![]() The LOS yield measurements show that the neutron angular distributions are generally not isotropic with variations in neutron intensity with direction approaching ☑0%. ![]() The yield along each line-of-sight (LOS) is determined from the shot-induced activity after correcting each measurement for the loss of neutrons to intervening material in the LOS. The shot-induced activity is inferred by extrapolating the measured counts back to shot time. The 89Zr decays by emitting a 909.2 keV gamma with a 78.4-h half-life allowing the disks to be removed after the shot and taken to a separate facility to be counted in sequence by a single high-resolution Ge spectrometer. The 12 MeV threshold for this reaction naturally favors the interaction of 14 MeV neutrons over lower energy scattered neutrons. 2 DT neutrons from the shot create unstable 89Zr in the disks through the reaction 90Zr(n, 2n) 89Zr. Neutron angular distributions were first measured at NIF by placing Zr disks against the port covers of the NIF target chamber (TC) at 15 to 20 locations around the chamber. ![]() This paper will describe the RTNAD hardware and analysis procedures. In addition, these data combined with measurements of the down-scattered neutrons can be used to derive fuel areal density distributions. The large number of detectors and the high statistical accuracy of the array enable the spherical harmonic modes of the neutron angular distribution to be measured up to L ≤ 4 to provide a better understanding of implosion dynamics. Automatic analysis algorithms were developed to handle the non-stop data stream. The neutron flux along the detector line-of-sight at shot time is determined by extrapolating the fitted 909 keV decay curve back to shot time. The neutrons produce activated 89Zr through an (n, 2n) reaction on 90Zr, which is insensitive to low energy neutrons. The measured decay rate dramatically increases during a DT implosion in proportion to the number of 14 MeV neutrons striking each Zr cup. The array consists of 48 LaBr 3(Ce) crystal gamma-ray spectrometers mounted outside the NIF target chamber, which continuously monitor the slow decay of the 909 keV gamma-ray line from activated 89Zr located in Zr cups surrounding each crystal. ![]() The uniformity of the neutron distribution is an important indication of implosion symmetry and DT shell integrity. The Real Time Nuclear Activation Detector (RTNAD) array at NIF measures the distribution of 14 MeV neutrons emitted by deuterium-tritium (DT) fueled inertial confinement fusion implosions. ![]()
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