Nuclear Physics The Core of Matter, The Fuel of Stars (1999) / Chapter Skim
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Pages 104-127
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From page 104... ...
104 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS 104 5 The Nuclear Physics of the Universe INTRODUCTION: CHALLENGES FOR THE FIELD The rich tradition of collaboration between nuclear physics and astrophysics dates from the early work of Hans Bethe and Willy Fowler, nuclear physicists who won Nobel Prizes for their efforts to understand the nuclear reactions taking place in stars. Today this intersection remains especially vital, driven on the one hand by the rapid technological advances in astronomical observation, and on the other by the need to understand the underlying nuclear and atomic microphysics that govern most astrophysical objects and phenomena.
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From page 105... ...
THE NUCLEAR PHYSICS OF THE UNIVERSE 105 mine whether massive neutrinos play a crucial role in cosmology. What detectors might nuclear physicists construct for this purpose?
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From page 106... ...
106 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS BOX 5.1 Solar Neutrinos from Homestake to the Sudbury Neutrino Observatory In the summer of 1965, workers deep in the Homestake gold mine, Lead, South Dakota, completed the excavation of a 30 × 60 × 32 ft3 cavity at a depth of 4,850 ft. This was the first step in bringing to life a new detector proposed by Ray Davis, Jr., and his Brookhaven National Laboratory collaborators.
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From page 107... ...
THE NUCLEAR PHYSICS OF THE UNIVERSE 107 The chlorine detector was literally 20 years before its time. It exploited a marvelous circumstance: solar neutrinos would convert a few atoms of 37Cl into 37Ar which, because argon is a noble gas, could be quantitatively flushed from a large volume of fluid by a helium-gas purge.
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From page 108... ...
108 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS occurring in heavy water -- neutrino absorption on deuterium to produce two protons and an electron -- can only be initiated by the electron neutrinos, and thus will count the electron neutrino portion of the solar neutrino flux. But a second reaction -- neutrino scattering off deuterium to produce a neutron and a proton -- will count all of the solar neutrinos.
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From page 109... ...
THE NUCLEAR PHYSICS OF THE UNIVERSE 109 cause argon is a noble gas, the few atoms produced, roughly one every two days, could be extracted from the Homestake detector and counted by observing their subsequent decay. This experiment, which continues to operate and improve in accuracy, produced the first evidence that the Sun was not producing as many neutrinos as predicted by our theories of stars and nuclear reactions.
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From page 110... ...
110 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS who first described it. The dependence of this enhancement on the energy of the neutrino provides a natural explanation for the experimental results discussed above.
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From page 111... ...
THE NUCLEAR PHYSICS OF THE UNIVERSE 111 is similar in design, except that the water is "heavy," with the hydrogen replaced by deuterium. This difference will allow the nuclear physicists building the detector to measure solar neutrinos whether or not they have oscillated into a different type (or flavor)
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From page 112... ...
112 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS The resolution of the solar neutrino problem is important not only to nuclear physics, but also to particle physics, astrophysics, and cosmology. In many theories that attempt to generalize our current Standard Model of particle physics, small neutrino masses are related to entirely new physics that is otherwise beyond our grasp: current and planned accelerators fall far short of the energies where this physics can be seen directly.
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From page 113... ...
THE NUCLEAR PHYSICS OF THE UNIVERSE 113 the raw material for a future generation of stars and planets. Indeed, were it not for this continuing synthesis, we would lack the essential elements that make up our Earth and allow it to sustain life.
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From page 114... ...
114 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS us deduce more accurate primordial values from present-day abundance measurements. In the later synthesis that begins with the formation of the first stars, there is a close coupling between the macroscopic astrophysics issues, models of stars and of galactic chemical evolution, and the nuclear microphysics, the networks of nuclear reactions that govern this evolution.
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From page 115... ...
THE NUCLEAR PHYSICS OF THE UNIVERSE 115 and other metals, a signature of a primary process not dependent on preexisting metals. This has focused attention on nucleosynthesis by supernova neutrinos, a process suggested by nuclear physicists: energetic neutrinos scatter off nuclei in the outer layers of the exploding star, knocking out nucleons and transmuting the target nuclei into different elements.
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From page 117... ...
THE NUCLEAR PHYSICS OF THE UNIVERSE 117 tic hierarchy of neutrino temperatures: the muon and tauon neutrinos are about twice as energetic, on average, as the electron neutrinos. It has been emphasized recently that, if the tauon neutrino has a mass large enough to be important in cosmology, oscillations between electron and tauon neutrinos might be expected in supernova explosions.
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From page 118... ...
118 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS and (n,p) reactions at medium-energy nuclear facilities, as described in the next section.
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From page 119... ...
THE NUCLEAR PHYSICS OF THE UNIVERSE 119 the same spin-flip transitions that govern neutrino-nucleus interactions. (The same measurements can be made for radioactive species of interest by illuminating a hydrogen target at a radioactive ion beam facility.)
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From page 120... ...
120 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS detectors. In fact, it led to the discovery that the original cross section estimates for the chlorine detector were based on a flawed assumption; fortunately, it proved possible to correct the error.
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From page 121... ...
THE NUCLEAR PHYSICS OF THE UNIVERSE 121 FIGURE 5.5 At the upper right, the nuclear reaction network of the carbon-nitrogenoxygen (CNO) cycles, including possible pathways for breaking out of these cycles into heavier nuclei, is shown.
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From page 122... ...
122 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS as 68Se. Our current poor knowledge of the disintegration energy of this nucleus makes the stellar lifetime of 68Se uncertain by about a factor of 10,000.
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From page 123... ...
THE NUCLEAR PHYSICS OF THE UNIVERSE 123 FIGURE 5.6 The rp process can be halted by encountering a nucleus that too easily emits protons, and thus lives only a very short time. Nuclides resulting from the breakup of 78Kr in an in-flight radioactive beams facility are shown in the panel above.
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From page 124... ...
124 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS constitutes the outer surface of neutron stars. But as one descends below the surface, the matter is squeezed more and more tightly by gravity (Figure 5.7)
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From page 125... ...
THE NUCLEAR PHYSICS OF THE UNIVERSE 125 features. For example, when sufficiently compressed, the nuclei of this matter could join to form thin and long spaghetti nuclei, which in turn might merge at even higher densities into thin flat sheets of "lasagne." Finally, at a density of about 1014 g/cc, the nuclei are fully dissolved and matter becomes a nuclear fluid.
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From page 126... ...
126 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS neutron stars depend on knowledge of nuclear interactions gained in the laboratory. In the next few years, new progress is expected.
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From page 127... ...
THE NUCLEAR PHYSICS OF THE UNIVERSE 127 OUTLOOK Present efforts in nuclear astrophysics may soon lead to the solution of the solar neutrino problem, the successful modeling of the supernova explosion mechanism, an understanding of the nucleosynthesis of heavy elements, and more quantitative constraints on the structure and dynamics of neutron stars. If the solution of the solar neutrino problem involves massive neutrinos, nuclear physics will have demonstrated the need for physics beyond the current Standard Model of particle physics.
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