Nuclear Physics (1986) / Chapter Skim
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1 Introduction to Nuclear Physics
1 Introduction to Nuclear Physics
Pages 9-36
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From page 9... ...
Studies of the nucleus can thus be viewed as a link between the worlds of the infinitesimal and the astronomical. Collectively, the various nuclei can be regarded as a laboratory for investigating the fundamental forces that have governed our universe since its origin in the big bang.
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From page 10... ...
Protons and neutrons are collectively called nucleons, and for many years it was thought that nucleons were truly elementary particles. We now know, however, that they are not elementary but have an internal structure consisting of smaller parti
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From page 11... ...
This defines its chemical identity, because the proton number (equal to the number of unit electric charges in the nucleus) is balanced, in a neutral atom, by the electron number, and the chemical properties of any element depend exclusively on its orbital electrons.
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From page 12... ...
Their aim is not only to explain all the known facts of nuclear physics but to predict new ones whose experimental verification will confirm the correctness of the theory and extend the bounds of its applicability. A similar approach applies to the study of nuclear reactions, in which experimentalists and theorists seek to understand the changing nature and mechanisms of collisions between projectile and target nuclei at the ever-increasing energies provided by modern accelerators.
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From page 13... ...
Doing so brings out the other half of the essential challenge of nuclear physics: that nucleons are strongly interacting particles. THE FUNDAMENTAL FORCES In nature, the so-called strong force holds atomic nuclei together despite the very substantial electrostatic repulsion between all the
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From page 14... ...
Lying between gravitation and the strong force, but much closer to the latter in inherent strength, is the electroweak force. This rather complex force manifests itself in two ways that are so different that until the late 1960s they were believed to be separate fundamental forces just as electricity and magnetism, a century ago, were thought to be separate forces rather than two aspects of the one force, electromagnetism.
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From page 15... ...
This profound idea revolutionary in its time but now commonplace lies at the heart of quantum mechanics, the physical theory that underlies all phenomena at the submicroscopic level of molecules, atoms, nuclei, and elementary particles. The other manifestation of the electroweak force is the weak force, which is responsible for the decay of many radioactive nuclides and of many unstable particles, as well as for all interactions involving the particles called neutrinos, which we discuss below.
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From page 16... ...
To understand the atomic nucleus properly, therefore, we must take into account all the other particles that exist there under various conditions, as well as the compositions of the nucleons and of these other particles. Second, the theoretical framework for much of nuclear physics is now deeply rooted in the quantum field theories of the fundamental interactions, which are the domain of particle physics.
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From page 17... ...
Physicists now believe that there are three classes of elementary particles leptons, quarks, and elementary vector bosons and that every particle, elementary or not, has a corresponding antiparticle. Here we must make a short digression into the subject of antimatter.
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From page 18... ...
Neutrinos and antineutrinos are commonly produced in the radioactive process called beta decay (a weak-interaction process)
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From page 19... ...
Another odd property of quarks is that they have fractional electric charge; unlike all other charged particles, which have an integral value of charge, quarks have a charge of either -1/3 or + 2/3. Because free quarks have never been observed, these fractional charges have never been observed either-only inferred.
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From page 20... ...
Gluons belong to the third class of elementary particles, the elementary vector bosons, which we will examine shortly, after a brief introduction to the concept of spin. In addition to their mass and charge, all subatomic particles (including nuclei themselves)
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From page 21... ...
Most of the bosons to be discussed in the next section are elementary particles unlike mesons and are called vector bosons (because they have spin 11. Elementary Vector Bosons Earlier it was mentioned that the fundamental interactions are mediated by the exchange of certain particles between the interacting particles.
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From page 22... ...
22 NUCLEAR PHYSICS ~ ~ , >~ ___, l _~ ~ ~ _ i: ~ ,,' I' i/ FIGURE 1.2 The way in which force is transmitted from one particle to another can be visualized (crudely) through the example of two roller skaters playing different games of catch as they pass each other.
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From page 23... ...
The range of the strong force very short, yet much longer than that of the weak force is explained by the mesons' moderate masses, which are typically less than that of a nucleon and very much less than that of an intermediate vector boson. What is most significant for nuclear physics is that the nucleons interact via the exchange of virtual mesons, so the nucleus is believed always to contain swarms of these particles among its nucleons.
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From page 24... ...
The experimental and theoretical challenges posed by this goal are enormous, but so are the potential rewards in terms of our understanding of the nature of nuclear matter. CONSERVATION LAWS AND SYMMETRIES The total amounts of certain quantities in the universe, such as electric charge, appear to be immutable.
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From page 25... ...
For example, the fundamental symmetry of space and time with respect to the linear motions and rotations of objects leads directly to the laws of the conservation of linear and angular momentum. Similarly, the mathematical foundations of the quantum field theories imply certain symmetries of nature that are manifest as various conservation laws in the subatomic domain.
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From page 26... ...
Equally impotent are dynamical symmetries Lund in the physical laws governing aN natural phenomena. (By permission of the Escher Foundation, Hags Cemeentemuseum The ague.
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From page 27... ...
The efforts of theoretical physicists to construct Grand Unified Theories of the fundamental interactions, in which these interactions are seen merely as different manifestations of a single unifying force of nature, depend strongly on experimental observations pertaining to symmetries, conservation laws, and their violations. A most important observation in this regard would be any evidence of a violation of the conservation of baryon number, which may not be a universal law after all.
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From page 28... ...
Equally indispensable are the sophisticated detectors that record and measure the many kinds of particles and the gamma rays emerging from the nuclear collisions produced by the accelerator beams.' There are several different kinds of accelerators, differing mainly in the ways in which they provide energy to the particles, in the energy ranges that they can span, and in the trajectories followed by the accelerated particles. The most common kinds are Van de Graaff electrostatic accelerators, linear accelerators, cyclotrons, and synchrotrons; an example of a modern cyclotron is shown in Figure 1.4.
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From page 29... ...
(Courtesy of the Indiana University Cyclotron Facility.)
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From page 30... ...
The objective may be to use the projectiles to raise nuclei in the target substance from their lowest-energy ground state to higherenergy excited states in order to gain insight into the structures and dynamics of intact nuclei; in this way one studies nuclear spectroscopy. Alternatively, the objective may be to bombard the target nuclei in such a way that they undergo a nuclear reaction of some kind, possibly disintegrating in the process.
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From page 31... ...
For a collision involving the effects of the strong force to occur, the projectile must be energetic enough to overcome the Coulomb barrier and approach the target closely. Between about 10 and 100 MeV per nucleon is the medium-energy regime, where many studies of nuclear spectroscopy and nuclear reactions are carried out; these are the energies characteristic of the motions of nucleons within a nucleus.
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From page 32... ...
Nuclear Interactions The principal kinds of nuclear interactions in collisions are scattering, in which the projectile and target nuclei are unchanged except for their energy states; transfer, in which nucleons pass from one nucleus to the other; fusion, in which the two nuclei coalesce to form a compound nucleus; spallation, in which nucleons or nucleon clusters are knocked out of the target nucleus; and disintegration, in which one or both nuclei are essentially completely torn apart. Not all interactions that occur in collisions are equally probable, so it is important to know what does occur to an appreciable extent and what does not-and why.
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From page 33... ...
This is the "big-science" approach to nuclear-physics research. A highly noteworthy feature of nuclear physics, however, is that much research of outstanding value is still done by individuals or small groups working with more modest but nonetheless state-of-the-art facilities in many universities and laboratories throughout the world.
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From page 35... ...
I Major Advances In Nuclear Physics
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