Fast Pb-glass neutron-to-light converter for ICF target burn history measurements

Abstract

We are developing a streak-camera-based instrument to diagnose the fusion reaction rate (burn history) in laser-driven ICF targets filled with D-T fuel. Recently, we attempted measurements using the 16.7 MeV gamma ray emitted in the T(d,γ)5He fusion reaction. Pb glass acts as a gamma-ray-to-light converter. Gamma rays interact in the glass to form electron-position pairs that produce large amounts (1000 photons/gamma ray) of prompt (<10 ps) Cerenkov light as they slow down. In our experimental instrument, an f/10 Cassegrain telescope optically couples light produced in the converter to a streak camera having 20 ps resolution. Experiments using high-yield (1013 D-T neutrons), direct-drive targets at Nova produced good signals with widths of 200 ps. Time-of-flight measurements show the signals to be induced by neutrons rather than by gamma rays. The Pb glass appears to act as a fast neutron-to-light converter. We continue to study the interaction process and the possibility of using the 16.7 MeV gamma rays for burn time measurements.