Studying nuclear astrophysics at NIF

When scientists at the stadium-sized National Ignition Facility (NIF) attempt to initiate fusion next year, 192 powerful lasers will direct 1.2 MJ of light energy toward a 2-mm-diameter pellet of deuterium (²H, or D) and tritium (³H, or T). Some of that material will be gaseous, but most will be in a frozen shell. The idea is to initiate “inertial confinement fusion,” in which the two hydrogen isotopes fuse to produce helium-4, a neutron, and 17.6 MeV of energy.

The light energy will be delivered to the inside walls of a hohlraum, a heavy-metal, centimeter-sized cylinder that houses the pellet. The container’s heated walls will produce x rays that impinge on the pellet and ablate its outer surface. To conserve momentum, the outward-moving ablated particles must compress the pellet and its DT fuel. Ultimately, a hot spot develops at the pellet’s center, where fusion produces ⁴He nuclei that have sufficient energy to propagate outward and generate additional heating that triggers successive reactions and, finally, causes the frozen shell to react. Ignition should last several tens of picoseconds and generate more than 10 MJ of energy and roughly 10¹⁹ neutrons. The temperature will exceed 10⁸ K, and fuel will be compressed to a density of several hundred g/cm³, both considerably greater than at the center of the Sun.

The figure shows the major components of NIF. The extreme conditions that will be produced there are intended to simulate those in nuclear weapons and inside stars. The facility is thus an important part of the US Department of Energy’s Stockpile Stewardship Program, designed to assess the nation’s aging nuclear stockpile without doing nuclear tests. In this Quick Study we consider a third application of NIF—using the extraordinary conditions it will produce to perform experiments in basic science. We will focus on measurements of some of the nuclear reaction probabilities that are important to nuclear astrophysics, the field that relates energy production and nucleosynthesis from nuclear reactions in stars and in the Big Bang to the environments in which those nuclear reactions occur. NIF, unlike previous nuclear physics facilities, will enable measurements of nuclear reactions at temperatures, densities, and ionization states similar to those that occur in stars.

No need for large extrapolation

Data obtained in conventional nuclear experiments at accelerators must be corrected or extrapolated if they are to be applied to stellar environments. For example, to address the Coulomb repulsion between charged particles, experimenters at accelerators typically measure reaction probabilities at relatively high energies and then extrapolate them—often over many orders of magnitude—to the lower energies at which stars operate. Moreover, in contrast to the nuclei in accelerator targets, light nuclei in stellar plasmas are highly ionized. That discrepancy requires another set of corrections. At NIF, the conditions in the target will be much more like those in the cores of stars.

In the thermal-plasma environment of a star, electrons shield the positive charges of interacting nuclei and thus reduce the Coulomb repulsion. Screening occurs in accelerator experiments too—there it involves atomic electrons. Accelerator-based data often require rather large corrections for electron screening or for excited nuclear states present in the stellar plasma. Corrections will also be necessary for the NIF pellet, but they will be qualitatively different. Indeed, NIF experiments can help test the accuracy of shielding corrections that investigators have applied to accelerator results.

Still, nuclear astrophysicists will need to resolve both experimental and conceptual issues if they are to obtain useful information from NIF. One such issue concerns the short duration of NIF shots, which last less than 0.1 ns. To what extent does that minuscule time allow thermal equilibrium to be established and thus for the NIF environment to simulate that of stars? Conventional accelerator experiments determine reaction probabilities at many energies, which must then be averaged over the energy range that would characterize a thermal environment. If thermal equilibrium is established in the NIF target, that energy averaging would be performed automatically. In DT shots, however, the high-energy neutrons produced in the nuclear reactions certainly will not achieve thermal equilibrium. In some experiments, thermal equilibrium may occur, but in order to produce useful data, scientists will need to confirm that it does. In other experiments, thermal equilibrium is not crucial for obtaining meaningful results.

Slow burn

One nuclear reaction that scientists hope to study with NIF is indicated in the lower-right corner of the figure—proton capture by nitrogen-14 to yield oxygen-15. That reaction, the slowest one in the so-called primary CNO (carbon-nitrogen-oxygen) cycle, is the main bottleneck to hydrogen burning in stars somewhat more massive than the Sun. Reducing the uncertainties in the reaction rate would allow for improved age determinations of globular clusters—that is, clusters of stars that were all created shortly after the birth of our galaxy. Astronomers can identify the most massive cluster stars that are still burning their core H and determine their ages with the help of stellar evolution codes. The age of those stars and thus of the globular cluster provides a lower bound on the age of the universe.

The proton-capture reaction has been studied down to about 70 keV. However, experiments that have been run at low energies do not agree; newly determined reaction probabilities are smaller than the older ones by about 40%. The target for the NIF experiment would include H and ¹⁴N together, possibly in the form of ammonia. Investigators should be able to determine the yield of the product ¹⁵O by observing its decay in material collected after the shot. Nuclear reactions are very sensitive to temperature, so one can introduce additional nuclei such as lithium-6 or boron-10 in the NIF target at trace abundances, then use their proton-capture yields to determine the temperature at which the principal reaction occurs.

NIF scientists will also be able to study the stellar s-process, a series of slow neutron-capture reactions and β decays that synthesize half the nuclei above iron. The β decays occur much more rapidly than the captures until the process arrives at a “branch point” isotope for which the neutron-capture and β-decay rates compete. Because the capture probabilities depend on the neutron density, a branch point provides a measurement of that density. However, the hot s-process plasma includes low-lying excited states of at least some branch-point nuclei, and those states modify the capture probabilities. Experiments that don’t access the excited states will not correctly measure the stellar neutron-capture rates that are relevant to the s-process.

Thulium-171, for example, is a branch-point nucleus with an excited state at 5.025 keV above the ground state. The excited state, which is populated in the s-process environment, will also be populated in the NIF target. Thus NIF will, for the first time, enable measurement of the effective ¹⁷¹Tm neutron-capture probability, that is, the probability appropriate for the superposition of the ground and excited states of ¹⁷¹Tm found in an s-process plasma. To make the measurement, an experimenter could add ¹⁶⁹Tm and ¹⁷¹Tm to a target and collect their neutron-capture reaction products after the NIF shot. The neutron-capture probability for ¹⁶⁹Tm is known, so the ratio of ¹⁷²Tm to ¹⁷⁰Tm—obtained from β decays—determines the effective ¹⁷¹Tm neutron-capture probability. As with the ¹⁴N proton-capture reaction, one can add trace amounts of other nuclei to the target to determine the temperature. The temperature can also be deduced from the width of the 14-MeV neutron peak obtained when D and T fuse to yield ⁴He and a neutron.

Out with a bang

NIF may also enable scientists to test some aspects of Big Bang nucleosynthesis. BBN calculations predict the primordial abundances of ²H, ³He, and ⁴He fairly well at the universal baryon-to-photon ratio determined from the 2.7-K cosmic microwave background fluctuations. However, the predicted abundance of ⁷Li is three times higher than observed, and that of ⁶Li is two to three orders of magnitude lower. Physicists have made heroic efforts to reconcile the ⁷Li discrepancy but have not been completely successful. The observed ⁶Li abundance is less robust than that for ⁷Li, and astronomers are not in complete agreement about its value.

Although the density and time scales are different, the nuclei and reactions for BBN and for a NIF DT ignition shot are identical. Thus measurement of the ⁶Li, ⁷Li, and beryllium-7 (a BBN product that decays to ⁷Li) produced in a NIF shot could test scientists’ understanding of the processes that produce them. To eliminate background from the hohlraum, the experiment will require that NIF’s laser beams impinge directly on the target. And because Li is a common impurity, experimenters will have to design a system that collects only material generated at the target. The experiment, though, is important and ultimately might suggest that new physics is required to reproduce the observed primordial Li abundances.

NIF will provide unprecedented environments for nuclear astrophysics and several other fields of astrophysics when it begins its full facility tests later this year. We hope it motivates physicists to explore ways to stretch the boundaries of science.

This is article number LLNL-JRNL-414484.