  Quantifying the population of nanoscale defects that are formed in metals and
alloys exposed to extreme radiation environments remains a pressing challenge
in materials science. These defects both fundamentally alter material
properties and seed long-timescale performance degradation, which often limits
the lifespan of engineering systems. Unlike ceramic and semiconducting
materials, these defects in metals and alloys are not spectroscopically active,
forcing characterization to rely on indirect measurements from which the
distribution of nanoscale defects may be inferred. In this mini-review,
different experimental methodologies which have been employed for defect
inference are highlighted to capture the current state of the art. Future
directions in this area are proposed, which, by combining data streams from
multiple and complementary characterization methods in concert with multi-scale
modeling and simulation, will enable the ultimate goal of quantifying the full
spectrum of defects in irradiated metals and alloys.
