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 [pr] (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 [pr] 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 he Measured along different lines of sight. The ratio of two perpendicular measurements provides a quantitative measure of [pr] 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, 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 in 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 (similar to 1/1000) after a strong 14 MeV pulse needs to be tested (C) 2002 Elsevier Science B.V. All rights reserved.