Key Science Drivers for a Rare-Isotope Beam Facility
Chapter 1 presented a quick tour of nuclear physics, but more importantly it characterized the roots of some of the intellectual and technological drivers toward the future. This chapter explores the present-day investigations that would most directly be impacted by a facility for rare-isotope beams (FRIB)— and therefore would also most likely set the minimum performance requirements.
THE SCIENCE DRIVERS
A facility capable of producing intense beams of a wide variety of radioactive nuclei would clearly impact many areas of science and technology. This chapter presents the committee’s view of the principal scientific drivers in nuclear structure physics, nuclear astrophysics, fundamental symmetries, and some important technical applications. However, it is often the case with new world-class facilities that their most important scientific discoveries are not foreseen. The science drivers are briefly presented below, and each is then discussed in a more expanded presentation.1 A facility capable of executing the indicated research is referred to here as a FRIB.
1The Glossary in Appendix D provides additional discussion of key scientific terms.
Testing new nuclear structure concepts. A quantitative understanding of nuclear structure is important to problems ranging from the origin of the elements to the use of nuclei as laboratories for probing new interactions. The nuclear many-body problem—strongly interacting, with two kinds of particles (protons and neutrons), and with competing effects due to short-range multiple scattering and long-range collectivity—is also of broad intrinsic interest. The phenomena that arise—shell structure, pairing, superfluidity, collective motion and its connections with many-body symmetries, and spectral transitions from order to chaos—and the methods that nuclear physicists employ are also fundamental to fields such as atomic and condensed-matter physics and quantum chemistry. Nuclear structure theory has made significant progress in recent years by adapting numerical techniques for high-performance computing and through conceptual advances such as effective field theory and improved density functionals. However, the reexamination of old paradigms and subsequent development and validation of new nuclear models require data. This is a role for a FRIB: to test the predictive power of models by extending experiments to new regions of mass and proton-to-neutron ratio and to identify new phenomena that will challenge existing many-body theory. A FRIB’s rare-isotope beams of unprecedented intensity and its sophisticated detector arrays would allow experimentalists to explore the limits of nuclear stability. A FRIB’s technological developments would allow nuclear physicists, for the first time, to study nuclei that previously could be found only in the billion-degree explosions of distant supernovae.
Production and properties of superheavy nuclei. Theory predicts that super-heavy nuclei that do not exist anywhere else in the universe can be assembled. The nuclei would contain in excess of 120 protons; hence their stored Coulomb energy would be huge. However, with a large number of excess neutrons and an appropriate geometry, the attractive nuclear force could allow such a unique system to exist for times exceeding a day. The synthesis of such nuclei and their proper identification constitute an experimental challenge, but an advanced exotic-beam facility such as a FRIB is required if any meaningful search is to be carried out. These superheavy systems will provide great insight into the nuclear reactions and structure and, if they possess sufficient lifetimes, may reveal unusual chemical properties.
Probing neutron skins. Very-neutron-rich nuclei that can be reached by a FRIB offer the only laboratory access to matter made of pure neutrons. The
outer layer of those exotic nuclei consists of a neutron skin, which dramatically impacts their structure, reactions, and decays. Neutron skins can result in novel collective modes. Vibrations with respect to the inner proton-neutron core, for example, can impact neutron-capture rates, which are key to the astrophysical rapid neutron-capture process (r-process). With an improved understanding of strongly interacting matter in finite nuclei with large neutron excesses, scientists will be better equipped to model neutron stars: giant reservoirs of neutron matter.
The origin of the heaviest elements. At the extreme temperatures and pressures of fiery stellar explosions, new elements are forged by enormous fluxes of free neutrons (the r-process), energetic protons (the rapid proton-capture process, or rp-process), and gamma rays (the gamma process, historically referred to as the p-process). On timescales of seconds and less, these fluxes drive the original element abundance to the neutron or proton drip lines where even the most basic nuclear properties—binding energy and half-life—are, for the most part, unknown. Yet more than half of the elements in nature—mostly the ones heavier than iron—have been created this way. These same nuclear processes also power stellar thermonuclear explosions observed as classical novae and Type I x-ray bursts. They also provide the signatures for the diagnostics of core-collapse super-nova explosions. The measurement of the properties of these exotic short-lived nuclei in the pathway of these “extreme” processes therefore provides the key to a better understanding of nucleosynthesis and the conditions, timescales, and mechanism of stellar explosions.
Explosive nucleosynthesis. For nuclei in the iron group and lighter, nucleosynthesis also frequently proceeds through exotic parent nuclei. The iron in our blood and the calcium in our bones were produced by many generations of supernovae occurring since the big bang, in which these elements were originally formed as radioactive nickel and, in part, as radioactive titanium. Only about 10 percent of the isotopes in a typical modern calculation of explosive nucleosynthesis are stable. The rates for most of the key reactions are estimates based on uncertain extrapolation of theory. An exotic-beam facility would be able to measure many of the most critical rates and constrain the theoretical prediction of the rest.
Composition of neutron stars. There are roughly a billion neutron stars in the Milky Way Galaxy, yet their internal structure and the composition of their crusts are poorly understood. Produced by the explosive deaths of
massive stars, neutron stars are only a few times larger in size than the event horizons of black holes of the same mass. They produce a variety of high-energy phenomena—pulsars, x-ray bursts, some types of gamma-ray bursts—and are laboratories for general relativity. While an exotic-beam facility would not directly probe the high densities of neutron stars, it would be able to constrain the isospin dependence of the nuclear equation of state that determines neutron star structure. Moreover, using charge-exchange reactions on the most critical neutron-rich nuclei along the electron-capture chains that produce the critical nuclei in the crusts of neutron stars, a FRIB could enable the study of the central questions concerning the composition and energetics of their upper mantles.
Tests of fundamental symmetries with rare isotopes. The Standard Model of particle physics has been extraordinarily successful but has long been believed to be incomplete. The incompleteness is now demonstrated by the discovery of neutrino mass; modifications to the Standard Model are required. The Standard Model also leaves mysteries, failing to explain, for example, the asymmetry between matter and antimatter in the universe. Solving this problem seems to demand large effects of time-symmetry (T) violation, and there is little guidance from the Standard Model on this question. Among many experimental approaches for finding a new source of T violation, the search for a permanent electric dipole moment (EDM) is consistently cited as one of the most promising. While most particles have a finite magnetic dipole moment, a finite EDM violates time-reversal symmetry and has not yet been observed. The size of a possible EDM is expected to be dramatically enhanced in a few heavy radioactive nuclei with unusual pear-shaped deformations. Large numbers of such nuclei could be produced at a high-intensity FRIB, improving the sensitivity to an EDM by several orders of magnitude over existing experiments. Such measurements, free from backgrounds and many systematic effects, would be sensitive to the existence of physics at energy scales even higher than those that can be studied at the new Large Hadron Collider at the European Organization for Nuclear Research (CERN).
Other Scientific Applications
Applications from stockpile stewardship, materials science, medical research, and nuclear reactors. Applications in these areas have long relied on a wide
variety of radioisotopes. At the present time, each of these areas would be significantly advanced by a facility with high isotope production rates capable of producing high specific-activity (pure) samples for experimental use. In addition, the parallel advances in low-energy nuclear theory driven by a properly organized FRIB experimental program would provide better models for needed nuclear reactions in areas now beyond direct experimental reach.
In the case of stockpile stewardship, the complex nuclear reaction networks needed for understanding device performance would be greatly clarified.
Many materials science applications typically require high-purity radioactive isotopes for implantation to diagnose subtle but important phenomena at the few-atom level. Here, the growing demand, the relatively short half-lives, and the required purity of the desired range of isotopes argue strongly for a new high-production-rate facility.
Similarly, medical applications, such as the development of new alpha-and beta-emitter tagged antibodies that target and destroy cancer cells, have unmet requirements for high isotope production rates.
Lastly, in the reexamination of the nuclear fuel cycle as part of the “global nuclear energy partnership,” improved cross sections for neutrons on unstable fission fragments and actinides are required for the design of better fast-neutron reactors. The contributions of a FRIB to these questions would, in large part, be greatly enhanced by the availability of a suitable neutron source at the site.
A quantitative understanding of nuclear structure is important in problems ranging from the origin of the elements to the use of nuclei as laboratories for probing new interactions. Yet a general theory of nuclear structure remains elusive: the classical formulation of this problem, protons and neutrons interacting through a strong, short-range potential, is difficult to solve except for the lightest nuclei. Nor is it understood in any detail how such a formulation emerges from the underlying theory of quantum chromodynamics (QCD). For this reason, many of the tools for describing nuclei are based on models constructed to explain observations—such as quantum mechanical tunneling, symmetry breaking, both ordered and chaotic spectral properties, and rotations and vibrations—rather than being derived from fundamental theory. Thus, these tools are of limited utility in terms of both extrapolating power and predicting new phenomena.
However, much progress is being made. The first calculations of nucleon-
nucleon scattering properties have recently emerged from lattice QCD, and effective field theory, also motivated by QCD ideas, has provided controlled expansions for observables in few-body nuclei. The classical nuclear many-body problem can now be solved exactly through 12 nucleons, owing to the growth in computing power. There are methods for heavier nuclei being formulated that make direct connections with the underlying nucleon-nucleon interaction by defining how that interaction must be modified when used in model calculations.
The validation of improved models requires data. While a considerable body of information about nuclei at or near stability exists, a FRIB would test models by providing data in entirely new mass regions. This new information would stimulate further improvements by revealing the shortcomings of current models and uncovering new phenomena requiring conceptual advances in theory.
Figure 2.1 illustrates some of the progress that has been made in solving the classical nuclear physics problem, protons and neutrons interacting through a potential derived from two-nucleon scattering data, augmented by three-nucleon forces also constrained by experiment. The results were obtained from computationally intensive variational and Green’s function Monte Carlo calculations. This figure shows that in cases where the classical nuclear many-body problem can be solved, quantitative agreement with experiments is obtained for nuclear ground states and low-lying excitations. Significant in this figure is the important role of three-body forces. They are seen to provide approximately 15 percent of the binding energy, a uniquely large effect in physical systems.
A goal of nuclear structure theory is to extend such successes to the heavier nuclei that would be the focus of FRIB research. Such extensions cannot come about through growth in high-performance computing alone. The combinatorial growth of the complexity of the nuclear many-body problem with increasing nucleon number is too steep and the accuracy requirements too severe: typical nuclear binding energies may be 1 percent of the size of the canceling vector and scalar potentials operating within the nucleus. But there are paths forward that promise to combine exact techniques and present knowledge of the two- and three-nucleon potentials with models, thereby making model-based calculations far more reliable.
Much is known about the qualitative physics governing the structure of heavy nuclei. Nuclei exhibit a shell structure analogous to that found in atoms, despite the much stronger interactions among the nuclear constituents. Mass measurements show that nuclei with special “magic” numbers of neutrons or protons—2, 8, 20, 28, 50, 82, and 126—have particular stability. A spherical potential—representing the “mean field” that influences nucleon motion owing to the nucleon’s interactions with the rest of the nucleus—can reproduce this pattern and account for simple excitations of nuclei near magic numbers. But unlike in atoms, impor-
tant correlations between the nucleons arise from “residual” strong interactions beyond the mean field. The nuclear shell model, perhaps the most widely used microscopic nuclear model, superimposes such correlations on the shell structure, thereby directly accounting for that part of the residual interaction most important to the long-distance structure of the nucleus. The effects of short-distance correlations can also be treated, though indirectly.
The shell model, however, still requires solution of the nuclear many-body
problem for many active valence nucleons occupying the quantum states between the magic numbers. This problem also becomes numerically challenging for nuclei beyond nickel (56 nucleons). Thus, other models are needed in which only the most important degrees of freedom are identified and retained, so that a full treatment of all interactions among the valence nucleons can be avoided. This kind of approach to many-body quantum physics can be found in many other fields, such as condensed-matter physics, atomic and molecular physics, and quantum chemistry. Examples of nuclear physics models that have had success include those describing collective motion such as rotations and vibrations, those that simplify the interactions among valence nucleons by limiting interactions to small clusters of nucleons, and those that replace interactions among many nucleons by a density functional describing conditions locally around each nucleon.
One dramatic example of collective behavior is the breaking of spherical symmetry by deformation of the nuclear shape into a football or a pancake and the subsequent restoration of that symmetry by the collective rotation of the deformed nucleus, producing a spectrum characteristic of a rigid rotor. Models have been developed to describe the conditions for such shape changes and the resulting nuclear spectra characteristic of rotation.
The understanding of such phenomena is limited by the at-present restricted view of all possible nuclei. Most nuclear experiments are conducted with stable nuclei, a group of about 300 species that exist naturally on Earth. These nuclei can be viewed as forming the floor of a valley—called the valley of stability—in a two-dimensional landscape in N and Z (see Figure 2.2). That is, the stable nuclei are a one-dimensional path in (N,Z) through this two-dimensional landscape. Many properties of the stable nuclei have been measured, and most nuclear models have been designed to reproduce these properties. Thus, the important test of understanding nuclear structure will be the extent to which nuclear properties can be predicted in new regions of the landscape—properties of nuclei away from the valley of stability.
The effort to understand the broad spectrum of nuclei, stable and unstable, has important implications for other fields. In astrophysics, unstable nuclei play crucial roles in explosive environments such as supernovae and colliding neutron stars. In fact, it is believed that roughly half of the stable nuclei heavier than iron were synthesized as unstable nuclei in the core of an exploding supernova, then ejected into the interstellar medium. The stable r-process nuclei found on Earth are the “daughters” of these unstable parents, formed when the parents decayed back to the valley of stability after the supernova explosion.
Nuclear physicists would like to understand how far the nuclear landscape extends beyond the valley of stability: how exotic can a nucleus be while still remaining bound to strong interactions? The valley of stability follows a path that
begins, for light nuclei, with N ~ Z, then later veers toward nuclei with N > Z as the repulsive Coulomb force begins to favor heavy nuclei with fewer protons than neutrons. The walls of the valley are quite asymmetric (see Figure 2.2). Owing to the Coulomb force, only a few protons can be added to a heavy stable nucleus before the nucleus breaks apart. Thus, the valley walls on the proton-rich side are steep, and the proton drip line is not far from the stable valley floor. For this reason, experimentalists have already succeeded in “mapping” the “limit” of stable, proton-rich nuclei through bismuth (Z = 83). In contrast, the valley walls on the neutron-rich side are much less steep: many neutrons can typically be added to a nucleus without causing the nucleus to break apart immediately. Until the advent of radioactive-beam facilities, only relatively few of these neutron-rich nuclei at or near the drip line could be explored. A FRIB is an instrument designed to produce these nuclei, determine their masses, and measure their decay modes. Major sur-
prises could result. For example, theory suggests that there may be an undiscovered island of superheavy nuclei, significantly heavier than the most massive stable nucleus, uranium, lying beyond current experiments, but potentially accessible to a FRIB.
This description captures the essence of a FRIB’s role in nuclear structure physics: this facility would allow the mapping of a far greater region of the (N,Z) landscape than is currently accessible, thus testing the predictive power of nuclear models and provoking improvements in those models. The measurements from a FRIB would be immediately relevant to explosive environments important to astrophysics and could reveal unexpected nuclear properties, such as unusually long-lived superheavy nuclei. The following discussion expands on these points.
Testing Nuclear Structure Concepts
Several examples are discussed below to illustrate how a FRIB might probe aspects of nuclear structure not readily accessible with only stable nuclear beams.
Probing the Disappearance of Shell Structure
Perhaps the most important early advance in microscopic nuclear structure theory was the recognition that the observed regularities in nucleon separation energies with so-called magic numbers could be ascribed to properties of a mean field, despite the very strong short-range repulsion known to exist between nucleons. The shell structure of nuclei with N or Z near the magic numbers is manifested by gaps in the energy spacing and angular momentum of low-lying levels. But robust shell structure, or at least the familiar magic numbers, may prove to be a property only of nuclei near the valley of stability. Theory suggests that some of the known shell gaps close significantly as nuclei become very neutron-rich and/or extended in radius, as illustrated in Figure 2.3. If this behavior is confirmed by experiment, it will influence the distribution of heavy elements produced in the neutron-rich environment of a supernova.
One important goal of a FRIB is to produce new neutron-rich, doubly magic nuclei, that is, unstable nuclei where N and Z are both magic. If the shell gaps are unusual, this will demonstrate that the mean field, and thus the interaction of valence nucleons with the rest of the nucleus, differ from that of stable nuclei. Such nuclei are particularly simple probes of the effective internucleon interaction. Specifically, a FRIB is expected to produce the short-lived, doubly magic species 48Ni, 56Ni, 78Ni, 100Sn, and 132Sn and to allow the exploration of their single-particle structure through one-nucleon transfer and knockout reactions for testing if they exhibit the magic shell-structure behavior.
Pairing and Superfluidity
Any attractive interaction between fermions (above the degenerate Fermi sea) at sufficiently low temperatures generally leads to fermion pairing and, therefore, to superfluidity, analogous to the Cooper pairing of electrons in superconducting metals. It is not surprising, therefore, that pairing plays an important role in nuclear structure. As the number of nucleons can be precisely controlled at a FRIB, exotic nuclei accessible with a FRIB would offer many new opportunities to study pairing, including its influence on the structure of the diffuse, neutron-rich skin found in nuclei far from the valley of stability. Such studies are of potential importance to an understanding of the cooling of nature’s ultimate neutron-rich “nucleus,” the
neutron star. In extremely-neutron-rich nuclei and in heavier nuclei (A > 60) with an equal number of neutrons and protons, different superfluid phases may appear, characterized by nucleonic Cooper pairs carrying different isospin, spin, and total angular momentum. Pairing can be probed at a FRIB through a variety of reactions that add or subtract pairs of nucleons. Two-nucleon transfer studies to probe pairing properties could be carried out at a FRIB within a week, given beam intensities of 104 ions per second. Thus, experiments with 56Ni, 64Ge, 72Kr, and the heavier N = Z nuclei up through 88Ru and probably 92Pd would likely be possible. An important probe of proton pairing, the (3He,n) reaction, might be possible for species up to 88Ru. Two-nucleon knockout reactions could be performed with beams as modest as 10 ions per second.
The Evolution of Collective Motion in Complex Nuclei
The number of distinct nuclear configurations increases as a combinatorial of the number of interacting nucleons. A remarkable feature apparent in nuclear spectra is that, in spite of such complexity, heavier nuclei exhibit novel collective properties that may not be as readily apparent in few-body systems. Similar simplicity also arises in the complex systems of other fields, such as atoms, molecules, and materials. In many cases, these regularities arise from underlying symmetries that govern the systems, from which the relevant and usually simple collective coordinates can then be deduced. The goals of nuclear structure physics include identifying the relevant collective coordinates, understanding their connections to the approximate symmetries governing nuclear motion, and then understanding how these symmetries arise from the underlying microscopic theory based on the degrees of freedom of nucleons.
One example is the sharp structural change in nuclear ground states that occurs in certain mass regions under seemingly small changes in mass, such as the addition of a pair of neutrons. The nucleus may respond by altering its shape from spherical to deformed ellipsoidal. This phenomenon (see Figure 2.4) can be understood in terms of quantum mechanical tunneling, a transition between nearly degenerate minima in the energy corresponding to distinct shapes, or deformations. The resulting coexistence of distinct shapes determines the excitation spectra of such transitional nuclei. These excitations are governed by symmetries: the spherical symmetry that is destroyed by deformation is restored by the associated collective modes (rotation of the ellipsoid).
While such phenomena are seen in chains of isotopes near the valley of stability, FRIB experiments could map nuclear phases over a much larger region, including cases in which the valence protons and neutrons occupy very different shells. Key questions that could be addressed by looking at the extreme nuclei far
outside the valley of stability include what the consequences of the extended neutron radii (skins) in such nuclei are, whether the effective interactions will be weaker in this density regime, and what the effects of the large isovector densities in these species will be. It is unclear whether new candidate regions for spherical-to-deformed-shape transitions—regions exemplified by the neutron-rich nuclei 112Zr, 96Kr, and 156Ba or the proton-rich nucleus 134Sm—will exhibit the same kind of sharp shape transitions seen nearer the valley of stability. These nuclei, and their neighbors in the expected transition regions, would be available for study at a FRIB, given beam intensities ranging from a few to 10,000 ions per second. Such beams would allow experimenters to determine masses and lifetimes, and, for the
more intense beams, to study Coulomb excitation, nucleon transfer, and highly inelastic collisions of these nuclei.
The study of such shape transitions would be just one element of a FRIB program to map out the collective behavior of exotic nuclei. The data from a FRIB would span very large isotopic sequences, often covering several major shells. The proposed experiments would help improve the understanding of how the critical elements of nuclear collective motion—pairing, all possible kinds of deformation, vibrations, and associated decays such as fission—evolve as one alters the neutron-to-proton ratio and the aspects of the effective interaction that this ratio controls.
Probing Neutron Skins
It was noted previously that nuclear and electrostatic forces conspire to push the neutron drip line far from the valley of stability. Nuclei with large neutron excesses are known to exhibit distinctive properties, such as the extended neutron densities (see Figure 2.5) that develop as neutrons occupy weakly bound quantum levels. Such extended neutron halos and skins have consequences for the effective interaction, weakening the coupling of outermost neutrons to the rest of the nucleus. To the extent that the understanding of strongly interacting matter with large neutron excesses is improved, it will also be more possible to model the exotic neutron-rich environment of neutron stars.
One expects to find new collective modes that are a consequence of this extended neutron skin. One of these, a low-energy isovector vibrational mode, could alter neutron-capture cross sections important to r-process nucleosynthesis. The beam intensities at a FRIB would allow experimenters to study a range of neutron skins several times greater than it is currently possible to do.
Production and Properties of Superheavy Nuclei
The elements that are found naturally on Earth end with uranium. But others may be synthesized either in the laboratory or during stellar explosions. The question of what the heaviest nuclei are that can exist, particularly ones that might live long enough to be studied, is an intriguing one in nuclear physics. Will a FRIB be able to synthesize long-lived superheavy nuclei and allow experimenters to study their chemistry? Owing to the large electrostatic energy of superheavy nuclei, one would naively expect them to be highly unstable and to spontaneously fission. However, quantum mechanics enters here in a dramatic way: individual nucleon orbits in specific nuclear shapes can lead to reductions in energy that can overcome disruptive Coulomb effects, thus binding these nuclei. Theoretical predictions indicate that the short alpha-decay lifetimes (millisecond or less) of known
superheavy nuclei are due to a neutron deficiency, and that more-neutron-rich isotopes of the same elements might have very long lifetimes. However, theories disagree in their predictions for the location and extent of the region in (N,Z) where superheavy nuclei might exist.
A FRIB could play a crucial role in identifying such nuclei because the mechanisms by which superheavy nuclei can be produced in the laboratory have not been thoroughly explored. A FRIB would provide a range of options for synthesiz-
ing superheavy elements. One could collide two nuclei with a combined number of protons and neutrons very near that of a potential superheavy candidate and look for the requisite fusing. Alternatively, and perhaps more likely of success, would be the collision of neutron-rich nuclei. The resulting compound system could decay into the superheavy ground state via evaporation of the excess neutrons. As an example, no target-projectile combination of stable isotopes will directly lead to the center of the expected region of long lifetimes, thought to be around Z = 112 and N = 184 (see Figure 2.6). Intense beams from a FRIB would therefore complement studies of the heaviest nuclei with stable beams in at least two ways. First, in favorable cases, that is, instances in which the intensity of the rare isotope is large (90,92Kr, 90,92Sr >1011 ions per second), fusion reactions become feasible with reaccelerated beams of high intensity and precise energies. Second, there is also interest in exploring the chemistry and atomic physics of the longer-lived elements, in cases in which the heavy isotope is produced in sufficient quantity. The atomic and chemical properties of superheavies are likely to be novel because of the highly relativistic behaviors of the inner-shell electrons, which in turn would affect the overall density of states.
A FRIB would extend research in nuclear structure from the domain of stable or near-stable nuclei familiar in everyday life to nearly the full range of nuclei that exist in nature’s most exotic stellar environments. With its access to many new species, a FRIB would allow experimentalists to select beams that most readily map out how nuclei change as a function of N, Z, and binding energy.
The identified goals for a FRIB include testing the limiting values of N and Z in nuclei, determining properties of neutron skins, and searching for new superheavy systems at the limits of mass and charge. A FRIB, by enabling the exploration of the unknown regions of the nuclear landscape, would also have the potential to discover completely unanticipated phenomena in nuclear structure physics.
The nuclear physics of unstable nuclei is fundamentally important in three astrophysical contexts: in making determinations of the abundances of the elements and isotopes produced in stars and stellar explosions; in generating and releasing energy in such environments; and in helping develop the understanding of the behavior of matter at the extremes of neutron excess found in neutron stars and supernovae. Each of these areas poses robust problems in nuclear physics that have eluded solution for decades.
How Were the Elements from Carbon to Uranium Created?
The chemical elements and isotopes observed today were produced by nuclear processes from the big bang through star generations by a multitude of nuclear burning processes (see Figure 2.7). A complete understanding of the origins of the elements in the universe requires not only mastery of the observed current populations but also mastery of the plethora of nucleosynthesis processes that have taken place over time within the different families of stars within the universe.
The central problem of nucleosynthesis is that the elements found on Earth, the ones stable against weak decay, are only a small fraction of those transiently produced in stars along the reaction chains that create them. Nature frequently chooses paths that pass through the unstable isotopes for making the stable isotopes. Hence, to date it has been possible to study in the laboratory only a small fraction of the isotopes encountered in stars, particularly those created in key explosive events. The iron in our blood, for example, was made in supernovae as radioactive 56Ni, a doubly magic nucleus that is an abundant product of explosive burning whenever the reactants have equal numbers of neutrons and protons. Gamma rays from the decay of 56Co (the daughter of 56Ni) to iron were detected coming from Supernova 1987A. Similarly, theory predicts that part of potassium was made in supernovae as radioactive calcium, manganese from cobalt, cobalt from copper, and so on.
Explosive events such as novae, supernovae, and x-ray bursts tend to produce unstable nuclei either because they quickly fuse fuels that have equal numbers of neutrons and protons (as in the 56Ni example), or because they provide situations with large abundances of free protons or free neutrons at high temperature. A typical modern calculation of nucleosynthesis in a supernova carries 1,500 isotopes (only 10 percent of which are stable), coupled by about 15,000 possible reactions involving neutrons, protons, alpha particles, gamma rays, and neutrinos in entrance or exit channels. Such a calculation still does not include the larger set of nuclei and reactions needed to study the r-process (see below). As a result, perhaps the most challenging aspect of a quantitative theory of nucleosynthesis is the sheer volume of data it requires. The rates for most of these reactions are estimates from theory, and many will never be measured, but the most critical ones need to be measured to confirm the predicted reaction patterns and to provide a basis set for calibrating the theory of the rest.
One area in which a FRIB could contribute greatly is in terms of the understanding of nucleosynthesis of heavy elements by the r-, gamma-, and rp-processes (see Figure 2.8). Here “r” stands for rapid neutron addition, “rp” for rapid proton addition, and “gamma” for a series of photodisintegration reactions proceeding through unstable neutron-deficient nuclei. These rapid processes occur in nature
when there is a sufficiently large density of free neutrons, gamma rays, or protons at high temperature. Together, these processes are responsible for making over half of the isotopes heavier than iron—the r-process making the neutron-rich isotopes, the rp-process making some of the more abundant neutron-deficient ones from mass 60 to 120, and the gamma-process making the heavier neutron-deficient nuclei up to A ~ 200.
Each of these rapid processes occurs in an explosive environment. The r-process is believed to occur in the matter ejected by a merging binary pair of neutron stars, and in the “wind” blown by neutrinos from the surface of a neutron star when it first forms inside a supernova (the duration is only a few seconds). The rp-process can also occur in that neutrino-powered wind and additionally is the power source for Type I x-ray bursts on the surfaces of accreting neutron stars.
Rare Isotopes in Astrophysics—Example 1
The primary control points along the r-process path are the nuclei that are thought to possess closed neutron shells (N = 50, 82, and 126 are the most important). At these points, beta decay dominates neutron capture, which has been brought to a standstill by photoneutron ejection. The r-process slows down here and produces the prominent abundance peaks seen in observations. Access to these r-process nuclei, their masses, and half-lives is essential to the timescale of the entire process. An exotic-beam facility would enable measurements of the half-lives of the N = 126 r-process nuclei 192Dy, 193Ho, 194Er, 195Tm, and 196Yb, which are, according to current r-process models, the most important bottlenecks. Such lifetime measurements would be feasible with relatively limited intensities, perhaps on the order of 10 particles per second. Most of the important branchings for beta-decayed neutron emission and the related nuclear mass measurements are also within reach. With these measurements, astrophysical models would have a solid nuclear physics underpinning for investigating the synthesis of r-process nuclei in the region of the A ~ 195 peak and beyond to explain the production of the heaviest nuclei found in nature.
The rp-process may also play a role in classical novae. In both the r- and the rp-processes, temperatures of 0.5 billion to 2 billion K and neutron or proton densities of 100 to 106 gm cm–3 drive the composition to the neutron or proton drip line, respectively. The production of heavier nuclei depends on the binding energies (which determine the “waiting point”2 for a given capture chain), beta-decay lifetimes, and cross sections of nuclei so unstable that they are very difficult to produce in the laboratory. The gamma-process happens as the shock wave passes through the heavy-element shells of a supernova raising the temperature to 2 billion to 3 billion K. Neutrons, protons, and alphas are knocked off of heavy isotopes present in the star since its birth, changing them into a rarer, more neutron-deficient collection of species. Unlike the rp-process, the flows here do not reach the proton drip line, but proceed through unstable heavy nuclei whose neutron separation energies are large, that is, where (γ,p) and (γ,a) occur at rates comparable to (γ,n) (see Box 2.1).
A rare-isotope beam facility would provide access to the vast majority of the neutron-rich nuclei involved in the r-process for measurements of decay lifetimes, masses, and other properties—all of the essential information for reliable theoretical modeling of r-process nucleosynthesis. In particular, such a facility is needed to
access r-process nuclei near the shell closure at neutron number 126. As a major bottleneck in the r-process, this region is an important normalization point for model predictions of the synthesis of heavy r-process elements such as uranium and thorium. Results from an exotic-beam accelerator facility, coupled with astrophysical simulations, would constrain temperature, density, timescales, and neutrino fluxes at the r-process site from observations of elemental abundances. This information would in turn help to determine once and for all the sites in nature where the r-process occurs. Using isotope harvesting, an exotic-beam accelerator facility could also enable neutron-capture cross-section measurements of long-lived unstable nuclei produced in the slow neutron-capture process (s-process). These reactions are used to monitor temperature and convective mixing in the helium shells of asymptotic giant branch stars where most of the heavy isotopes not due to the r-process are made.
How Is Energy Generated in Stars and Stellar Explosions?
Ordinary stars are gravitationally confined thermonuclear reactors, with nuclear reactions providing the necessary power to keep the star from contracting. Because stars live a long time, the most important reactions involve stable nuclei and are not a goal of an exotic-beam accelerator facility.
By contrast, nuclear energy generation in explosive events, especially novae and x-ray bursts, comes from reactions involving unstable targets. A classical nova is the consequence of a critical mass of hydrogen and helium piled up on an accreting white dwarf star and experiencing a nuclear-powered runaway. An x-ray burst is the same phenomenon, but with a neutron star substituted for the white dwarf. In both instances, temperatures from 0.3 billion to 2 billion K are reached in dense, hydrogen-rich material (the lower temperature is more relevant to novae; x-ray bursts are hotter). Energy is initially generated from the carbon-nitrogen-oxygen (CNO) cycle, but as the temperature increases above about 0.5 billion K, alpha capture on unstable oxygen and neon nuclei (15O and 18Ne) leads to a breakout and an ensuing chain of proton-capture sequences that can go as far as the element tin. These proton captures, augmented at the highest temperatures by (α,p) reactions, proceed along the proton drip line. The rate at which heavier elements are produced depends on the binding energies, lifetimes, and cross sections of these very-short-lived, proton-rich nuclei. Energy is generated from a combination of helium burning, hydrogen burning by the CNO cycle, and the rapid proton captures on heavy elements, with proton capture dominating in the x-ray burst case (see Box 2.2).
At present, it is uncertain if novae ever get hot enough for a substantial breakout and rp-process, but this definitely occurs in x-ray bursts where the life-
Rare Isotopes in Astrophysics—Example 2
Certain reactions are more critical than others in understanding astrophysical events. The reaction 15O(α,γ) results in a breakout of material from the carbon-nitrogen-oxygen (CNO) cycle and starts a rapid proton-capture (rp) process that leads to nucleosynthesis possibly as far as tin. The reaction rate determines the temperature at which breakout occurs, triggering the NeNa cycle in novae or the rp-process in x-ray bursts. Within the current range of uncertainty in this reaction, breakout for high-temperature nova explosions cannot be excluded, and the question about the on-site production of the observed Ne abundances cannot be addressed. The predictions of x-ray burst models also depend critically on this particular rate. Recent simulations suggest significant differences in the burst amplitude and sequence, depending on the present uncertainties in the rate.
An experimental verification of the predicted low-energy resonance parameters in the 15O(α,γ) reaction is desperately needed; these parameters can only be measured in the laboratory with a rare-isotope facility. The required intensities range from on the order of 106 to 108 particles per second for alpha scattering measurements to 1011 to 1012 particles per second for the necessary studies of resonant capture. Both this level of intensity and the requisite beam quality would be compatible with a next-generation facility.
times and binding energies of proton-rich waiting-point nuclei are reflected in the observed light curve (see Figure 2.9). In the most energetic of these, light pressure blows a wind from the neutron star surface, possibly contributing to the nucleosynthesis of some rare isotopes.
How Would an Exotic-Beam Accelerator Facility Help Improve Understanding of Neutron Star Structure, Supernovae, and Gamma-Ray Bursts?
There are roughly one billion neutron stars in the Milky Way Galaxy, yet their structures and crusts are very poorly understood. Produced in supernovae at the deaths of massive stars, neutron stars are the sites of radio pulsars, x-ray pulsars, and exotic binaries that are laboratories for general relativity. Of particular interest is the physics of the neutron star crust. The properties of neutron-rich nuclei far from stability are important to probing the thermal and electromagnetic characteristics of matter at extreme density. Material accreted onto the neutron star envelope will be buried in layers with increasing density as new material piles on. Electron capture will make the nuclei progressively more neutron-rich. The same thing happens to the ashes of x-ray bursts. Eventually neutron drip occurs at a density ~4 × 1011 g/cm3 and internal energy is released, heating the neutron star crust. The timescale and internal energy production depends on the electron-
capture rates and the neutrino losses in neutron star crust matter. These electron-capture rates can be studied with an exotic-beam accelerator facility using charge-exchange reactions on the most critical radioactive neutron-rich nuclei along the dominant electron-capture chains between A = 56 and A = 104. The measurement of the Gamow-Teller strength distribution will also provide information about the neutron release and the subsequent neutronization of neutron star crust matter.
A neutron star is, in some ways, just a huge stellar-mass-sized nucleus with a very large neutron-to-proton ratio. Unlike the case with ordinary atomic nuclei, however, gravity is important in confining the nucleons, and the central density in neutron stars is much greater than in ordinary nuclei. As a result, new aspects of the nuclear force (and particle physics) are needed to fully describe the system. A
key uncertainty is the resistance to compression offered by such matter at nuclear and supernuclear densities. This uncertainty affects the maximum mass of neutron stars, the strength of the initial shock wave in the most common variety of supernovae (those derived from iron-core collapse in massive stars), and the dynamics of neutron star mergers (see Figure 2.10). Most studies of nuclear compressibility are, of necessity, carried out on stable nuclei. For neutron stars, the phases, nuclear masses, electron-capture rates, and equation of state in the outer crust (which geometrically can be quite large) are not known in the sense that there is little experimental confirmation of the physics inputs in model crusts. With an exotic-beam accelerator facility, the range of neutron excesses available would be much larger, so the neutron-to-proton ratio dependence of the nuclear equation of state could be determined.
Exotic Beams: An Urgent Need of the Nuclear Astrophysics Community
The key feature of an exotic-beam accelerator facility (such as a FRIB) for applications in nuclear astrophysics is its ability to produce high fluxes of unstable nuclei across a broad range of masses and particle-separation energies—it is the general-purpose nature of the facility that becomes its primary asset for nuclear astrophysics (see Box 2.3). Ultimately, one wants to understand the origin of all nuclei and then to use that understanding to diagnose stellar explosions and the chemical evolution of galaxies of all sorts. That is, in order to get leverage on the specific problem, scientists need first to sample and then understand the general case. Scientists have worked toward that goal for at least 50 years and have made some progress.
The vast majority of the elements heavier than helium are made in stars, with supernovae making the majority. The processes of nucleosynthesis have been defined, and one or more probable sites exist for each. Models agree qualitatively with the abundances seen in the Sun and in stars of varying ages in the Galaxy, but the theory is only as reliable as the nuclear data it employs. Major investments are being made in space- and ground-based observations of abundances in all astronomical environments. These measurements are carried out across the spectrum— from gamma-ray lines emitted by nuclear gamma decay in space, to infrared—and in objects nearby and at high redshift. The complexity and realism of numerical simulations on large, massively parallel machines is starting to approach the precision of the best and most recent observational data—and comparisons have yielded great insights. To enable scientists to pursue these questions fully then, an investment parallel to that in the astronomical observational facilities is necessary for expanding the nuclear data that comprise the physical basis for these simulations.
Specific Examples of Astrophysical Processes That a Rare-Isotope Facility Might Illuminate
Following is a list of astrophysics problems that an exotic-beam accelerator facility would uniquely address. A strength of such a facility is that as these problems are solved and new ones take their place, the same machine can address them.
Studies of fundamental interactions aim to understand the nature of the most elementary constituents of matter and the interaction forces between them. With the exception of the recent and dramatic discovery that neutrinos have mass, most of what has already been learned about elementary particles and interactions is embodied in the Standard Model of particle physics, a framework that has been astonishingly successful, with three decades of experimental tests that have sup-
ported its predictions with ever-increasing precision.3 How much of a change in the Standard Model will be required by the discovery of neutrino mass is not yet understood. Other and perhaps related defects in the Standard Model are that it fails to account for the dominance of matter over antimatter observed in the universe, does not include gravitational interactions, and contains many parameters that must be taken from experiment. Understanding the properties of the universe at a deeper level than the Standard Model is one of the greatest challenges facing science.
Historically, many features of fundamental interactions have been discovered in nuclear physics experiments. The existence of neutrinos was first proposed by Wolfgang Pauli to explain an apparent loss of energy and momentum in nuclear beta decays. The first observation of parity violation came from studies of 60Co beta decays, showing that the laws of physics are not the same if viewed in a mirror. Nuclear experiments have resulted in the first direct detection of neutrinos, the establishment of the vector/axial-vector structure of the weak interactions, the demonstration of mixing between different flavors of neutrinos, and the establishment of a 2 eV/c2 limit on the electron neutrino mass. This limit is presumed to apply to the other neutrinos, given the small mass differences observed in the recent nuclear experiments that discovered neutrino oscillations. Experiments exploiting nuclei as laboratories can have the powerful advantage that, with a large range of different isotopes to choose from, a specific isotope can often be selected with unique properties that isolate or amplify important physical effects. For example, recent measurements at the TRIUMF laboratory at the University of British Columbia and the On-Line Isotope Mass Separator (ISOLDE) at CERN of positron-neutrino correlations in pure Fermi 0+ → 0+ beta decays put stringent constraints on a possible scalar contribution to weak interactions, while a measurement of the same correlation in 3/2+→ 3/2+ beta decays, recently completed at the Lawrence Berkeley National Laboratory, is also sensitive to tensor interactions.
Among the most striking facts that the Standard Model cannot explain is the dominance of matter over antimatter in the universe. The leading proposed explanation for this vital fact is that an asymmetry between matter and antimatter developed as the universe cooled after the big bang, owing to a violation of time-reversal symmetry of physics laws, or, equivalently, to a violation of charge-parity (CP) symmetry. While the ingredients necessary for CP violation exist in the Standard Model, the level of CP violation is far too small to account for the observed
amount of matter in the universe. One of the best ways to look for a sufficient source of CP violation is by searching for a permanent electric dipole moment in subatomic particles. Other methods for searching for excess CP violation in the quark section are also being actively pursued, including major efforts at B-factories at the Stanford Linear Accelerator Center (SLAC) and the High Energy Accelerator Research Organization (KEK) in Japan. The discovery of neutrino mass opens up the possibility of CP violation for the leptons.
Most particles (with spin) have a finite magnetic dipole moment in their ground state; these moments have no particular significance for fundamental symmetries. However, the presence of an analogous electric dipole moment in their ground state violates time-reversal and CP symmetry and has never been observed. At the level of present experimental sensitivity, an EDM could be a signal of the excess CP violation beyond that allowed by the Standard Model to explain the matter-antimatter asymmetry. Many searches for an EDM have been conducted over the years, putting extremely tight bounds on its possible size. The absence of an observable EDM played a role in establishing the mechanism of CP violation in the Standard Model involving the mixing of the three generations of quarks. As a result, the Standard Model predicts negligibly small EDMs, while most extensions of the Standard Model can naturally generate much larger EDMs. Present EDM experiments are already sensitive to the existence of new particles with large CP-violation at the TeV scale and place stringent constraints on many theories proposed to explain the matter-antimatter asymmetry of the universe.4
Existing techniques for laboratory-based EDM searches are beginning to reach their limits, although several new ideas have emerged. One of the most promising methods for expanding the reach of EDM searches is to choose nuclei with special properties that could enhance the effect of CP-violating interactions. A handful of such nuclei have been identified over the years: for example, 229Pa, 223Ra, 225Ra, and 223Rn. The CP-violating effects are enhanced in these radioactive nuclei because they have a static octupole deformation and closely spaced levels of opposite parity, increasing the mixing of quantum states due to CP-odd nuclear forces. Such pear-shaped nuclei occur only rarely and only in special regions of the nuclear chart. Several theoretical calculations have confirmed that the size of the EDM (if it exists) is expected to be enhanced in these nuclei compared with 199Hg, the most sensitive stable nucleus currently used in EDM searches, by a factor of several hundred to several thousand. Developing better estimates of the enhancement
factors is an important problem for nuclear structure physics that will become particularly crucial if a finite EDM is observed.
EDM searches with radioactive nuclei require the development of new experimental techniques. The most promising approach for Ra isotopes is based on recently developed laser cooling and trapping techniques. As recently as 2005, laser trapping of 225Ra was demonstrated at the Argonne National Laboratory. For the EDM measurement, the atoms will be cooled and collected in a magneto-optical trap, spin polarized, transferred into an optical dipole trap, and placed into a region of high electric field. A permanent EDM would then result in a precession of the nuclear spin proportional to the strength of the electric field. A very different technique is being developed for Rn isotopes at TRIUMF and at the University of Michigan. It involves collecting Rn atoms in a glass cell where they are polarized by spin-exchange collisions with optically pumped alkali atoms, and their precession in an electric field is monitored using gamma- or beta-decay asymmetry.
While current EDM searches are very susceptible to various environmental noise sources and often have to contend with significant systematic effects, experiments using radioactive isotopes with large intrinsic sensitivity to CP violation will be much less affected by these problems. Therefore, there is a strong expectation that they will be able to make clean EDM measurements; optimistic forecasts suggest that these results might only be limited by the statistical uncertainty determined by the number of available atoms and the integration time. Currently, 225Ra is produced from a radioactive Th source, while 223Rn will be produced with an Isotope Separator On-Line (ISOL) target at TRIUMF using a 50 kW proton beam. Present sensitivity projections indicate that EDM experiments with radioactive isotopes can improve on current EDM limits by about two orders of magnitude using existing sources. As these new experimental techniques for EDM measurements mature, they will need more intense sources to realize their full potential. Existing ISOL targets are limited by thermal effects due to beam heating, but new concepts that can handle higher-power beams, such as tilted targets and beam rastering, are being developed. It will also be crucial for EDM experiments that future facilities have multiuser capabilities and allow months-long data-collection periods. To assist in advancing this frontier, a FRIB should incorporate these characteristics.
Searches for new sources of CP violation are just one example of fundamental interaction studies that can be done with radioactive nuclei. Another important interaction that is still poorly understood is the parity-violating interactions that lead to a nuclear spin distribution called an anapole moment. A nonzero anapole moment has been detected so far in only one nucleus, 133Cs, and its size is not consistent with theoretical estimates. The size of parity violation is enhanced in heavy atoms, making it possible to perform anapole measurements on a string of
Fr isotopes. Additional such measurements would continue to expand the horizons of parity-violation studies in nuclear matter.
The interdisciplinary nature of fundamental interaction studies also leads to a significant stimulus for other branches of physics and science. For example, the experimental techniques for EDM and anapole-moment measurements come largely from atomic physics, while their results will directly affect theoretical particle physics. New experimental techniques developed for these measurements, such as efficient laser trapping and detection of radioactive atoms, have led to significant improvements in radioactive dating and trace-isotope detection.
OTHER SCIENTIFIC APPLICATIONS
Applications of a facility for rare-isotope beams fall into four categories: stockpile stewardship and inertial fusion, medical and biological research, materials science, and advanced fuel development for nuclear power. The chief advantages of a FRIB for these applications are very high isotopic production rates (~100 times those of existing facilities), fairly complete N,Z coverage, and high specific activity. Readers who may be unfamiliar with the material and terms covered in this section are referred to the Glossary (Appendix D).
Because the nation’s nuclear weapons stockpile stewardship program is aimed at maintaining confidence in the U.S. nuclear deterrent without testing, there is greatly increased emphasis on gaining better scientific understanding of all the input information and computational tools used to evaluate the status of the stockpile. In the context of microscopic physics, relevant nuclear data such as cross sections, branching ratios, and transition rates take their place along with other data including radiation opacities and material equations of state in overall detailed assessments of performance uncertainties.5 Because of the extreme operating regimes of nuclear weapons, much of the nuclear input originates from theoretical calculations due to the difficulty in carrying out experiments on unstable nuclear species. This situation led to the consideration of the role for advanced
facilities such as rare-isotope beam facilities that can give experimental access to this unique regime.6
In the specific arena of the application of nuclear physics to stockpile stewardship and (to some extent) inertial fusion, radiochemical analysis is a powerful tool for evaluating performance. In the analysis of the performance of devices, a wide array of nuclear species has been employed and inferences made from the recovery of samples after nuclear tests. In general, understanding the test results required the modeling of the diverse reaction pathways driven by both neutrons and charged particles spanning an energy spectrum from about 0.1 to 16 MeV. Thus, the required cross sections involve processes such as (n,γ), (n,n′), and (n,2n) on the ground, excited, and isomeric states of stable and radioactive isotopes. The yttrium neutron reaction network and its charged-particle entrance and exit branches shown in Figure 2.11 is a fairly typical example. In addition, fission and fission fragment reactions are an important area of study.
All of this is analogous to the r-process, except that (n,2n) is absent because the incident neutron energy is too low. As in astrophysics, high-leverage kinetic paths have been identified and are the subject of many investigations.
As most of the needed cross sections have not been measured, statistical reaction models such as that of Walter Hauser and Herman Feshbach are applied. Such statistical models require parameterized nuclear-level densities, angular momenta, and values for the compound-state pre-equilibration cross sections, adding further uncertainty. These parameterizations are typically obtained by fitting existing experimental data on stable species. Importantly, in many cases where the direct cross section cannot be measured, it is also possible to apply a variant of the compound nucleus ansatz using inverse kinematics on related reactions (known as surrogates), thereby allowing experimental tests of key cross sections. The surrogate method is useful in cases both in which the target lifetime is too short for practical scattering experiments and in which a neutron-scattering source is unavailable.
A facility capable of isotope production rates significantly greater than those now available could improve this situation in two powerful ways. First, rare-isotope experiments can directly measure cross sections on important radioactive
species and also pin down the needed parameters in compound nucleus calculations directly on actual nuclei of interest. Second, in the event that a suitable neutron-scattering source was not available, it would still be possible to extend the surrogate method over a wider range of the relevant (N,Z) space by examining appropriate inverse reactions on unstable species. The main leverage of a high-flux exotic-beam experimental program is likely the ability to pin down a large fraction of the steps in an important network (such as the Y network and its charged particle feeders) as distinct from a few measurements on a handful of nuclei.
Real improvement in the knowledge of relevant cross sections would rely on complementary aspects of both the proposed ISOL and fragmentation options. The main issues are the production and harvesting rate of sufficient isotopes in competition with their decay, and the purity of the collected samples.
Because the stockpile stewardship reaction sets are similar to those needed to study the s- and r-processes, a parasitic collection scheme for radiochemically relevant isotopes, running in parallel with basic science experiments, is in order. As was indicated above, the addition of a monoenergetic, tunable, intense neutron source covering the full energy range would be very useful to the study of the wide variety of (n,x) reactions on the exotic species created at a FRIB. The utility of such a neutron source depends on production rates, target isotope decay times, and the development of both activation analysis and prompt diagnostics (Figure 2.12). From the low-energy (~50 keV) (n,γ) reactions to the higher-energy (n,xn) reactions unique to stewardship (≥3 to 4 MeV), with generic neutron partial (channel specific) cross sections from 0.1 to 1 barn, both high fluences and pure samples are necessary to suppress background. Therefore, for radiochemistry, in contrast to r-process astrophysics, effective experimentation requires high-purity samples relatively near the valley of stability. A neutron source would also be very valuable for s- and r-process studies.
Turning to inertial fusion, radiochemistry is applicable to the determination of the density-radius product of capsules at maximum compression.7 These parameters are inferred from the flux and range of charged particles and neutrons that are made in thermonuclear reactions and react on tracer nuclei placed in the capsule. Because the overall level of radiochemical activation is an integrated function of the entire capsule’s time history, better knowledge of the cross sections will help disentangle the details of the capsule implosion, subsequent ignition, and runaway burn.
Significantly, a FRIB’s greatest impact on the broad national security arena might be through the reinvigoration of low-energy nuclear physics. At present, while stockpile stewardship has a continuing need for people conversant with the phenomenology of nuclear physics, homeland security’s nuclear physics and
nuclear chemistry needs are rapidly growing. The anticipated homeland-security-funded activities could absorb all of the nuclear chemists and many of the nuclear physicists trained in the United States. Unless there is an increase in the number of nuclear physicists, perhaps spurred by a new U.S. initiative in low-energy nuclear physics, there is likely to be a surge in unfulfilled demand before 2010 in the number of such applied scientists and engineers.8
Medical and Biological Research Applications of Radionuclides
The applications of radionuclides to the medical sciences and biological research fall into the three overlapping categories of imaging, targeted therapy, and radiotracers. In each of these areas, radionuclides offer the capability of imaging local conditions as a function of metabolism as well as delivering site-specific therapies.9 In this subsection the committee discusses some of the broader impacts of rare-isotope science; it should be noted that a U.S. FRIB would not serve as a primary element of medical research; rather, it might advance the science of rare isotopes and that, in turn, could have implications for clinical practices.
All three applications mentioned above share the characteristic of requiring isotopes with short lifetimes (<1 day). This is because one wants the radiotracer/ radiopharmaceutical to result in a low integrated dose to the patient, match the lifetime to the metabolic uptake under study, and minimize hazardous waste. Also, rapid-turnaround serial diagnostic tests of patients require short tracer lifetimes. As with other applications of short-lived radionuclides for chemically specific in situ probes, local, high specific activity is also desired, as it leads to the highest site-specific dose.
In contrast to medical imaging done with collimated, externally defined sources as in computerized axial tomography (CAT) scanning, imaging with radioactive species can track the local rate of metabolism or of a biological function. Examples of the latter are positron emission tomography (PET) and single photon
This estimate is supported in an unpublished paper from the Lawrence Livermore National Laboratory. See also a recent study by the nuclear energy industry that projected great difficulty in replacing the expected retirement of more than 23,000 skilled workers in the next decade (available at http://www.nei.org/index.asp?catnum=3&catid=1295; accessed January 26, 2007). See additional discussion in Chapter 4 of this report.
For further reading, see T.J. Ruth and D.J. Schlyer, “The Uses of Accelerator Produced Radioisotopes,” Ch. 2 in Review of the Applications of Isotopes in Medicine and Biology, forthcoming; or N. Oriuchi, T. Higuchi, H. Hanaoka, Y. Ida, and K. Endo, “Current Status of Cancer Therapy with Radio-Labeled Monoclonal Antibodies,” Ann. Nucl. Med. 19, 355-365, 2005.
emission computed tomography (SPECT). Typical isotopes applied to these methods are respectively 11C (T1/2 ~ 20.4 minutes) for PET, and 99mTc (T1/2 ~ 6 hours) for SPECT. The very short lifetimes of the PET nuclei require on-site accelerator production, while the SPECT mainstay 99mTc is primarily made via reactor-produced 99Mo (T1/2 ~ 66 hours).
These examples also typify the trade-offs between reactor and accelerator production of medical isotopes. Reactors are applied to produce radioisotopes either by (n,γ) reactions in target cells or by the harvesting of fission fragments. Their advantages of low cost and parasitic collection are weighed against several disadvantages, including contamination of samples with multiple isotopes of the same element, resulting in low specific activity; lifetime limitations on the distance to the point of application; and the inability to make some isotopes. In contrast, accelerators have long offered the possibility of using charged-particle reactions to drive production, as well as the applicability of in-flight product filtering to produce high specific activities. The main drawbacks of accelerator production are its high cost and low overall production rates. Of course, one should not assume that a FRIB would produce isotopes at a commercially viable level, but it certainly could produce specific activities that readily allow useful research on applied topics. For instance, a recent experiment in Europe using a novel radioisotope produced at the CERN ISOLDE facility showed significant enhancement in cancer-drug effectiveness; see Appendix E for details.
Moving to radiopharmaceutical therapy, there is a variety of radioactive “scalpels” in various stages of development. Beginning with Goldenberg’s original work in 1978, the basic idea is to attach appropriate radionuclides to compounds that are preferentially taken up at the target site (e.g., localized lymphoma cells), and emit decay products (alpha, beta, Auger electron) with appropriate specific activities and range and energy-loss characteristics for the type of diseased tissue in question.
As with other applications, the main advantages of the proposed facility for rare-isotope beams for both imaging and radiopharmaceuticals are both the very high isotopic production rates (estimated at approximately 10 times greater than at ISOLDE or TRIUMF) at high specific activity and the complete coverage of almost all candidate nuclei. Given the enormous production rates, parasitic harvesting of appropriate radioisotopes may be attractive.
Materials Science Applications of Radionuclides
Generally speaking, rare isotopes have broad applications in condensed-matter and materials science as low-density, very-high-signal-to-noise in situ detectors of local atomic environments. Radioactive isotopes offer the synergistic vir-
tues of chemical specificity with the emission of decay products (gamma, beta) whose angular and spectral content can carry a faithful imprint of local field gradients and crystalline anisotropy. Examples include varieties of photoluminescence of implanted ions, perturbed angular correlation gamma decays, Mössbauer spectroscopy, beta-nuclear magnetic resonance (beta-NMR; see the Glossary in Appendix D), and electron (beta) channeling.10 Radioactive probes can give improvement of many orders of magnitude over conventional probes in detectable defect or impurity densities.
In several respects, beta-NMR exemplifies the development of this field and the key role of very high specific activity beams. It is natural to compare beta-NMR with the established technique of muon-spin-resonance (µSR). Both offer as much as a 10-orders-of-magnitude improvement in signal over conventional NMR, through the combination of high polarization and beta-decay anisotropy. They therefore can probe “rare” structures, such as superconducting vortices, local magnetic relaxation, and behaviors at nanostructure material interfaces. However, unlike muons, which are produced well polarized, in beta-NMR one usually needs to produce high-purity beams of the requisite nuclei, then polarize and implant them. This has only recently been possible with the ISOL method and has now been successfully implemented at TRIUMF.11 Beta-NMR has the advantage over µSR because of the former’s much higher intensities of implantable ions and because the nuclei have much longer lifetimes.
The study of semiconductors is another key application of radionuclides, where their potential for detecting low-density crystalline defects, impurities, and weak doping gradients is proving very important in the development of higher-performance materials.
The great potential of radioactive probes for materials science is currently limited by the capacity to produce pure isotopes. There is, potentially, a very large materials science user community for these applications. Other key issues are the polarization of the beam—it must be quite high—and the intensity requirements of 106 particles per second; the latter is not as challenging as the need for the availability of significant beamtime. Typical experiments require systematic stud-
ies of many samples as a function of temperature, magnetic fields, pressure, and so on and do not benefit from higher intensities. Hence, a new facility for rare-isotope beams would be of great value for these applications if it met certain requirements for multiuser capabilities and offered long run times.
Exotic-Beam Applications to Advanced Reactor Fuel Cycles for Transmutation of Waste
The transmutation of waste as a key part of future nuclear power fuel cycles is an active area of study in the United States, Japan, Western Europe, Russia, India, and China. Given the likely future growth of fission power, ideas such as fast neutron reactors and accelerator transformation of waste (ATW) for the mitigation of long-lived radioactive waste will certainly be investigated with much greater urgency. Both fast neutron reactors and ATW use high-energy neutrons either to burn or to irradiate waste, thereby favoring fission over (n,γ) processes causing the net destruction of unwanted actinides. In order to accomplish this goal, however, a wide variety of neutron cross sections, including many on unstable neutron-rich isotopes, are required for the improved designs of the detailed operating regime, determining the required levels of isotopic separation and purity. Many of the required cross sections could be measured at a rare-isotope facility in a manner analogous to needs for stockpile stewardship and astrophysics, either by using direct neutron reactions (if available) or by application of the surrogate method. For an application such as this one, the utility of a rare-isotope facility is not in its production of highly exotic nuclei but in the high-volume production of isotopes from which high-precision cross sections can be extracted.