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 D-D fusion neutrons through the 115In(n,n')115mIn reaction.  Outside the chamber, zirconium and copper are used to measure 14 MeV D-T fusion neutrons via 90Zr(n,2n), 63Cu(n,2n) and 65Cu(n,2n) reactions.  An array of sixteen 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 may reveal apparent bulk fuel velocities as high as 300 km/s in addition to areal density variations as high as 20% between the pole and equator of the compressed fuel.

Neutron activation diagnostics at the National Ignition Facility

244th National Meeting (2012)

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