Investigation of the use of secondary protons and neutrons for studying fuel areal density in imploded, D_2-filled capsules

Data from experiments on OMEGA and from Monte-Carlo simulations illustrate the perils of using yields of secondary neutrons (Y_2n) or protons (Y_2p) (per primary neutron) to infer fuel areal density (ρR_fuel) of imploded, D_2-filled capsules. Problems include artificially diminished Y_2p-implied ρR_fuel due to Y_2p saturation at high ρR_fuel and to mix; artificially enhanced Y_2n-implied ρR_fuel due to effects of mix; and uncertainties due to the temperature dependence of Y_2n. As a result, ρR_fuel values deduced from Y_2p and Y_2n using simple models may disagree with each other and may even both be wrong. We show that Y_2p- and Y_2n-implied values of ρR_fuel agree with each other in experiments with high temperature and low ρR_fuel, where the problems mentioned above should not be important, and we use simulations to estimate how the secondary yields and spectra should be interpreted in these and other cases. This work was performed in part at the LLE National Laser Users' Facility, and supported in part by the US DoE (contract W-7405-ENG-48 with LLNL, grant DE-FG03-99DP00300 and Cooperative Agreement DE-FC03-92SF19460), LLE (subcontract P0410025G), and LLNL (subcontract B313975).