Penn state american nuclear society atomic energy merit bagde program final report



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Figures in Radiation History

(Enrico Fermi)


Enrico Fermi's first significant accomplishment in nuclear physics was providing a mathematical means for describing the behavior of certain types of subatomic particles, a process concurrently developed by Paul Dirac and which came to be known as Fermi-Dirac statistical mechanics. His next major accomplishment was to successfully explain beta decay by incorporating into the process the production of a new particle which he named the neutrino. Despite the significance of his contributions to theoretical physics, Fermi is best known for his experimental work. When Frédéric and Irène Joliot-Curie first produced artificial radionuclides by bombarding aluminum with alpha particles, Fermi recognized that James Chadwick's recently discovered neutron offered a means to create radionuclides from targets of higher atomic number. In the course of doing so, Fermi noticed that greater activity was induced when the neutron bombardment was performed on a wooden table. He deduced that the neutrons became more effective because they slowed down after being scattered by the wood. He also recognized that neutron bombardment of uranium had the potential to produce a new class of elements, referred to as the transuranics. For his discovery of new radioactive elements and his work with slow neutrons, Fermi was awarded the 1938 Nobel Prize in physics. However, unknown to Fermi and the Nobel Prize Committee, the "new elements" Fermi characterized (with one exception) weren't new at all, they were fission products, i.e., radioisotopes of known elements produced by splitting uranium. Shortly after receiving his Nobel Prize, Fermi left Italy to join the faculty of Columbia University in the United States. Here he supervised a series of experiments that culminated in construction of the CP-1 Pile, the first controlled self-sustaining nuclear chain reaction. This momentous event took place in a squash court under the west stands of Stagg Field at the University of Chicago on December 2, 1942. Fermi thus became the first to control nuclear fission, the very process that in 1934 had led him to the false conclusion that he had discovered the transuranic elements!

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Figures in Radiation History

(Otto Hahn)

Otto Hahn was the chemist whose discovery of nuclear fission ultimately led to the ending of WW II. The story of Hahn's discovery began in 1938 with a report by Irène Joliot-Curie that bombarding uranium with neutrons had resulted in the production of a radionuclide of thorium, which they later speculated was a transuranium element similar to lanthanum. The astounded Hahn told Irène's husband, Frédéric, that such a thing was nonsense and that he would perform an experiment to prove as much. In the process of duplicating her work, Hahn and co-worker Fritz Strassmann discovered that, among other things, three isotopes of barium had been produced. This was incredible because the mass of barium is about half that of uranium. No known reaction could explain such a huge change. When they published their results (Jan. 6, 1939) Hahn and Strassmann noted that such a thing was "in opposition to all the phenomena observed up to the present in nuclear physics." Hahn, conscious of the fact that as a chemist he was treading in the domain of physics, did not offer an explanation. Instead, he left it up to Lise Meitner, his longtime collaborator, to whom he had sent a letter (December 19, 1938) describing his findings and asking "Perhaps you can suggest some fantastic explanation," which she explained as nuclear fission. Nevertheless, despite the contributions of Strassmann and Meitner, it was Hahn who was awarded the 1944 Nobel Prize in chemistry for the discovery. Unfortunately, Hahn was not at the awards ceremony to receive his prize. At the time he learned of the award, he was being held by the British who were seeking information from him about the failed German effort to develop an atomic bomb. As the Chairman of the Nobel Committee for Chemistry reported "Professor Hahn . . . has informed us that he is regrettably unable to attend this ceremony."

Thanks to the following group for allowing us to reprint this information:

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Tel: 703-790-1745


Fax: 703-790-2672

Figures in Radiation History

(Lise Meitner)

Lise Meitner, forever linked in people's minds with the monumental discovery of nuclear fission, made many significant contributions to science throughout a long and productive career. Upon receiving a doctorate in physics in 1906, Meitner went to the University of Berlin where she began her collaborations with Otto Hahn. The first significant result of this collaboration was an important technique for purifying radioactive material that took advantage of the recoil energy of atoms produced in alpha decay. Later, at the Kaiser Wilhelm Institute in Austria, she was the first to explain how conversion electrons were produced when gamma ray energy was used to eject orbital electrons. She also provided the first description of the origin of auger electrons, i.e., outer-shell orbital electrons ejected from the atom when they absorbed the energy released by other electrons falling to lower energies. When Nazi Germany annexed Austria in 1938, Meitner, a Jew, fled to Sweden. In her absence, Hahn and Fritz Strassmann continued experiments they had begun earlier with Meitner and demonstrated that barium was produced when a uranium nucleus was struck by neutrons. This was absolutely startling because barium is so much smaller than uranium! Hahn wrote to Meitner, "it [uranium] can't really break up into barium . . . try to think of some other possible explanation." While visiting her nephew Otto Frisch for the Christmas holidays in Denmark, she and Frisch proved that a splitting of the uranium atom was energetically feasible. They employed Niels Bohr's model of the nucleus to envision the neutron inducing oscillations in the uranium nucleus. Occasionally the oscillating nucleus would stretch out into the shape of a dumbbell. Sometimes, the repulsive forces between the protons in the two bulbous ends would cause the narrow waist joining them to pinch off and leave two nuclei where before there had been one. Meitner and Frisch described the process in a landmark letter to the journal Nature with a term borrowed from biology: fission.


Thanks to the following group for allowing us to reprint this information:

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1313 Dolley Madison Blvd., Suite 402
Mclean, Virginia 22101

Tel: 703-790-1745


Fax: 703-790-2672



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