Use of 41Ar production to measure ablator areal density in NIF beryllium implosions

For the first time, Ar-41 produced by the ( n,gamma) reaction from Ar-40 in the beryllium shell of a DT filled Inertial Confinement Fusion capsule has been measured. Ar is co-deposited with beryllium in the sputter deposition of the capsule shell. Combined with a measurement of the neutron yield, the radioactive (41) Ar then quantifies the areal density of beryllium during the DT neutron production. The measured 1.15 +/- 0.17 x 10 (vertical bar) (8) atoms of (41) Ar are 2.5 times that from the best post-shot calculation, suggesting that the Ar and Be areal densities are correspondingly higher than those calculated. Possible explanations are that (1) the beryllium shell is compressed more than calculated, (2) beryllium has mixed into the cold DT ice, or more likely (3) less beryllium is ablated than calculated. Since only one DT filled beryllium capsule has been fielded at NIF, these results can be confirmed and expanded in the future. VC 2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license.