Observation of high-energy deuterium–tritium fusion gamma rays using gas Cherenkov detectors

Abstract

As fusion ignition conditions are approached using the National Ignition Facility (NIF), independent high-bandwidth gamma-ray fusion-burn measurements become essential complements to neutron-based methods. Time resolution ∼20 ps (10–30 GHz), energy discrimination, and significant stand off distance may be needed for credible burn history measurements. The 16.75 MeV gamma rays that accompany deuterium–tritium (d+t) fusion provide a high-bandwidth alternative to 14 MeV fusion neutrons for d+t burn-history measurements. A thresholding detector, based on the Cherenkov effect, resulting from Compton and pair production electron interactions in gaseous carbon dioxide, offers a means of separating the high-energy fusion gamma rays from other background sources. When coupled to appropriate streak camera recorders, a gas Cherenkov system response of 15 GHz is feasible. As a first step toward this goal, a gas Cherenkov system, employing a 1 GHz photodiode detector, was fielded at the Omega laser facility to demonstrate unambiguous detection of fusion gamma rays from high-yield d+t implosions. With the carbon dioxide Cherenkov production threshold at 12 MeV, the observed high-energy gamma-ray signal is proportional to the observed 14 MeV fusion neutron yield and independent of neutron-induced gamma rays.