An Atomic History Chapter 1

An Atomic History 0-3 8/11/02 7:30 AM Page 6 Chapter One 7 Between 1898 and 1911, this work was continued by Ernest Rutherford, who studied the nature of the radiation emitted by uranium and thorium. Rutherford was the first to discover and name alpha and beta radiation, and link them with Thompson’s electrons. Rutherford also discovered that radioactive elements, whether they were uranium, thorium, or radium, would all spontaneously disintegrate by emitting alpha and beta particles. The 1 Nuclear Awakenings longevity of these elements was determined in "half-lives."6 Albert Einstein not only provided more pieces of the puzzle; he put the puzzle in a new frame. In 1905, while working in the Swiss Patent Office, Einstein prepared five papers on the nature of modern physics, any one of which would have secured his fame. One of the five, and the one for which he later received a Nobel Prize, dealt with the "pho- toelectric effect." In it, Einstein theorized that light is made of discrete packets or "quan- ta," and that the energy of each packet is determined by the wavelength of the light, not its intensity. Two of the five papers dealt with new evidence for the existence and size of atoms and molecules. Another two expounded a radical new theory on the relationship of The work done at the Savannah River Site is the culmination of over a hundred time and space: one dealt with the theory of relativity, while the other posited that mass years of nuclear research. Modern physics, the study of the properties, changes, and inter- has energy—expressed as the equation "E=mc2."7 This equation became one of the hall- actions of matter and energy, is basically the study of the atom and its components. It marks of modern physics. It stressed the interchangeability of matter and energy, and sug- began with the discovery of radiation in 1895, even though it was not then known that gested the huge amount of energy that could come from a small amount of matter.8 It also radiation came from the atomic decomposition of radioactive elements. Slowly at first, paved the way for an understanding of gamma rays, the third and most penetrating type of and then with increasing speed, physicists unlocked the secrets of atomic structure, the radiation emitted by radioactive elements. Albert Einstein, 1922. Burndy awesome power of fission, and finally, thermonuclear reactions. From the smallest com- By the time Einstein published his theory of relativity, the notion of atoms as the Library, courtesy of AIP Emilio Segré Visual Archives. ponents of the natural world came what could be its biggest threat and perhaps its greatest building blocks of all elements was coming into acceptance. Before then, the idea of the opportunity. This chapter will show how these momentous events occurred, beginning atom, which went back to the ancient Greeks, was useful only as a concept. Prior to the with the discovery of x-rays at the end of the nineteenth century. 1900s, no one actually knew anything about atoms, or if they even existed. Although John Dalton had hypothesized their existence in the early 1800s, at the end of that century, even as respected a physicist as Max Planck could publicly doubt the existence of atoms as EARLY DEVELOPMENT OF NUCLEAR PHYSICS actual particles.9 By the early 1900s, this was no longer the case. Not only were atoms thought to exist, but Rutherford’s increasingly sophisticated experiments also indicated Wilhelm Roentgen discovered x-rays in 1895 as a mysterious emanation from the flu- that the area of each atom was not solid, but mostly empty space. In 1911, Rutherford orescing glass wall of a cathode ray tube.1 Even though the source and the nature of the showed by bouncing alpha particles off atoms at large angles, that the bulk of an atom was new rays were unknown, they found almost immediate medical application as a means of contained in a central nucleus that had a positive charge.10 This experimentation identified seeing into the human body.2 The discovery of x-rays led to the testing of various natural- the basic electrical structure of the atom, with negatively charged electrons orbiting a posi- ly fluorescing materials to see which, if any, produced the mysterious rays. Henri tively charged nucleus. Becquerel soon discovered that uranium salts also produced these rays, and Marie and This scheme, which can be visualized as a small solar system with the nucleus at the Pierre Curie continued his work. Marie Curie named the mysterious force "radioactivity."3 center surrounded by orbiting electrons, was only a useful way to describe what was really The Curies devoted their lives to the study of radioactive elements, or unstable elements unknown in 1911. The bond that tied an electron to the nucleus was recognized to be that emitted radiation. In addition to the previously known uranium and thorium, the much greater than a gravitation field. The nature of the force that bonded the components Curies discovered radium and polonium. By the early 1900s, their studies led to the iden- of an atom remained a mystery until Danish physicist Niels Bohr wrote On the tification of some 30 radioactive isotopes.4 Constitution of Atoms and Molecules in 1913. According to Bohr, the forces that made for Marie Curie, 1906. Courtesy of AIP While the Curies studied radioactivity, English physicist J. J. Thompson discovered a stable atom could not be explained by the laws of classical physics, but only by the Emilio Segré Visual Archives. the first component of the atom, the electron, in 1897. This new particle was identified in quantum principles pioneered by Planck and Einstein.11 Ernest Rutherford, circa 1920. the course of Thompson’s work with cathode rays, which Thompson showed were actually In 1919, after the First World War, Rutherford, working at the Cavendish Laboratory Nature, courtesy of AIP Emilio Segré Visual Archives. small negatively charged particles he called "corpuscles." The new particles were lighter at Cambridge, became the first person to artificially transmute an element. Using alpha than atoms of hydrogen, the lightest of the elements. For this reason, Thompson guessed particles, he bombarded atoms of nitrogen to create hydrogen atoms.12 As a result of this that they must be a part of an atom that was somehow ejected from the whole.5 work, he also discovered, within the nucleus, a high-energy positively charged particle he called a "proton."13 This discovery completed the electrical make-up of the atom, with a An Atomic History 0-3 8/11/02 7:30 AM Page 6 Chapter One 7 Between 1898 and 1911, this work was continued by Ernest Rutherford, who studied the nature of the radiation emitted by uranium and thorium. Rutherford was the first to discover and name alpha and beta radiation, and link them with Thompson’s electrons. Rutherford also discovered that radioactive elements, whether they were uranium, thorium, or radium, would all spontaneously disintegrate by emitting alpha and beta particles. The 1 Nuclear Awakenings longevity of these elements was determined in "half-lives."6 Albert Einstein not only provided more pieces of the puzzle; he put the puzzle in a new frame. In 1905, while working in the Swiss Patent Office, Einstein prepared five papers on the nature of modern physics, any one of which would have secured his fame. One of the five, and the one for which he later received a Nobel Prize, dealt with the "pho- toelectric effect." In it, Einstein theorized that light is made of discrete packets or "quan- ta," and that the energy of each packet is determined by the wavelength of the light, not its intensity. Two of the five papers dealt with new evidence for the existence and size of atoms and molecules. Another two expounded a radical new theory on the relationship of The work done at the Savannah River Site is the culmination of over a hundred time and space: one dealt with the theory of relativity, while the other posited that mass years of nuclear research. Modern physics, the study of the properties, changes, and inter- has energy—expressed as the equation "E=mc2."7 This equation became one of the hall- actions of matter and energy, is basically the study of the atom and its components. It marks of modern physics. It stressed the interchangeability of matter and energy, and sug- began with the discovery of radiation in 1895, even though it was not then known that gested the huge amount of energy that could come from a small amount of matter.8 It also radiation came from the atomic decomposition of radioactive elements. Slowly at first, paved the way for an understanding of gamma rays, the third and most penetrating type of and then with increasing speed, physicists unlocked the secrets of atomic structure, the radiation emitted by radioactive elements. Albert Einstein, 1922. Burndy awesome power of fission, and finally, thermonuclear reactions. From the smallest com- By the time Einstein published his theory of relativity, the notion of atoms as the Library, courtesy of AIP Emilio Segré Visual Archives. ponents of the natural world came what could be its biggest threat and perhaps its greatest building blocks of all elements was coming into acceptance. Before then, the idea of the opportunity. This chapter will show how these momentous events occurred, beginning atom, which went back to the ancient Greeks, was useful only as a concept. Prior to the with the discovery of x-rays at the end of the nineteenth century. 1900s, no one actually knew anything about atoms, or if they even existed.

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