Scattered and (n,2n) neutrons as a measure of areal density in ICF capsules

The fraction of low-energy neutrons created from 14 MeV neutrons by elastic scattering and (n,2n) reactions on D and T has been proposed as a measure of the areal density 〈ρr〉 (radial integral of density) of ICF targets. In simple situations the fraction of neutrons between 9.4 (the upper energy of T+T neutrons) and 13 MeV (below the Doppler broadened 14.1 MeV peak) is proportional to the 〈ρr〉 at the time of neutron production. This ratio does not depend upon the temperature of the fuel, as does the number of reaction-in-flight neutrons. The ratio of neutrons elastically scattered at a specific energy (e.g. 13 MeV) to the total number of neutrons can be measured along different lines of sight. The ratio of two perpendicular measurements provides a quantitative measure of 〈ρr〉 asymmetry. A detector can be placed inside the target chamber to measure these low-energy neutrons. If it is close enough to the target that measurements are made before the 14 MeV neutrons reach the chamber wall, gamma rays can be a negligible background. Calculated gamma ray and scattered neutron backgrounds from a cryogenic target support or a typical diagnostic instrument also do not calculate to pose a problem. A GaAs detector 2.5 m from the target in the NIF chamber appears to have enough sensitivity and sufficiently rapid time response to make this measurement, but measurement of a weak signal (∼11000) after a strong 14 MeV pulse needs to be tested.