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

Abstract 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 (∼ 1 1000 ) after a strong 14 MeV pulse needs to be tested.