The fluence of high‐energy (>14 MeV) ‘‘secondary’’ neutrons, produced in D–T reactions involving suprathermal fuel ions created by elastic scattering with 14‐MeV ‘‘primary’’ neutrons, yields information about the fuel density‐radius product (&rgr;R) and the hydrodynamic stability of inertial‐confinement fusion (ICF) targets. The suprathermal ions, produced in proportion to the fuel &rgr;R, create secondary neutrons with energies ≤30 MeV when fusing with a thermal ion. The ratio of secondary to primary neutrons determines the mean fuel &rgr;R. Since secondary neutrons, which are mainly formed near the fuel tamper interface with energies >24 MeV, are radially directed, the detection of their angular distribution provides a unique measurement of the deviation from spherical geometry of the fuel &rgr;R. Valid for fuel &rgr;R>0.1 g/cm2and D–T yield above 1014, this diagnostic would be valuable in upcoming ICF experiments.