranium with slowed neutrons, Hahn wrote to colleague Lise Meitner and revealed the news the two scientists encountered. Lise, Otto Frisch's aunt, collaborated over this process with Frisch. Lise and Frisch were able to cultivate a theoretical justification on the basis of the process. Lise and Frisch were the first to understand this process that Hahn and Strassmann had observed and vindicated that it was nuclear fission that occurred. Frisch and Lise justified this process as the explosion of the uranium nucleus caused by the bombardment of a neutron. This phenomena was a new type of nuclear disintegration and was radioactively more powerful than ever seen before. Frisch and Lise calculated this energy released to be approximately 200,000,000 electron volts. This process resembled the division of biological cells, so they decided to name it 'fission'.

News of this enlightenment traveled through the science community and was soon understood that the rare isotope ,²³⁵U from ²³⁸U, was the optimal selection towards fissionable material. ²³⁵U was calculated to potentially hold the power to cause a self-sustaining nuclear chain reaction. The problem was that only .7% of ²³⁸U is fissionable material, the remaining 99.3% of the mass is not efficient fissile material. In order to create a self-sustaining reaction, the rare isotope must be separated from ²³⁸U to create the critical mass needed for nuclear fission. Estimating the several cross-sections, Francis Perrin calculated the critical mass to be thirteen tons. Peierls calculated the mass in a theoretical paper written in 1939 to be "of the order of tons".

Uranium Enrichment

Rudolf Peierls, a profound theorist who studied under Arnold Sommerfield, was a German scientist working in Britain at Birmingham University who decided to remain in Britain when Adolf Hitler came to power . Through the years, Peierls became the founder of three different schools of theoretical physics, Manchester, Oxford, and Birmingham. When fission was discovered, Peierls was atomically interested in understanding the process. His first task was aimed at solving how to create a chain reaction. Peierls indicated that the multiplication of neutrons can only take place if the path taken by each individual neutron in the system is long enough to give it an optimally high chance of colliding with another neutron. Peierls understood the significance the size the critical mass must be to allow a proper chain reaction to take place. In the interior of the critical mass sphere, neutrons are being produced in the fissionable material. Also, within the sphere of critical mass, a portion of the neutrons are colliding with other neutrons while a separate portion of the neutrons are escaping through the surface of the sphere. Peierls aimed at calculating the equilibrium of the system, where the number of neutrons being produced equaled the number of neutrons escaping the mass.

The two men responsible for aggressively investigated the questions associated with critical mass were Rudolf Peierls and Otto Frisch. In the article,” Resonance of uranium and thorium disintegrations and the phenomena of nuclear fission”, Neils Bohr developed a captivating theory. Bohr was aware that the main factor in the fission process is the formation of a compound nucleus after the incident neutron has been absorbed. The common isotope for ²³⁸U is ²³⁹U. For ²³⁵U, the rare isotope, is ²³⁶U. To make ²³⁸U fission, energy of at least 1 MeV must be resourced from the incident neutron. The energy in ²³⁵U is supplied by the mass defect, allowing neutrons of all energies to be able to cause fission. This energy sustained within the material or lack of energy can be the difference between a fissionable and fissile nucleus. Frisch and Bohr both settled on the notion that the dominant isotope of uranium could not produce a self-sustaining chain reaction. The quantity of emitted neutrons would be below the threshold energy for fission to occur. For a chain reaction to occur, uranium would require the separation of isotopes on an industrial scale.

In order to estimate the critical mass, the mean free path of neutrons must be taken into account.  This was initially estimated to be the average distance between fissions, approximately the magnitude of the critical radius. Peierls theory corrected this to be r_f=1/nσ_f. Where n is number density of the uranium nuclei in the sphere and σ_f is the fission cross section. Peierls left the estimation of the critical mass cross-section of ²³⁵U to Frisch. Frisch approximated the cross-section of the critical mass to be approximately 10^-23 cm². This evaluation is reflected by the critical mass cross-section Frisch estimated. The cross-section Frisch estimated was determined as the cube of the radius and hence the inverse of the cross-section. After doing various calculations with this mass estimation, Frisch assessed the mass to be in the range of a couple pounds. Once Frisch approximated the critical mass, he informed Peierls of his new discovery. Frisch and Peierls were able to revise the initial belief of critical mass needed for nuclear fission in uranium to be substantially less than the previously assumed critical mass. They estimated a metallic sphere of ²³⁵U with a radius of 2.1 cm could suffice to be explosive. This amount resembled approximately 1 kilogram of ²³⁵U. Now that they had an understanding of the mass they began to investigate processes in which they could successfully separate the uranium isotopes to obtain this mass. These developments from the two scientists would lead to the cultivation of a memorandum regarding the possibility of the production of a nuclear bomb.

The Memorandum

These results led to the infamous memorandum that would be developed by Peierls and Frisch, Frisch-Peierls Memorandum. The results developed from Frisch and Peierls were discussed with Mark Oliphant, Henry Tizard, Chairman of the Committee on the Scientific Survey of Air Defense, and Prof. G.P. Thomson of Imperial College, Chairman of the government committee concerned with the possibility of a nuclear chain reaction. These men set up a government committee, the Military Application of Uranium Detonation (MAUD) Committee, which in time became Britain’s Tube Alloy program.  The committee’s purpose was to examine the potential of Frisch and Peierls quantitative and qualitative analysis regarding critical mass and uranium separation. After the investigation of the report, the committee endorsed the Frisch and Peierls memorandum. They concluded from the report that a super weapon was now feasible and possible. This memorandum, written by Peierls and Frisch, was the initial step in the development of the nuclear arms program in Britain. Marked the beginning of an aggressive approach towards uranium enrichment and the development of an atomic bomb.

Oliphant's Visit to the United States

June 22, 1941, Germany invaded the Soviet Union.  The next month, the MAUD Committee approved its final report. The Committee reached the conclusion that it is possible to manufacture a productive uranium bomb. The final report urged the cooperation with the United States should be continued in the research of nuclear fission. Charles C. Lauritsen, a Caltech physicist working at the National Development and Reform Commission (NDRC), was in London during this time and was invited to sit in on the MAUD meeting. The committee pushed for rapid development of nuclear weapons using gaseous-diffusion as their isotope separation device. Once he returned back to the U.S., he was able to brief Dr. Vannevar Bush, Director of the Office of Scientific Research and Development (OSRD), concerning the details discussed during the meeting. The Americans refused to acknowledge the significance of the information and insisted it was in America’s best interest to decline their referral.

In August of 1941, Mark Oliphant, Director of the physics department at the University of Birmingham and original member of the MAUD Committee, was sent to the U.S. to work with the NDRC on radar. Upon his visit to the U.S., Oliphant met with William D. Coolidge. Coolidge was shocked when Oliphant informed him that the British had predicted that only ten kilograms of ²³⁵U would be efficient to supply a chain reaction effected by fast moving electrons. While in America, Oliphant revealed that Briggs had locked away the reports transferred from Britain entailing the initial discoveries of the MAUD. That being said, Oliphant became aware that the Uranium Committee in America was little informed of the progress the MAUD Committee had been making in Britain. Oliphant took the initiative himself to enlighten the scientific community in the U.S. of the recent ground breaking discoveries the MAUD Committee had just exposed. While in the U.S., Oliphant also traveled to Berkley to with meet with Ernest Lawrence, inventor of the cyclotron. After Oliphant had informed Lawrence of his report on uranium, Lawrence met with Conant, Pegram, and Arthur Compton to relay the details which Oliphant had directed to Lawrence. Oliphant was not only able to get in touch with Lawrence, but he met with Conant, Director of the NDRC, and Bush to inform them of the significant data the MAUD had discovered.  Oliphant’s ability to inform the Americans led to Oliphant convincing Lawrence, Lawrence convincing Compton, and then Kistiakowsky convincing Conant on the importance of the MAUD findings. These actions from Oliphant resulted in Bush taking this report directly to the president.

Tizard Mission

In 1940, a British commission,(Tizard and his mission members), were sent to America to create relations and help advance the research towards war technology with the Americans. The most important device transferred through the Tizard mission was the Cavity Magnetron. This inquiry from the Tizard mission made it possible for the Americans to create a radiation laboratory. This lab would later be used as a model for the Los Alamos laboratory. Tizard mission members were involved in advising scientists on construction of the lab and its relations with the armed forces. A barrier had been broken and a pathway to exchange technical information between the two countries had been developed. The mission did not discuss the development of nuclear fission because at the time the process of fission was deemed impractical and was not a main priority for Tizard Mission members.

Isotopic Separation

During the initial stages of development from Frisch and Peierls, Peierls turned to Franz Simon, a chemist shielded in Britain, to consult methods of isotope separation. Frisch chose to perform gaseous thermal diffusion using Clusius tubes because it seemed the most simplistic method to perform isotopic separation. Under Frisch’s calculations, there would need to be 100,000 Clusius tubes to extract the desired separation amount. Simon proceeded in a different direction, Simon wanted to find a method suitable for mass production. With an isotope ratio of 1:139, uranium separation would have to be achieved at a large scale.

When P.B. Moon examined the suggestion that gaseous thermal diffusion be the method of choice to the Thomson committee, there was not a majority agreement upon the decision to move forward with this method (339). After the committee had consulted with Peierls and Simon over the separation method, they came to the agreement that “ordinary” gaseous diffusion was the most optimal method to pursue (339). This method inquires that the gases diffuse through porous materials at rates that are determined by their molecular weight. This method is examined through the analysis of the rate of diffusion of different gasses (339-340).

Francis Ashton applied this method in 1913 when he separated two isotopes of neon by diffusing a sample thousands of times through a pipe clay. Thick materials like pipe clay proved to slow to be efficient on an industry scale (340).  Simon proposed using a metal foil punctured with millions of microscopic holes would allow the separation process to move faster (340). In 1940, Simon developed a report, “Estimate of the size of an actual separation plant”, its intent was to offer details concerning the size and costs of the plant, if built. Simon estimated a plant that separated 1 kg per day of ²³⁵U from pure uranium would cost about £5,000,000(343).

In 1941, Frisch moved to London to work with Chadwick and his cyclotron. Frisch built a Clusius tube there to study the effects of uranium hexafluoride. Frisch and Chadwick discovered the gas, uranium hexafluoride, is of one the gases that the Clusius method will not work (345). This was only a minor setback due to the fact that Simon was already in progress of establishing the alternative method of separation through ordinary gaseous diffusion ( 345).