  LUX-ZEPLIN (LZ) is a second-generation direct dark matter experiment with
spin-independent WIMP-nucleon scattering sensitivity above $1.4 \times
10^{-48}$ cm$^{2}$ for a WIMP mass of 40 GeV/c$^{2}$ and a 1000 d exposure. LZ
achieves this sensitivity through a combination of a large 5.6 t fiducial
volume, active inner and outer veto systems, and radio-pure construction using
materials with inherently low radioactivity content. The LZ collaboration
performed an extensive radioassay campaign over a period of six years to inform
material selection for construction and provide an input to the experimental
background model against which any possible signal excess may be evaluated. The
campaign and its results are described in this paper. We present assays of dust
and radon daughters depositing on the surface of components as well as
cleanliness controls necessary to maintain background expectations through
detector construction and assembly. Finally, examples from the campaign to
highlight fixed contaminant radioassays for the LZ photomultiplier tubes,
quality control and quality assurance procedures through fabrication, radon
emanation measurements of major sub-systems, and bespoke detector systems to
assay scintillator are presented.

Instrumentation and Detectors;High Energy Physics - Experiment