Atomic Bomb

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.. s. On January 1, 1896, his first paper on this subject was published. Many found it unbelievable; photographs of bones inside of hands and bullets that were lodged within bodies provided the proof. The implications for medicine intrude the mind almost unbiddingly. Rontgen, receiving the first Nobel Prize for Physics in 1901, was unable to determine what caused this new phenomenon.

Nature did not want to give up its secret of radiation yet. Hernri Bacquerel (1852 - 1908) came one step closer to understanding this phenomenon. His experiments consisted of taking a photographic plate and wrapping it in thick sheets of black paper. He placed a phosphorescent substance, made form a compound of uranium salt, on the plate and left it in the sun for several hours. When he developed the photograph, it had a the image of the substance that produced the phosphorescence. After performing the experiment several times, he concluded that the phosphorescent substance is able to emit radiation that penetrates the paper. The weather became overcast and he stored the materials, including a blank photographic plate, in a drawer

Several days passed and he developed the photograph. He discovered that the same image was on the photograph. He deduced immediately that the process is independent of the substance actually florescing. It was not the fluorescence causing the images on the photographs, but something else entirely. Marie Sklodowska Curie (1867 - 1934), about two years after Becquerel's discovery, in 1897, was ready to do her doctoral thesis.

She sought her husband's -- Pierre -- advice. He suggested that she undertake a study of the 'new phenomenon.' She developed ways to measure the phenomenon with greater precision. She was the first to use the term radioactive for this phenomenon. They discovered several new radioactive elements. They also developed ways to extract the radioactive substances from the samples she used.

She and her husband worked in this field for the rest of their lives. VII. Structure of the Atom Shifting the story again brings us to Ernest Rutherford (1871 - 1937) whose experiments in 1898 lead to the conclusion that there were two types of radiation emissions. He called them alpha rays and beta rays. In a few years they discovered that beta rays were electrons moving at high speeds. Also, P. V. Villard, in France, discovered gamma rays, which is a 'much further penetrating x-ray.' Rutherford, in 1903 and 1904, through continued experimentation felt that alpha rays are helium nuclei being expelled from the nucleus of these radioactive materials.

His observations lead him to counting individual alpha particles. Hans Geiger (1882 - 1945) worked with Rutherford and together were able to determine several important universal constants. The experiments also helped to confirm that matter is discrete, and not continuously distributed. Rutherford studied the passage of alpha particles through other objects. Several students helped Rutherford. Ernest Mardsden (1889 - 1970) around 1904 witnessed that occasionally alpha particles were deflected when traveling through a thin metal foil.

When Marsden related this observation to Rutherford, he desired to see the experiment himself. (Segre, 'From X-rays..', 20-57) The following excerpt explains Rutherford's findings: 'The big deflections had greatly amazed Rutherford. He later said that it was as if someone had told him that having fired a pistol at a sheet of paper, the bullet had bounced back! 'Several weeks passed. Then one day in 1911 Rutherford announced that now he knew why Marsden's particles were deflected at wide angles. And, moreover, he knew the structure of the atom.' (Segre, 'From X-rays..', 104) At this time, several models of the atom were already hypothesized. Rutherford's experiments had provided solid scientific evidence that the idea of the atom being like a small planetary system was essentially correct.

Rutherford hypothesized that the nucleus contained the positive charges. These charges were concentrated in a comparatively small volume of space. This nucleus was circled by a similar number of negative charges. (He knew there were problems with this theory, but he used this theory in the same way that Newton was willing to use action-at-a-distance. It was close enough to make useful calculations.) The alpha particles that shot into the foil and bounced back were deflected by the nucleus. This deflection was the result of the mutual repulsion two protons have for each other.

It is governed by the mathematical description of Coulomb's law. Without field theory, Rutherford would have had to figure out how two very small protons are able to feel each other's presence inside an atom. But with field theory, he did not need to concern himself with it too much. Rutherford's next problem dealt with finding the neutron. The neutron had been hypothesized from the fact that helium has a weight of four protons but an electrical charge of only two.

The question of the extra weight was perplexing. The idea of a neutral particle, with the properties that are associated with what is now known as the neutron, was first proposed by Rutherford in 1920. James Chadwick (1891 - 1974) and Rutherford performed a search for this theoretical particle, but were unable to prove its existence. Shortly, we will see what had to happen first to make the discovery of the neutron possible. Thus, the atom could be shown to exist. Shortly after Rutherford's evidence that the atom is like planetary system, but on a very small scale, was made known, many people commenced work in this new field which later became known as nuclear physics.

Some, such as Rutherford and the Curies, made this topic their lifes' work. The experiments lead to quantum mechanics, which was also worked on steadily through this time period. It is still pursued today, but unfortunately, we will not look at quantum mechanics in this paper. VIII. Fission Frederic Joliot (1900 - 1958) and Irene Curie (1897 - 1956), his wife, were performing experiments in 1931 with polonium, which had been discovered by her mother, Marie Curie. Their experiments produced very strange results; literal transmutations of elements were occurring at the atomic level for which they could not account. They published these results on January 18, 1932.

When Chadwick saw the report he repeated the experiments, using additional elements, and proved that the radiation contained a neutral particle whose mass was approximate to that of a proton. He called it a neutron in a report sent to Nature on February 17, 1932. Continuing his work found that slow moving neutrons were more apt at producing these transmutations than protons. When he received the Nobel Prize in 1935, he discoursed on the usefulness of the neutron as a catalyst to fission. A small excerpt from his lecture follows. 'The great effectiveness of the neutron in producing nuclear transmutations is not difficult to explain. In the collisions of a charged particle with a nucleus, the chance of entry is limited by the Coulomb forces between the particle and the nucleus; these impose a minimum distance of approach which increases with the atomic number of the nucleus and soon becomes so large that the chance of the particle entering the nucleus is very small.

In the case of collisions of a neutron with the nucleus there is no limitation of this kind. The force between a neutron and a nucleus is inappreciable except at very small distances, when it increases very rapidly and is attractive. Instead of the potential wall in the case of the charged particle, the neutron encounters a potential hole. Thus even neutrons of very small energy can penetrate into the nucleus. Indeed slow neutrons may be enormously more effective than fast neutrons, for they spend a longer time in the nucleus.' (Weaver, 733) As stated in the quote, slow moving neutrons have a greater incidence of affecting the nuclei of the material than fast moving neutrons. By bombarding of the elements, and determining the reactions that took place, physicists found the neutron to proton ratio of a wide range of these elements.

They also found that the neutron to proton ratio increased as the number of protons in the nucleus increased. The element with the most protons known at the time was uranium. It has 92 protons and 146 neutrons. (It is usually known as uranium-238.) Bombarding uranium yielded the most spectacular results yet. The uranium atom was actually split into two atoms of approximately the same size -- and fission was accomplished. This released significant amounts of energy. It was found that an isotope of uranium, uranium-235, easily fissioned with slow neutrons to yield krypton and barium.

(Taylor, 353) Unfortunately, uranium-235 is found in naturally occurring uranium only about 1 part in 137. Extracting it is not an easy process. (Segre, 'From X-rays..', 210) This provides the last piece of information needed to deduce the possibility of an atomic bomb. IX. Sustained Reactions - The Atomic Bomb In 1940, Otto Frisch and Rudolph Peierls posed an important question. From 'Nuclear Fear', we may read this question. 'Exactly what would happen, they asked themselves, if you could cull from natural uranium a mass composed purely of the rare uranium-235? Bohr and others had told the public that there could be enough energy there to blow up a city, but nobody had worked it out as a serious technical possibility. Now Frisch and Peierls realized that with fissionable uranium-235 atoms all crammed together, there would be no need for a moderator to slow the neutrons down, since even the fast neutrons emitted in each fission would have a good chance to provoke another fission.

The whole chain reaction would go so swiftly that, before the mass had a chance to blow itself apart, a run away [reaction would allow] many of the uranium-235 atoms [to] split and release energy.' (Weart, 84) This question may have been left academic for years had it not been for World War II. As the awesome power of an atomic bomb was realized by leaders of several countries, a race began to be the first to make a working bomb. As a result, a simpler method was discovered than separating uranium-235 from uranium-238. This simpler method starts when uranium-238 absorbs a single neutron a new element, called neptunium-239, is created. (Neptunium-239 has 93 protons and 146 neutrons.) This element decays into plutonium-239 (94 protons and 145 neutrons). Plutonium is stable and also has the property of undergoing fission with slow neutrons.

Hence, the atom bomb was conceivable. Plutonium was produced in a reactor. (Weart, 87) The United States was one of the nations was one of the countries searching for the technology to make the atomic bomb a reality. On July 16, 1945, they succeeded when the first atomic bomb was detonated. 'In an isolated spot named Alamogordo, moments before first light.., night exploded noiselessly into day. Searing colors - gold, purple, blue, violet, gray - illuminated everything in sight.

From the floor of the desert, a ball of fire rose like the sun (only brighter, one report read, 'equal to several suns in midday'). .. Thirty seconds later came a blast of burning air, followed almost instantaneously by an awesome roar. A cloud the shape of an immense mushroom ascended nearly eight miles, was caught by the desert winds, and curled into a giant question mark.' (Stoff, 1) This was the realization of a long trek through history. Thought and experiment combined with field theory, a knowledge of chemical properties of the elements, and the discovery of radioactivity. This gave people the ability to answer the question: What is the structure of the atom? Not only was the structure determined, but it was found that the number of protons and neutrons could change. (Protons and neutrons together are known as nucleons -- particles that inhabit the nucleus.) Changing the number of nucleons has several names: radioactivity, fission and fusion according to how the atom is changing and what is causing the change.

Generally energy is released as a result of this change. Using Einstein's bold statement that E=mc^2, the nature of this energy became known. The energy is a direct conversion from part of the mass of the atom. As we saw, it was a short technological step to use the same source of energy for the sun, as a source of energy on the earth. -------------------------------------------------- ------------------------------Endnotes Taylor, John R.; Zafiratos, Chris D.; 'Modern Physics for Scientists and Engineers', (Engelwood Cliffs, New Jersey: Prentice Hall, 1991) Sachs, Mendel, 'Einstein Versus Bohr', (La Salle, Illinois: Open Court, 1988) Segre, Emilio, 'From Falling Bodies to Radio Waves', (New York: W. H.

Freeman and Company, 1984) Segre, Emilio, 'From X-rays to Quarks', (New York: W. H. Freeman and Company, 1980) Stoff, Michael B.; Fanton, Jonathan F.; Williams, R.; 'The Manhattan Project: A Documentary Introduction to the Atomic Age', (Philadelphia, PA: Temple University Press, 1991) Weart, Spencer R., 'Nuclear Fear', (Cambridge, Mass.: Harvard Press, 1988) Weaver, Jefferson Hane, 'The World of Physics', Vol 1 (New York: Simon and Schuster, 1987) -------------------------------------------------- ------------------------------Bibliography Asimov, Issac, 'The History of Physics', (New York: Walker and Company, 1983) Taylor, John R.; Zafiratos, Chris D.; 'Modern Physics for Scientists and Engineers', (Engelwood Cliffs, New Jersey: Prentice Hall, 1991) Sachs, Mendel, 'Einstein Versus Bohr', (La Salle, Illinois: Open Court, 1988) Segre, Emilio, 'From Falling Bodies to Radio Waves', (New York: W. H. Freeman and Company, 1984) Segre, Emilio, 'From X-rays to Quarks', (New York: W.

H. Freeman and Company, 1980) Stoff, Michael B.; Fanton, Jonathan F.; Willans, R.; 'The Manhattan Project: A Documentary Introduction to the Atomic Age', (Philadelphia, PA: Temple University Press, 1991) Weart, Spencer R., 'Nuclear Fear', (Cambridge, Mass.: Harvard Press, 1988) Weaver, Jefferson Hane, 'The World of Physics', Vol 1 (New York: Simon and Schuster, 1987).

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