Abstract
The Real Time Nuclear Activation Detector (RTNAD) array at NIF measures the distribution of 14MeV neutrons emitted by deuterium-tritium (DT) fueled inertial confinement fusion implosions. The uniformity of the neutron distribution is an important indication of implosion symmetry and DT shell integrity. The array consists of 48 LaBr3(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 measured decay rate dramatically increases during a DT implosion in proportion to the number of 14MeV neutrons striking each Zr cup. The neutrons produce activated 89Zr through an (n, 2n) reaction on 90Zr, which is insensitive to low energy neutrons. 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. Automatic analysis algorithms were developed to handle the non-stop data stream. 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. In addition, these data combined with measurements of the down-scattered neutrons can be used to derive fuel areal density distributions. This paper will describe the RTNAD hardware and analysis procedures.
Highlights
AND MOTIVATIONSuccessful ignition of Inertial Confinement Fusion (ICF) capsules requires that the shell of deuterium-tritium (DT) ice lining the inside of the capsule be uniformly and symmetrically compressed.1 The uniformity of the compressed shell is reflected in the angular distribution of the 14 MeV neutrons produced by DT reactions in the central hotspot as the emitted neutrons scatter more frequently in regions of higher areal density lowering the number of MeV neutrons observed in that direction.Neutron angular distributions were first measured at NIF by placing Zr disks against the port covers of the NIF target chamber (TC) at to 20 locations around the chamber.2 DT neutrons from the shot create unstable 89Zr in the disks through the reaction90Zr(n, 2n)89Zr
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
The array consists of 48 LaBr3(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
Summary
Successful ignition of Inertial Confinement Fusion (ICF) capsules requires that the shell of deuterium-tritium (DT) ice lining the inside of the capsule be uniformly and symmetrically compressed. The uniformity of the compressed shell is reflected in the angular distribution of the 14 MeV neutrons produced by DT reactions in the central hotspot as the emitted neutrons scatter more frequently in regions of higher areal density lowering the number of 14 MeV neutrons observed in that direction. The uniformity of the compressed shell is reflected in the angular distribution of the 14 MeV neutrons produced by DT reactions in the central hotspot as the emitted neutrons scatter more frequently in regions of higher areal density lowering the number of 14 MeV neutrons observed in that direction. 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.. DT neutrons from the shot create unstable 89Zr in the disks through the reaction. The yield along each line-of-sight (LOS) is determined from the shotinduced activity after correcting each measurement for the loss of neutrons to intervening material in the LOS. The LOS yield measurements show that the neutron angular distributions are generally not isotropic with variations in neutron intensity with direction approaching ±10%. Scitation.org/journal/rsi by permanently mounted detector packages [called RTNADs (Real Time Nuclear Activation Detectors)] and expanded to 48 locations around the chamber
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