Exploiting the 27Al(n,alpha)24Na reaction to supplement RTNAD yield and hot-spot velocity measurements at the NIF

The RTNAD[1] (Real Time Neutron Activation Diagnostic) array measures neutron angular distributions from Inertial Confinement Fusion implosions at the National Ignition Facility by monitoring the decay of 89Zr produced in the (n,2n) reaction of 14 MeV neutrons on 90Zr. The cross section for this reaction rises rapidly for neutrons around 14 MeV causing an increase in apparent yield in the direction of the hot-spot center of mass velocity[2] (and a decrease in the opposite direction) which must be corrected using velocity measurements from the Neutron Time-of-Flight detectors. The corrected yield distribution is subject to errors in the velocity measurement. The RTNADs also observe the decay of 24Na produced by the (n, alpha) reaction on 27Al the bulk of which is in the target chamber wall. The cross section for this reaction is nearly flat around 14 MeV and requires no velocity correction. Thus, we can use the data from both reactions to untangle shell morphology measurements from hot-spot velocity effects.

[1] R. M. Bionta, G. P. Grim, K. D. Hahn, E. P. Hartouni, E. A. Henry, H. Y. Khater, A. S. Moore, D. J. Schlossberg, “Real-Time Nuclear Activation Detectors for Measuring Neutron Angular Distributions at the National Ignition Facility”, Rev. Sci. Instrum. 92, 043527 (2021).

[2] Hans G. Rinderknecht, R. Bionta, G. Grim, R. Hatarik, H. Khater, D. Schlossberg, and C. Yeamans, “Velocity correction for neutron activation diagnostics at the NIF”, Rev. Sci. Instrum. 89, 10I125 (2018).