Measurement of areal density in the ablators of inertial-confinement-fusion capsules via detection of ablator (n, n′γ) gamma-ray emission

We report the first gamma-ray-based measurements of the areal density of ablators in inertial-confinement-fusion capsule implosions. The measurements, made at the OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)], used observations of gamma rays arising from inelastic scattering of 14.1-MeV deuterium-tritium (DT) neutrons on C-12 nuclei in the compressed plastic ablators. The emission of C-12(n,n'gamma) gamma rays from the capsules is detected using the Gamma Reaction History instrument [H. W. Herrmann et al., J. Phys.: Conf. Ser. 244, 032047 (2010)] operating at OMEGA. From the ratio of a capsule's C-12(n,n'gamma) emission to the emission from the same processes in an in situ reference graphite puck of known mass and geometry [N. M. Hoffman et al., in IFSA 2011 proceedings (submitted)], we determine the time-averaged areal density of C-12 in the capsule's compressed ablator. Measured values of total ablator areal density for thirteen imploded capsules, in the range 23 +/- 10 to 58 +/- 14 mg/cm(2), are comparable to values calculated in 1D radiation-hydrodynamic simulations, and measured by charged-particle techniques. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4799799]