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1 Introduction to Nuclear Physics All phenomena in the universe are believed to arise from the actions of just three fundamental forces: gravitation and the less familiar strong force and electroweak force. The complex interplay between these last two forces defines the structure of matter, and nowhere are the myriad manifestations of this interplay more evident than in the nucleus of the atom. Much of the substance of the universe exists in the form of atomic nuclei arranged in different ways. Within ordinary nuclei, the weak gravitational attraction between the constituent particles is overwhelmed by the incomparably more powerful strong nuclear force, but gravitation's effect is large indeed in neutron stars- bizarre astrophysical objects whose properties are very much like those of gigantic nuclei. 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. Indeed, as this report illustrates, the study of nuclear physics is becoming ever more deeply connected with that of cosmol- ogy as well as elementary-particle physics. Before venturing into these exciting realms, we will quickly survey the field of nuclear physics at an elementary level in order to learn the language. Although nuclear physics has the reputation of being a difficult subject, the basic concepts are relatively few and simple. 9
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10 NUCLEAR PHYSICS 10-2 m 10-14 m ~ Raspberry _ - - - ~ Nucleus - i Quark - _ _ _, - _- _ __ _ _ · ~ ,~ Proms _. ~ , - __ Nuc: - _- ~. ~ - _ -_ -10 m -15 m FIGURE l.l Approximate dimensions for the structure of matter from raspberries to quarks (the cellular and molecular levels of structure have been omitted). THE ATOMIC NUCLEUS The atomic nucleus is an extremely dense, roughly spherical object consisting primarily of protons and neutrons packed fairly closely together (see Figure 1.1~. 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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INTRODUCTION TO NUCLEAR PHYSICS 11 cles and that there are other particles in the atomic nucleus along with them. These aspects of the nucleus are discussed below. Protons and neutrons are very similar, having almost identical physical properties. An important difference, however, lies in their electric charge: protons have a unit positive charge, and neutrons have no charge. They are otherwise so similar that their interconversion in the decay of radio- active nuclei is a common occurrence. The character of the nucleus provides the diversity of the chemical elements, of which 109 are now known, including a number of man-made ones. (The cosmic origin of the elements is a different question-one that is addressed by the specialized field of nuclear astrophysics.) Each element has a unique proton number, Z. 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. The smallest and lightest atom, hydrogen, has one proton and therefore one electron; the largest and heaviest naturally occurring atom, uranium, has 92 protons and 92 electrons. In a rough sense, this is all there is to the diversity of the chemical elements and the fantastic variety of forms inanimate and animate that they give rise to through the interactions of their electron clouds. To explain the stability of the elements, however, and to study nuclear physics, we must also take into account the neutron number, N. of each nucleus. This number can vary considerably for the nuclei of a given element. The nucleus of ordinary hydrogen, for example, has one proton and no neutrons, the latter fact making it unique among all nuclei. But a hydrogen nucleus can also exist in a form that has one proton and one neutron (Z = 1, N = 1~; this nucleus is called a deuteron, and the atom, with its one electron, is called deuterium. Chemically, however, it is still hydrogen, as is the even heavier, radioactive form tritium, which has one proton and two neutrons (Z = 1, N = 21; a tritium nucleus is called a triton. These separate nuclei of a single chemical element, differing only in neutron number, are the isotopes of that element. Every element has at least several isotopes stable and unstable (radioactiveWand some of the heavier elements have already been shown to have more than 35. Although the chemical properties of the isotopes of a given element are the same, their nuclear properties can be so different that it is important to identify every known or possible isotope of the element unambigu- ously. The simplest way is to use the name of the element and its mass number, A, which is just the sum of its proton and neutron numbers:
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12 NUCLEAR PHYSICS A = Z + N. Because different combinations of Z and N can give the same value of A, nuclei of different elements can have the same mass number (chlorine-37 and argon-37, for example). To emphasize the uniqueness of every such separately identifiable type of nucleus, scientists refer to them as nuclides. There are about 300 naturally occurring stable nuclides of the chemical elements and about 2400 radioactive (i.e., spontaneously decaying) ones. Of the latter, the great majority do not exist naturally but have been made artificially in particle accelerators or nuclear reactors. These machines of modern physics can also create experi- mental conditions that are drastically unlike those ordinarily existing on Earth but that are similar, perhaps, to those characteristic of less hospitable corners of the universe. Thus they enable us, in our efforts to understand the laws of nature, to extend our intellectual grasp into domains that would otherwise be inaccessible. Experimental and theoretical investigations of the broad range of nuclides available to us represent the scope of nuclear physics. In the study of nuclear spectroscopy, for example, experimentalists perform many kinds of measurements in order to characterize the behavior of the nuclides in detail and to find patterns and symmetries that will allow the huge amounts of information to be ordered and interpreted in terms of unifying principles. The theorists, on the other hand, search for these unifying principles through calculations based on the available facts and the fundamental laws of nature. 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 accelera- tors. The many ways in which target nuclei can respond to the perturbations produced by energetic projectile beams provide a rich fund of experimental data from which new insights into nuclear structure and the laws of nature can be gained. In extreme cases, new states of nuclear matter may be found. THE NUCLEAR MANY-BODY PROBLEM The essential challenge of nuclear physics is to explain the nucleus as a many-body system of strongly interacting particles. In physics, three or more mutually interacting objects whether nucleons or stars are
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INTRODUCTION TO NUCLEAR PHYSICS 13 considered to be "many" because of the tremendous mathematical difficulties associated with solving the equations that describe their motions. With each object affecting the motions of all the others through the interactions that exist among them, and with all the motions and hence all the interactions changing constantly, the prob- lem very quickly assumes staggering proportions. In fact, this many- body problem is now just barely soluble, with the largest computers, for three bodies. For four or more, however, it remains generally insoluble, in practice, except by methods relying on various approxi- mations that simplify the mathematics. What nuclear physicists try to do-within the constraints imposed by the many-body problem is to understand the structure of nuclei in terms of their constituent particles, the dynamics of nuclei in terms of the motions of these particles, and the fundamental interactions among particles that govern these motions. Experimentally, they study these concepts through nuclear spectroscopy and the analysis of nuclear reactions of many kinds. Theoretically, they construct simplifying mathematical models to make the many-body problem tractable. These nuclear models are of different kinds. Independent-particle models allow the motion of a single nucleon to be examined in terms of a steady, average force field produced by all the other nucleons. The best-known independent-particle model is the shell model, so called because it entails the construction of "shells" of nucleons analogous to those of the electrons in the theory of atomic structure. At the other extreme, collective models view the nucleons in a nucleus as moving in concert (collectively) in ways that may be simple or complex- just as the molecules in a flowing liquid may move smoothly or turbulently. In fact, the best-known collective model, the liquid-drop model, is based on analogies with the behavior of an ordinary drop of liquid. The above descriptions are necessarily oversimplified. The actual models in question, as well as related ones, are very sophisticated, and their success in explaining most of what we know about nuclear structure and dynamics is remarkable. As we try to push this knowl- edge to ever deeper levels, however, we must take increasingly detailed account of specific nucleon-nucleon interactions. 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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14 NUCLEAR PHYSICS positively charged protons. The distance over which the strong force is exerted, however, is extremely short: about 10- ~5 meter, or 1 femtometer-commonly called 1 fermi (fm) after the great nuclear physicist Enrico Fermi. A fermi is short indeed, being roughly the diameter of a single nucleon. The time required for light to traverse this incredibly short distance is itself infinitesimal: only 3 x 10-24 second. As we will see, the characteristic duration of many events taking place in the nucleus is not much longer than that: about 10-23 to 10-22 second, corresponding to a distance traveled, at the speed of light, of only about 3 to 30 fm. This is the domain- incomprehensibly remote from our everyday experience-of the strong force, which dominates the nucleus. Nucle- ons within the nucleus are strongly attracted to one another by the strong force as they move about within the confines of the nuclear volume. If they try to approach each other too closely, however, the strong force suddenly becomes repulsive and prevents this from happening. It is as though each nucleon had an impenetrable shield around it, preventing direct contact with another nucleon. The behav- ior of the strong force is thus very complex, and this makes the analysis of multiple nucleon-nucleon interactions (the nuclear many-body prob- lem) much more challenging. At the opposite extreme of the fundamental forces is gravitation, a long-range force whose inherent strength is only about 10-38 times that of the strong force. Since the gravitational force between any two objects depends on their masses, and since the mass of a nucleon is extremely small (about 10-24 g), the erects of gravitation in atomic nuclei are not even close to being measurable. Nonetheless, the universe as a whole contains so many atoms, in the form of hugely massive objects (stars, quasars, galaxies), that gravitation is the dominant force in its structure and evolution. And because gravitation is extremely important in neutron stars, as mentioned earlier, these supermassive nuclei are all the more interesting to nuclear astrophys- icists. 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. Now we know that electromagnetism itself is but a part of the electroweak force; it is therefore no longer considered to be a separate fundamental force of nature.
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INTRODUCTION TO NUCLEAR PHYSICS 15 Electromagnetism is the force that exists between any two electri- cally charged or magnetized objects. Like gravitation, its influence can extend over great distances, and it decreases rapidly in strength as the distance between the objects increases. Its inherent strength is rela- tively large, however, being about 0.7 percent of that of the strong force at separation distances of about 1 fm. Electromagnetism is the basis of light and all similar forms of radiation (x rays, ultraviolet and infrared radiation, and radio waves, for example). All such radiation propagates through space via oscillating electric and magnetic fields and is emitted and absorbed by objects in the form of tiny bundles of energy called photons. In some radioactive decay processes, extremely energetic photons called gamma rays are emitted by the nuclei as they change to states of lower total energy. A photon is considered to be the fundamental unit of electromagnetic radiation: a quantum. This pro- found 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. The weak force in nuclei is feeble compared with the electromagnetic and strong forces, being only about 10-5 times as strong as the latter, but it is still extremely strong compared with gravitation. The distance over which it is effective is even shorter than that of the strong force: about 10-'8 m, or 0.001 fm roughly 1/1000 the diameter of a nucleon. The weak force governs processes that are relatively slow on the nuclear time scale, taking about 10- ~° second or more to occur. As short as this time may seem, it is about one trillion times longer than the time required for processes governed by the strong force. The prediction in 1967 and its subsequent experimental confirma- tion that the electromagnetic and weak forces are but two aspects of a single, more fundamental force, the electroweak force, were tri- umphs of physics that greatly expanded our understanding of the laws of nature. However, because these two component forces are so different in the ways in which they are revealed to us (their essential similarities start to become clear only at extremely high energies, far beyond those of conventional nuclear physics), it is usually convenient to discuss them separately, just as we often discuss electricity and magnetism separately. Thus they are still often described as though each were fundamental. In this book, we will let the circumstances
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16 NUCLEAR PHYSICS decide how they should be discussed: as electromagnetic and weak, or as electroweak. For the remainder of this chapter, we will discuss them separately. The fundamental forces are often called fundamental interactions, because the forces exist only by virtue of interactions that occur between particles. These interactions, in turn, are mediated by the exchange of other particles between the interacting particles. This may seem like Chinese boxes, but as far as we know, it stops right there: in the realm of elementary-particle physics, which we must now briefly introduce in order to see where the foundations of nuclear physics lie. THE ELEMENTARY PARTICLES The experimental study of elementary-particle physics also known by the inexact name high-energy physics~iverged from that of nuclear physics around 1950, when developing accelerator technology made it relatively easy to search for other and ultimately more basic "elementary" particles than the proton or the neutron. An enormous variety of subnuclear particles has by now been discovered and characterized, some of which are truly elementary (as far as we can tell in 1984), but most of which are not. Along with the discovery of these particles came major theoretical advances, such as the electroweak synthesis mentioned above, and mathematical theories attempting to classify and explain the seemingly arbitrary proliferation of particles (several hundred by now) as accel- erator energies were pushed ever higher. Chief among these theories, because of their great power and generality, are the quantum field theories of the fundamental interactions. All such theories are relativ- istic, i.e., they incorporate relativity into a quantum-mechanical frame- work suitable to the problem at hand. They thus represent the deepest level of understanding of which we are currently capable. We will return to these theories shortly, but first let us see what classes of particles have emerged from the seeming chaos. This is essential for two reasons. First, the nucleus as we now perceive it does not consist of just protons and neutrons, and these are not even elementary particles to begin with. 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. Aspects of the two fields are rapidly
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INTRODUCTION TO NUCLEAR PHYSICS 17 converging, after their long separation, and it is no longer possible to investigate many fundamental problems of nuclear physics except in the context of the elementary particles. Much of the material in this book, in fact, deals with the ways in which this new view of nuclear physics has come about and the ways in which it will accelerate in the future. 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. An antiparticle differs from its ordinary particle only in having some opposite elementary properties, such as electric charge. Thus, the antiparticle of the electron is the positively charged positron; the antinucleons are the negatively charged antiproton and the neutral antineutron. The antiparticle of an antiparticle is the original particle; some neutral particles, such as the photon, are considered to be their own antiparticles. In general, when a particle and its corresponding antiparticle meet, they can annihilate each other (vanish completely) in a burst of pure energy, in accord with the Einstein mass-energy equivalence formula, E = mc2. Antiparticles are routinely observed and used in many kinds of nuclear- and particle-physics experiments, so they are by no means hypothetical. In the ensuing discussions of the various classes of particles, it should be remembered that for every particle mentioned there is also an antiparticle. Leptons Leptons are weakly interacting particles, i.e., they experience the weak interaction but not the strong interaction; they are considered to be pointlike, structureless entities. The most familiar lepton is the electron, a very light particle (about 1/1800 the mass of a nucleon) with unit negative charge; it therefore also experiences the electromagnetic interaction. The muon is identical to the electron, as far as we know, except for being about 200 times heavier.* The tan particle, or taNon, is a recently discovered lepton that is also identical to the electron except for being about 3500 times heavier (making it almost twice as *The muon is still occasionally called a mu meson its original name which can be confusing because the term "meson" is now restricted to a very different kind of particle; thus a "mu meson" is not a meson in the modern sense.
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18 NUCLEAR PHYSICS heavy as a nucleon). The very existence of these "heavy electrons" and "very heavy electrons" is a major puzzle for physicists. Associated with each of the three charged leptons is a lepton called a neutrino: thus there is an electron neutrino, a muon neutrino, and a tauon neutrino. Neutrinos are electrically neutral and therefore do not experience the electromagnetic interaction. They have generally been assumed to have zero rest mass (see page 31 for an explanation of this term) and must therefore move at the speed of light, according to relativity, but the question of their mass is currently controversial. If the electron neutrino, in particular, does have any mass, it is very slight indeed. The possible existence of such a mass, however, has great cosmological significance: because there are so many neutrinos in the universe, left over from the big bang, their combined mass might exert a gravitational effect great enough to slow down and perhaps halt the present outward expansion of the universe. Neutrinos and antineutrinos are commonly produced in the radioac- tive process called beta decay (a weak-interaction process). Here a neutron in a nucleus emits an electron (often called a beta particle) and an antineutrino, becoming a proton in the process. Similarly, a proton in a nucleus may beta-decay to emit a positron and a neutrino, becoming a neutron in the process. Neutrinos and antineutrinos thus play an important role in nuclear physics. Unfortunately, they are extremely difficult to detect, because in addition to being neutral, they have the capability of passing through immense distances of solid matter without being stopped. With extremely large detectors and much patience, however, it is possible to observe small numbers of them. We have now seen that there are three pairs, or families, of charged and neutral weakly interacting leptons, for a total of six; there are therefore also six antileptons. Let us next look at the quarks, of which there are also three pairs-but there the similarity ends. Quarks Quarks are particles that interact both strongly and weakly. They were postulated theoretically in 1964 in an effort to unscramble the profusion of known particles, but experimental confirmation of their existence was relatively slow in coming. This difficulty was due to the quarks' most striking single characteristic: they apparently cannot be produced as free particles under any ordinary conditions. They seem instead always to exist as bound combinations of three quarks, three antiquarks, or a quark-antiquark pair. Thus, although they are believed
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INTRODUCTION TO NUCLEAR PHYSICS 19 to be truly elementary particles, they can be studied-so far-only within the confines of composite particles (which are themselves often inside a nucleus). This apparent inability of quarks, under ordinary conditions, to escape from their bound state is called quark confine- ment. There are six basic kinds of quarks, classified in three pairs, or families; their names are up and down, strange and charm, and top and bottom. Only the top quark has not yet been shown to exist, but preliminary evidence for it was reported in the summer of 1984. The six varieties named above are called the quark flavors, and each flavor is believed-to exist in any of three possible states called colors. (None of these names have any connection with their usual meanings in every- day life; they are all fanciful and arbitrary.) Flavor is a property similar to that which distinguishes the three families of leptons (electron, muon, and tauon), whereas color is a property more analogous to electric charge. 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. They are consistent, however, with everything we know about quarks and the composite particles they constitute. These relatively large composite particles are the hadrons, all of which experience the strong interaction as well as the weak interaction. Although all quarks are charged, not all hadrons are charged; some are neutral, owing to cancellation of quark charges. There are two distinctly different classes of hadrons: baryons and mesons. Baryons which represent by far the largest single category of subnuclear particles-consist of three quarks (antibaryons consist of three antiquarks) bound together inside what is referred to as a bag. This is just a simple model (not a real explanation) to account for the not yet understood phenomenon of quark confinement: the quarks are assumed to be "trapped" in the bag and cannot get out. Now, finally, we can say what nucleons really are: they are baryons, and they consist of up (u) and down (d) quarks. Protons have the quark structure bud, and neutrons have the quark structure add. A larger class of baryons is that of the hyperons, unstable particles whose distinguishing characteristic is strangeness, i.e., they all contain at least one strange (s) quark. In addition, there are dozens of baryon resonances, which are massive, extremely unstable baryons with lifetimes so short (about 10-23 second) that they are not considered to be true particles.
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26 \~ ~3 ~ -a FIGURE 1.3 a, ~ woodcut by M. C. Escher, provides an example of complex geomethca1 symmetries, which underDe many aspects of nuclear structure. Equally impotent are dynamical symmetries Lund in the physical laws governing aN natural phenomena. (By permission of the Escher Foundation, Hags Cemeentemuseum The ague. Reproduction rights arranged courtesy of the Vows OaDer~s, New York San Franc~co, and Laguna Beach.)
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INTRODUCTION TO NUCLEAR PHYSICS 27 many years, it was believed that parity was an exact (universal) symmetry of nature. In 1956, however, it was discovered by nuclear and particle physicists that this is not so; parity is not conserved in weak interactions, such as beta decay. However, it is conserved, as far as we know, in all the other fundamental interactions and thus represents a simplifying principle of great value in constructing math- ematical theories of nature. A similar, albeit isolated, example of symmetry violation has been found for the equally fundamental and useful principle called time- reversal invariance, which is analogous to parity except that it entails a mirror imaging with respect to the direction of time rather than to the orientation of particles in space. This symmetry has been found to be violated in the decays of the neutral kaon. No other instances of the breakdown of time-reversal invariance are known yet but physi- cists are searching carefully for other cases in the hope of gaining a better insight into the underlying reason for this astonishing flaw in an otherwise perfect symmetry of nature. The implications of such discoveries extend far beyond nuclear or particle physics; they are connected to basic questions of cosmology, such as the ways in which the primordial symmetry that is believed to have existed among the fundamental interactions at the instant of the big bang was then "broken" to yield the dramatically different inter- actions as we know them now. 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. Certain of the proposed Grand Unified Theories predict, in fact, that such a violation should occur, in the form of spontaneous proton decay not in the sense of a radioactive beta decay, in which a proton would be converted to a neutron (thus conserving baryon number) but rather as an outright disappearance of a baryon (the proton) as such. Extensive searches have been mounted to find evidence for proton decay, so far without success. Also of great importance would be any violation of the conservation of lepton number. This law, which is also obeyed in all currently known cases, is analogous to the conservation of baryon number, but with an added twist: lepton number ~ + 1 for leptons, -1 for antileptons) appears to be conserved not only for leptons as a class but also for each
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28 NUCLEAR PHYSICS of the three families of leptons individually (the electron, muon, and tauon, with their respective neutrinos). Any violation of lepton-number conservation would mean that neutrinos are not, in fact, massless and that they can oscillate (change from one family to another) during their flight through space. Exactly these properties are also predicted by certain of the proposed Grand Unified Theories, and this provides the impetus for searching for them in various types of nuclear processes. Such searches for violations of conservation laws represent an impor- tant current frontier of nuclear physics as well as of particle physics. ACCELERATORS AND DETECTORS The principal research tools used in nuclear physics are accelera- tors- complex machines that act as powerful microscopes with which to probe the structure of nuclear matter. 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 syn- chrotrons; an example of a modern cyclotron is shown in Figure 1.4. Most of the details of these machines need not concern us here, but a survey of some basic ideas is necessary for an appreciation of how nuclear physics research is actually done. Additional information on accelerators in general and on several important accelerators of the future can be found in Chapter 10, and a survey of the major operating accelerators in the United States is given in Appendix A. Projectiles and Targets The basic principle of all accelerators is the same: a beam of electrically charged projectile particles is given a number of pulses of energy in the form of an electric or electromagnetic field to boost the particles' velocity (and hence kinetic energy) to some desired value before they collide with a specified target. Typically, the projectiles are electrons, protons, or nuclei. The latter are often called ions, because they are generally not bare nuclei, i.e., they still retain one or more of the orbital electrons from the atoms from which they came. Nuclei of the two lightest elements, hydrogen and helium, are called the light
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INTRODUCTION TO NUCLEAR PHYSICS 29 FIGURE 1.4 Top view of the main cyclotron of the Indiana University Cyclotron Facility, a modern accelerator used for basic nuclear-physics research. The field produced by the four large magnets (note the physicist standing between two of them) confines the projectile particles light ions up to mass number 7 to a series of roughly circular orbits of ever-increasing size as they are accelerated to energies in the range of 40 to 210 MeV. After about 300 orbits, the beam is extracted and directed at targets in nearby experimental areas. (Courtesy of the Indiana University Cyclotron Facility.)
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30 NUCLEAR PHYSICS ions; they include the often-used alpha particle, which is just the nuclide helium-4 (Z = 2, N = 21. Nuclei from those of lithium (A = 6 or 7) to those with a mass number of about 40 can be called medium ions, and those with a mass number from about 40 on up through the rest of the periodic table are called heavy ions. (This classification is useful but necessarily somewhat arbitrary; the definition of heavy ion, for example, is sometimes extended all the way down to lithium.) Accelerators can also produce beams of exotic or unstable charged projectiles such as muons, mesons, antiprotons, and radioactive nuclides. These are made in reactions occurring at the target of a primary beam and are then focused into a secondary beam. Even neutral particles, such as neutrons and neutrinos, can be produced and used as secondary beams. The target struck by the accelerated projectile in a typical nuclear- physics experiment is a small piece of some solid chemical element of particular interest, although liquid and gaseous targets can also be used. The objective may be to use the projectiles to raise nuclei in the target substance from their lowest-energy ground state to higher- energy excited states in order to gain insight into the structures and dynamics of intact nuclei; in this way one studies nuclear spectros- copy. 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. The above descriptions pertain to the traditional fixed-target ma- chines (a stationary target being bombarded by a projectile beam), but accelerators can also be constructed as colliding-beam machines, or colliders. Here two beams collide violently with each other, nearly head-on, in a reaction zone where the beams intersect. Colliders have been pioneered by elementary-particle physicists because of the huge amounts of energy that can be deposited in the collision zone when both beams have been accelerated to high velocities. Their use is becoming increasingly important to nuclear physicists for the same reason, as described in Chapter 7. Energies The kinetic energies to which particles or nuclei are accelerated are expressed in terms of large multiples of a unit called the electron volt (eV), which is the amount of energy acquired by a single electron (or any other particle with unit electric charge, such as a proton) when it is accelerated through a potential difference of 1 volt (V) as in a 1-V
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INTRODUCTION TO NUCLEAR PHYSICS 31 battery. The characteristic particle beam energies in modern nuclear- physics accelerators are of the order of mega-electron volts (1 MeV = 106 eV) and giga-electron volts (1 GeV = 109 eV). When dealing with accelerated nuclei, which contain more than one nucleon, it is custom- ary to give the energy per nucleon rather than the total energy of the nucleus. For convenience, not only the energies of particles but also their masses are customarily given in terms of electron volts. Any mass can be expressed in terms of an equivalent energy, in accord with E = mc2. Thus the mass of an electron is 0.511 MeV, and the mass of a proton is 938 MeV. These are the rest masses of these particles, i.e., the masses that they have when they are not moving with respect to some frame of reference (such as the laboratory). When they are moving, however, their kinetic energy is equivalent to additional mass. This effect becomes significant only when their velocity is very close to the speed of light; then their kinetic energy becomes comparable to or greater than their rest mass, and they are said to be relativistic particles (or nuclei), because the dynamics of their reactions cannot be accu- rately described without invoking relativity theory. It is convenient to classify nuclear processes in terms of different energy regimes of the projectiles, although any such classification, like that of the projectile masses, is somewhat arbitrary and not likely to find universal acceptance. Bombarding energies of less than about 10 MeV per nucleon, for example, produce a rich variety of low-energy phenomena. It is in this regime (at about 5 MeV per nucleon) that the effects due to the Coulomb barrier are particularly important; the Coulomb barrier is a manifestation of the electrostatic repulsive force between the positively charged target nucleus and any positively charged projectile. 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. In the high-energy regime, between about 100 MeV per nucleon and 1 GeV per nucleon, high temperatures are produced in the interacting nuclei; also, some of the collision energy is converted to mass, usually in the form of created pions, which have a rest mass of 140 MeV. Above about l GeV per nucleon is the relativistic regime, where extreme conditions, such as the formation of exotic states of nuclear matter, are explored. [It is worth noting here that for electrons the transition to relativistic
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32 NUCLEAR PHYSICS behavior occurs at much lower energies (about 0.5 MeV), owing to the electron's small rest mass.] Nuclear Interactions The principal kinds of nuclear interactions in collisions are scatter- ing, 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. The probability of occurrence of a given interaction is expressed by a quantity called its cross section, which can be measured experimentally and compared with theoretical pre- dictions. Another quantity whose experimental measurement is important is the half-life of a radioactive species the time it takes for half of all the nuclei of this nuclide in a sample to decay to some other form or state. Normally, this decay is by the emission of alpha or beta particles or gamma rays; less commonly, it is by spontaneousission, in which a nucleus simply splits in two, with the emission of one or more neutrons. After half of the nuclei have decayed, it will take the same length of time for half of the remaining nuclei to decay, and so on. The characteristic half-lives of radioactive nuclides vary over an enormous range of values: from a small fraction of a second to billions of years. Particle Detectors Accelerators would be useless if there were no way to record and measure the particles and gamma rays produced in nuclear interac- tions. The detectors that have been invented for this purpose represent a dazzling array of ingenious devices, many of which have pushed high technology to new limits. Some are designed to detect only a specific particle whose presence may constitute a signature of a particular kind of event in the experiment in question. They may be designed to detect this particle only within a certain limited range of angles of emission with respect to the beam direction or over all possible angles of . . emission. Other detectors are designed to detect as many kinds of particles as
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INTRODUCTION TO NUCLEAR PHYSICS 33 possible, simultaneously again either for limited angles or for all angles. This kind of detector is necessarily complex, owing to the many kinds of particles that must be observed and to the number of particles actually produced. This latter number, called the multiplicity, is as small as one or two for many kinds of events, but in the catastrophic collisions of relativistic heavy ions, it may be several hundred. Yet another consideration in the design of detectors is whether they are to be used at a fixed-target accelerator or a collider; the requirements are often very different. Among the simplest detectors are those in which a visible track is left in some medium by the passage of a particle. Examples of such visual detectors are the streamer chamber (in which the medium is a gas), the bubble chamber (liquid), and photographic emulsions (solid). Most detectors, however, rely on indirect means for recording the particles, whose properties must be inferred from the data. The operating principles of the great majority of such detectors are based on the interactions of charged particles with externally applied magnetic fields or on the ionization phenomena resulting from their interactions with the materials in the detectors themselves. The largest of these detector systems may consist of thousands of individual modules and are used in the study of very complex events. Sophisticated, dedicated comput- ers are required to store and analyze the torrents of data from such instruments. At the largest accelerators, the efforts of many physicists, engineers, and technicians may be required for many months to plan and execute one major experiment, and months more of intensive effort may be required to process and analyze the data and interpret their meaning. 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. It is the cumulative effort of all these scientists and their colleagues working at the accelerators together with that of the nuclear theo- rists that advances our knowledge of nuclear physics.
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I Major Advances In Nuclear Physics
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Representative terms from entire chapter: