Nuclear Physics (1986) / Chapter Skim
Currently Skimming:
5 Nuclear Astrophysics
5 Nuclear Astrophysics
Pages 107-119
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 107... ...
Some of the most active areas of nuclear astrophysics today are concerned with the mechanisms of supernova explosions, where nucleosynthesis of the heavy elements occurs, and the formation of neutron stars. The latter represent nuclear matter under conditions of high temperature and density, from which a unique insight can be gained on the fundamen 107
|
From page 108... ...
In this chapter we look at a few of the most active current topics in nuclear astrophysics research, which epitomize the ways in which progress in basic nuclear physics benefits the development of other sciences and, ultimately, of our technological society as a whole. NUCLEI UNDER EXTREME ASTROPHYSICAL CONDITIONS The most extreme condition of matter imaginable existed for only an instant at the beginning of our universe, but a plausible account of this awesome event and its aftermath has been reconstructed from data available today.
|
From page 109... ...
Measurements of nuclear spallation reactions at cosmic-ray energies have recently become sufficiently extensive to allow a meaningful test of the astrophysical model, and it has been found that these cosmic-ray nuclides are produced in roughly their observed relative cosmological abundances. The four big bang nuclides mentioned above are the only four that can be attributed to that stage of the evolution of the universe.
|
From page 110... ...
FIGURE 5.1 From the observed abundances of the four big bang nuclides, it is possible to infer the present baryon density of the universe. The shaded bar for each nuclide represents the range of values calculated from its abundance, and the solid vertical line represents the best fit to these data.
|
From page 111... ...
The increased neutrino flux produced by the collapsing star increases the rate of energy loss by the star; this, in turn, decreases its internal pressure and hastens the collapse. At a later stage of the collapse, however, neutrinos will become trapped inside the star because of the greatly increased mass density of the star, which decreases its transparency to neutrinos; this trapping inhibits further electron capture and halts the synthesis of heavy elements.
|
From page 112... ...
field, the supernova collapse is eventually halted by the repulsive part of the strong force at very short internucleon distances. One effect of this compression to about twice normal nuclear density is an intense, rebounding pressure wave that forms a gigantic, outward-moving shock wave.
|
From page 113... ...
The ability of the shock wave to blast away the outer layers, for example, depends critically on the temperature, density, and composition of the original star; these factors are, in turn, highly sensitive to the rates of electron capture by the various nuclei present and to the rate of cooling by the accompanying neutrino emission. Refining the model is hampered by inadequate knowledge of the properties of nuclei and of the equation of state of hot, dense nuclear matter.
|
From page 114... ...
Recent theoretical work on beta decay of nuclei far from stability has emphasized the role of the spacing of highly excited energy levels in the product nucleus. The half-life for beta decay is quite sensitive to this quantity, and the half-lives are a crucial ingredient for calculating the production of heavy elements in supernovas.
|
From page 115... ...
In 1970 a solar-neutrino detector built by Brookhaven National Laboratory began operating in a South Dakota gold mine, a mile underground to help shield against cosmic-ray background counts. In experiments carried out during the past 14 years, the average counting rate has been about three neutrino captures per week, roughly one fourth the rate predicted by solar models.
|
From page 116... ...
_ . \ /_ \\ \ ( 037CI ~ ~37Ar/week l FIGURE 5.3 The solar-neutrino experiment being conducted in a South Dakota gold mine (see the text for details)
|
From page 117... ...
sensitively reflects conditions at the core of the Sun. The solar-neutrino problem represents the only major failure of the otherwise extremely successful standard solar model, and this discrepancy between the predicted and measured neutrino counting rates has prompted critical re-examinations of various aspects of solar physics and nuclear physics.
|
From page 118... ...
This is consistent with recent nuclear-physics measurements that suggest that red giant stars and novas are more likely sources of 26Al than are supernovas. Another example of the value of nuclear physics in furthering our understanding of stellar evolution is that of very hot stars, such as white dwarfs.
|
From page 119... ...
. These explosive phases of nucleosynthesis are thought to occur on the surfaces of white dwarfs and neutron stars that are accreting fresh hydrogen on their surfaces.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.