Neutron activation diagnostics at the National Ignition Facility (invited)

Abstract

Neutron yields are measured at the National Ignition Facility (NIF) by an extensive suite of neutron activation diagnostics. Neutrons interact with materials whose reaction cross sections threshold just below the fusion neutron production energy, providing an accurate measure of primary unscattered neutrons without contribution from lower-energy scattered neutrons. Indium samples are mounted on diagnostic instrument manipulators in the NIF target chamber, 25-50 cm from the source, to measure 2.45 MeV deuterium-deuterium fusion neutrons through the (115)In(n,n')(115 m) In reaction. Outside the chamber, zirconium and copper are used to measure 14 MeV deuterium-tritium fusion neutrons via (90)Zr(n,2n), (63)Cu(n,2n), and (65)Cu(n,2n) reactions. An array of 16 zirconium samples are located on port covers around the chamber to measure relative yield anisotropies, providing a global map of fuel areal density variation. Neutron yields are routinely measured with activation to an accuracy of 7% and are in excellent agreement both with each other and with neutron time-of-flight and magnetic recoil spectrometer measurements. Relative areal density anisotropies can be measured to a precision of less than 3%. These measurements reveal apparent bulk fuel velocities as high as 200 km/s in addition to large areal density variations between the pole and equator of the compressed fuel.

Applications of nuclear physics

“…For this reason the distribution of 85 Kr is not uniform around the globe, but it is useful in examining groundwater known to be less that 50 years old. The very low abundances and low water solubility of 39 Ar, 81 Kr, and 85 Kr, combined with their long half--lives, make measurements of their concentrations in water extremely challenging. For example, the atmospheric 81 Kr/Kr ratio is (5.2+/--0.6)x10 --13 , and one liter of water contains 10 3 atoms of 81 Kr, which corresponds to a decay rate of 4x10 --7 disintegrations per hour [65].…”

“…A detailed discussion of the technical issues involved is presented in the IAEA book Isotopes Methods for Dating Old Groundwater [64]. The combination of their chemical inertness, low water solubility, and half--lives allows one to use the noble gases 39 Ar (t1/2= 269 yr), 81 Kr (t1/2= .23Myr) and 85 Kr (t1/2= 10.76 yr) as probes in a range of Earth science applications [65]. Atmospheric 39 Ar and 81 Kr are produced primarily by cosmic ray spallation.…”

“…A suite of activation pucks located at various 3--D positions around the NIF chamber is used to measure the angular dependence of the 14 MeV neutron emission [39]. Differences in the 14 MeV yields as a function of the (θ,φ) location provide evidence for 3--D differences in the areal density of the capsule fuel.…”

“…Several standard isotopes are used in geochronology and cosmochronology, with some of the most common being 3 H, 14 C, 39 Ar, 36 Cl, 81,85 Tremendous understanding has been achieved in the different fields of isotope chronology over the past century. For example, cosmochronology has mapped out much of the early history of our Solar system using many of these isotope pairs in analysis of meteorites that were present at the time of planet formation.…”

“…Dating groundwater systems is important for assessing the long term availability of the water as well as its recharge rates, refining hydrologic models, and for understanding the transport of and/or vulnerability to pollutants. Several tracer methods have been developed for dating groundwater, including 3 H, 14 C, 36 Cl, 39 Ar 81 Kr, 85 Kr, and 234 U/ 238 U. When in contact with the atmosphere, concentrations of these isotopes reflect atmospheric levels.…”