ICF reaction rates using fusion gamma rays.$^{1}$

The fusion reaction rate in ICF capsules has been measured with fusion neutrons for a dozen years. The 16.7-MeV gamma rays produced in DT fusion can be used for targets producing $>$10$^{15}$ neutrons. Gamma rays make the measurement independent of target-to-detector distance, but their low branching ratio ($<$10$^{-4})$ makes the task difficult at current ICF facilities where neutron yields are $<$10$^{14}$. A detector system based on a Cherenkov gas cell is being tested at the OMEGA laser facility. Incident gamma~rays produce forward-directed, relativistic electrons in a converter foil at the entrance to a high-pressure (100 psi) gas cell containing CO$_{2}$ gas. The electrons generate Cherenkov light when they travel faster than the speed of light through the CO$_{2}$ gas. Reflective lenses collect the light at the output of the cell and relay it to a streak camera several meters away. Fast, 130-ps fwhm signals have been recorded. Signal width and time correspond to the burn width and bang time recorded with neutrons for the same targets. The relative contribution of hard x rays and gamma rays to these fast signals is being investigated. $^{1}$This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.