Nuclear Physics

NIF will provide a unique environment in which to perform nuclear physics experiments. The key to these measurements will be the details of the spectrum of the extraordinary density of neutrons, possibly as high as 10²⁶ per cm³, which will accompany NIF ignition shots.

Neutron Knockout Reactions

In one case, that of a deuterium-tritium (DT) target pellet, the neutrons will be relatively high energy, 14 MeV, from the ²H+³H–>⁴He+n (fusion) reaction. Classical Image of an Atom This will provide the capability to study multiple nuclear reactions on seed nuclei inserted at a low abundance in the pellet, which have been promoted to excited states by one neutron and then reacted with by another neutron. The second reaction will select out those nuclear states that have lifetimes comparable to the length of the neutron pulse from NIF; states with much shorter lifetimes will decay before they can interact with a second neutron down to states that are longer-lived, and will provide as much time as required for the second reaction to occur. While such reactions are interesting in themselves, they also allow inference of the very short lifetimes – possibly on the order of ten picoseconds or less – of the nuclear states involved.

Neutron Capture Reactions

In another instance the pellet might be pure deuterium doped with a few seed nuclei, in which case the neutrons produced from the 2H+2H->3He+n reaction would have lower energy, around 4 MeV or less. These neutrons would not have sufficient energy to perform the inelastic excitations or the neutron “knockout” reactions discussed above. However, they might instead be captured by the seed nuclei, with the potential to produce nuclei with more neutrons than the seed nuclei had.

This would present an opportunity to perform measurements in which neutrons are captured on nuclear excited states; these have never before been possible. If the excited states had an excitation energy that was comparable to the NIF target temperature, of order 10 keV, they would be populated in the high temperature environment of the NIF target. Then neutron capture would occur on these excited state nuclei as well as on the ground state nuclei. Theoretical estimates have suggested that the resulting “effective neutron capture cross section” could be quite different in high temperature environments from those in low temperature environments.

This could be very important to some aspects of stellar nucleosynthesis and stellar evolution. The s-process (slow) of nucleosynthesis, consisting of neutron captures on nuclei that exist along the neutron-rich edge of stability, occurs mostly during helium burning in stars, the second stage of stellar evolution, and in some cases at the beginning of carbon burning, the third stage. If this occurs during helium burning, the temperatures in the s-process environment are 10-30 keV, whereas if it occurs during carbon burning, the temperature is higher, around 90 keV. In either case, there are some interesting situations that require the neutron capture cross sections on the excited states in order to be understood. One such case involves the Thulium isotopes, shown in the figure. There the path of the s-process has multiple branch points, at each of which either beta decay or capture of another neutron can occur. Since the neutron capture possibility is proportional to the neutron density, full characterization of the neutron capture cross sections and beta decay lifetimes of these nuclei thus affords an opportunity to determine the neutron density in the regions deep inside a star in which this s-process occurs, a stunning possibility. However, several of the Thulium isotopes have excited states that could greatly influence the neutron capture cross section; their effect simply cannot be measured with any facilities other than NIF.

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