Lawrence Livermore National Laboratory
Nuclear Astrophysics
The conditions that are expected to be produced at NIF when it achieves ignition are extraordinarily well matched to the conditions that exist in stars in several phases of their evolution. The temperature and density, 100 million Kelvin (180 million degrees Fahrenheit) and 1,000 grams per cubic centimeter, are definitely in the stellar range, and the neutron density at ignition, possibly as high as 100 septillion (1026) per cubic centimeter, exceeds that of the r-process in stellar nucleosynthesis. It is natural, then, that nuclear astrophysicists would want to perform research at NIF.
Reactions of Stellar Hydrogen Burning
Some of the reactions that could be studied at NIF do not require ignition; they may not even need the deuterium/tritium pellets that will be used in the shots that are directed toward ignition. One such reaction is the 3He+3He 
4He+1H+1H reaction, which is one of the nuclear reactions involved in the main hydrogen burning cycle that powers the sun. The reactions in this main cycle are shown here:
This reaction has a relatively large cross-section, or production probability, at stellar temperatures and densities, so it has been studied to high accuracy in laboratory experiments using low-energy accelerators. Thus it is a good "first experiment" to do at NIF, since it will provide NIF physicists with a test of their ability to do precision measurements with NIF. It is anticipated that the NIF pellet will not contain the 2H and 3H that would normally be used in the ignition shots, but rather would contain just the 3He constituents of this reaction. There would be no neutrons produced in this study. Detection of the protons or of the 4He nuclei produced might be achieved by observing them directly using a magnetic analysis system that has been developed as one of the NIF diagnostics.
Nucleosynthesis experiments designed for the National Ignition Facility will generate a high neutron flux in a short burn time to replicate the process that produces the heavier elements created by a dying star.
Several other reactions associated with hydrogen burning in stars, particularly those of the CNO cycle, may be amenable to study with NIF. These include 12C(p,y)13N and 14N(p,y)15O, which in both cases would involve detecting radioactive reaction products. The latter reaction is particularly interesting, as it is the slowest reaction in the primary CNO cycle, and therefore determines the rate at which hydrogen can be burned in that cycle. This is directly related to the age of the Universe, as it can be determined when the stars in “globular clusters,” clusters of stars that all appear to have been created at the same time soon after the Universe formed, complete their hydrogen burning phase. By means of a code that describes stellar evolution, the ages of all the stars in the globular cluster can then be determined by calculating the ages of the stars that have just finished burning their core hydrogen.
Study of the 59Fe+n 
60Fe+gamma reaction
Recent evidence from the space borne gamma-ray detector INTEGRAL of the existence of 60Fe (half-life = 1.5 My) in the Galaxy (see the figure) has highlighted the importance of measuring the reactions, involving successive neutron captures, in the sequence
58Fe+n 59Fe+n 60Fe +n 61Fe
that synthesize 60Fe in supernovae. Gamma rays resulting from the decay of 60Fe are shown in the figure. Accurate determination of these reaction rates, plus the amount of 60Fe in the Galaxy, then places a strong constraint on models of supernovae, even at the level of the specific locations within the supernovae in which 60Fe is produced1. The 58Fe +n 59Fe reaction has been measured, as 58Fe is stable. However, 59Fe has a half-life of 44.47 days, so measurement of 59Fe +n 60Fe , which might involve making some 59Fe and putting it near an intense neutron source for some time, is not possible, since most of the target 59Fe would decay away during the lengthy exposure. The 60Fe +n 61Fe reaction is also important, as it is the balance between the reactions that make 60Fe and those that destroy it that determines its abundance. That reaction has been studied recently.2
Preliminary calculations suggest that it would be possible to study the
59Fe + n 60Fe reaction from an appropriately designed NIF shot. By making 59Fe from a separate accelerator bombardment, and then using it to dope the NIF target, the neutrons from the NIF target should produce sufficient 60Fe for it to be measured using the LLNL Center for Accelerator Mass Spectrometry facility. Accurate measurement of the neutron spectrum produced in the NIF target would also be required. At present no other way of measuring the 59Fe +n 60Fe reaction cross section appears to be feasible. 
Figure: INTEGRAL/SPI 60Fe signal from the inner Galaxy (Wang et al., arXiv:0704.3895v1 [astro-ph] 30 Apr 2007) Data from both gamma-ray lines (at 1173.23 and 1332.49 keV) from decay of 60Fe have been superimposed. The solid line represents a Gaussian fit with its width fixed from nearby instrumental background lines.
1
M. Limongi and A. Chieffi, Astrophys. J. 647, 483 (2006)
2
E. Uberseder et al., Phys. Rev. Lett. 102, 151101 (2009)
More Information:
Basic Research Directions for User Science at the National Ignition Facility, National Nuclear Security Administration and U.S. Department of Energy Office of Science, November 2011
"Studying Nuclear Astrophysics at NIF," R.N. Boyd, L. Bernstein, and C. Brune, Physics Today 62, no. 8, 60 (2009)
"A Closer Look at Nucleosynthesis," Science & Technology Review, July/August 2007
