• whether massive neutrinos play a crucial role in cosmology. What detectors might nuclear physicists construct for this purpose?
  • Our Earth and its rich biology depend on the many heavy elements synthesized during stellar evolution and in violent events like supernovae. What are the nuclear processes responsible for nucleosynthesis, and when and where do they take place?
  • What exotic forms of nuclear matter exist at the extraordinary densities characteristic of neutron stars? What connections can be established between the observed properties of such stars—their masses, radii, rotation rates, electromagnetic emissions, and so forth—and the behavior of nuclear matter under exotic conditions?
  • As the early universe cooled, a hot plasma of unconfined quarks and gluons coalesced into a gas of mesons and nucleons. Can ultrarelativistic heavy-ion collisions provide new insight into the consequences of this phase transition?
  • Earth is bathed in a sea of cosmic radiation, much of it emanating from nuclear processes occurring in our galaxy. How can further measurements of nuclear properties—lifetimes, gamma-ray lines, and so forth—help in determining the origin and consequences of this radiation? How can we exploit unstable nuclei as cosmological clocks of past events in our galaxy?

The efforts under way to address these challenges are described below.

The Solar Neutrino Problem

Stars, to sustain themselves against the force of gravity, must maintain the pressure of their gases by constantly producing energy. In our Sun, this energy is generated in a series of nuclear fusion reactions in which four hydrogen atoms are converted into helium. These reactions take place deep in the solar core, where temperatures are sufficiently high to allow nuclear fusion to occur. Although we cannot see into the solar core by conventional means, these reactions do produce one form of radiation, neutrinos, to which the Sun is transparent. Passing through the cooler outer layers of the Sun without scattering, these neutrinos carry, in their flux and energy distribution, a detailed record of the reactions by which they were produced. Thus, they offer a unique opportunity to view the nuclear processes that power stars like our Sun.

But the reason neutrinos can pass so easily through the Sun—their remarkably weak interactions with matter—also means that detecting them on Earth is a formidable experimental challenge. After almost three decades of effort, the tools to answer that challenge may be in hand. In the summer of 1965, a group of nuclear scientists began excavations for the first experiment, deep within the Homestake gold mine in Lead, South Dakota (see Box 5.1). With a detector filled with 610 tons of cleaning fluid, the experimentalists patiently waited for rare reactions of neutrinos that would convert a chlorine atom into argon. Because

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