Introduction to Understandable Physics
Volume IV - Modern and Frontier Physics
by
Book Cover & Preview Text
33-1. Introduction
We have learned about the behavior of atomic electrons that orbit a positively charged nucleus, finding that quantum physics is needed to explain our observations. Atoms have diameters measured in units of 10-10 m or angstroms Å, but experiments show that the nucleus is much smaller, with diameters measured in units of 10-15 m or fm, which stands for femtometers or fermis. Thus, it should not be surprising that the very small nucleus also requires quantum physics to characterize its behavior, which depends on the attractive nuclear forces between neutrons and/or protons along with the repulsive electrical force of the protons. Similar to the Periodic Table for atoms, there is a Chart of the Nuclides that denotes quantum features of the nuclei of the isotopes. In particular, we will find that the neutrons and protons revolve about each other with quantized angular momenta that define a shell structure of the nucleus; thus, the nucleus is spherical and particularly stable when it has completely filled subshells, as was the case for the noble gas atoms. Furthermore, less stable nuclei with partial subshell fillings may be distorted into the shape of an ellipsoid such as a football. Such a distorted nucleus is somewhat analogous to an atomic molecule, as it can rotate and vibrate as a whole similarly to a drop of liquid. A liquid drop model of the nucleus actually preceded the shell model, and it is useful for understanding why some nuclei are stable while others decay into smaller components. For example the fission of uranium can be understood with this model. The features of both the liquid drop model and shell model have been successfully combined into a collective model of the nucleus.
The above structure of the nucleus is deduced from transitions from one state of the nucleus to another, analogously to how atomic structure is deduced from energy transitions that produce photons for spectral analysis. Nuclear transitions can involve decay processes as well as nuclear reactions. In nuclear decay, a nucleus can radiate energy as photons called gamma ({g}) rays, electrons called beta ({b}) rays, or alpha particles called alpha ({a}) rays. In addition, some excited nuclei can emit neutrons (n) or protons (p), while some of the heavier nuclei can undergo fission. In each case the transition energies associated with the radiated particles help define the nuclear energy states that are involved. In a nuclear reaction, a particle such as a neutron, proton, or alpha particle smashes into a nucleus to modify its energy state, similarly to how an accelerated electron of an X-ray tube smashes into an atom to remove an inner shell electron, causing emission of an X-ray. Nuclear reactions include transmutation of one nucleus to another, fission that splits the nucleus into two major pieces, and the fusion of two nuclei to form a larger nucleus. Measurements of the events associated with nuclear transitions require special nuclear detectors, which will be examined in the next chapter. For the present, we will simply acknowledge that {a}-, {b}-,{g}-, and other detectors do provide information on the energy of the particle being detected. Similar to the spectral analyses in studying atomic structure, the energy spectra of these nuclear transition particles are very useful in analyzing nuclear structure.
There are many important applications of nuclear physics in the modern world. Nuclear reactors have been producing electricity for about 50 years and now contribute 20% of the electrical power in the United States. Nuclear weapons continue to be an important component in the balance of power in the world, and ever-improving nuclear safeguards to curtail their proliferation will likely be a constant challenge for global stability. By contrast many peacetime applications of nuclear isotopes have improved the quality of life on the planet. For example nuclear medicine incorporates radionuclides to image features within the human body, such as blood circulation in the heart. In addition, other radionuclides are chemically selected to concentrate in specific areas of the body to destroy cancerous tissue, an example being thyroid treatments with radioactive iodine. Industrial uses of radioactive tracers in process control and monitoring are also widespread. An important aspect of all these applications is the health concerns for receiving prohibitively large doses of radiation. The implementation of suitable shielding against such radiation is one of the common techniques for limiting human exposure.
In this chapter we will explore the basic nature of nuclear physics, while delaying an examination of its methods and applications until the next two chapters. However, prior to this, the next section presents a historical overview of the evolution of nuclear physics, beginning with its birth near the end of the nineteenth century. Some of this historical material has already been introduced earlier, particularly in Chapters 12 and 29.
33-2. History of Nuclear Physics
Discovery of Radioactivity
It is interesting that the discovery of X-rays by William Roentgen in 1895 actually opened the door to the study of nuclear physics. Roentgen's discovery prompted French Physicist Henri Becquerel (1852 - 1908) to explore X-rays in terms of fluorescent radiation, but in 1896 he discovered that a uranium compound could expose a photographic plate without any interaction from light or X-rays. Suspecting a form of radiation, he exposed the compounds to a charged electroscope, which was discharged by the ionizing effect of the radiation. In 1898, Polish-French scientist Marie Curie (1867 - 1934) showed that a variety of uranium compounds behaved similarly, concluding that uranium was the probable cause of the radiation. In addition, she showed that thorium was also radioactive. She and her husband Pierre Curie (1859 - 1906) subsequently discovered radioactive elements of radium and polonium, which was named after her native country. Marie Curie is also credited with coining the word radioactivity.
Notations:
{a} = Greek letter alpha
{b} = Greek letter beta
{g} = Greek letter gamma
Formats
Book Details
About the Book
Will Winn has written Introduction to Understandable Physics in a building-block fashion. Accordingly, Volume IV - Modern and Frontier Physics builds on the classical physics of the earlier volumes. Volume IV begins by studying the birth of quantum physics and relativity early in the twentieth century. These concepts then apply to atomic physics, explaining the periodic table relative to quantized electron shells. Similarly, nuclear physics explores the nucleus relative to its collective shell model. Atomic and nuclear applications are examined in medicine, power production and research, along with familiar items such as smoke detectors, cell phones and bar-code scanners.
Frontier physics examines both extremely small and large structures. Protons, neutrons, and many other particles can be classified into families. Each particle comprises quarks, which define a “genetic” family. A deeper substructure of stringshas also been theorized but experimental confirmation is problematic. For very large structures, cosmology explores the evolution of the universe, noting that the Big-Bang projects that “the very small” and “the very large” were “one-and-the-same” in their early development. This sameness argues that the four basic forces of nature were originally indistinguishable! Our understanding of the expansion of the universe has been impacted by the discoveries of dark matter and dark energy, The expansion rate projects the ultimate destiny of the universe - a “big crunch” or continued expansion. Much is yet to be explored!
Near the end of each chapter a Simple Projects section suggests experiments and/or field trips that can reinforce the physics covered. Some experiments are simple enough for students to explore alone, while others benefit from equipment available to physics instructors. Also optional text sections provide students with a deeper appreciation of the subject matter; however these are not required for continuity. Some of these optional topics can be candidates for term projects.
About the Author
Will Winn has enjoyed a 50-yr physics career, which includes various experiences in teaching, pure research, and industrial applications. Early in his career he taught physics at both secondary institutions and universities, namely Cornell and Virginia Commonwealth, and in later years he taught again at University of South Carolina Aiken. Upon receiving his Ph.D. in nuclear physics at Cornell in 1968, he had commenced a post-doctoral appointment at the Washington University Cyclotron facility to conduct research on nuclear energy levels, and he continued this work with another post-doctoral appointment at the MP Van de Graaff accelerator at the University of Rochester.
Seeking to apply his background to practical applications, he completed a masters degree program in Nuclear Engineering at University of Virginia in 1974. Then he began his research at the Savannah River Site, which included reactor experiments, nuclear chemistry, non-destructive testing, environmental monitoring, and non-proliferation studies. In 2000 he retired from SRS to teach math and physics at USCA, after which he commenced writing this text. Here, he refined physics notes he prepared for his USCA students, as well as drawing from his career writing experience, which includes over 60 published papers.
Dr. Winn has encountered a wide range of activities related to physics over the years. He has conducted research with university professors, chemists, biologists, geologists, engineers, and other physicists. As a former premed student who switched to physics, he also has had a continuing interest in the physics involved in medical applications. Accordingly, he has a good appreciation for the role of physics in a considerable number of fields, and this has influenced his teaching of students who require an introductory physics course. Furthermore, he has applied this experience to writing Introduction to Understandable Physics.