  The selection of low-radioactive construction materials is of the utmost
importance for rare-event searches and thus critical to the XENONnT experiment.
Results of an extensive radioassay program are reported, in which material
samples have been screened with gamma-ray spectroscopy, mass spectrometry, and
$^{222}$Rn emanation measurements. Furthermore, the cleanliness procedures
applied to remove or mitigate surface contamination of detector materials are
described. Screening results, used as inputs for a XENONnT Monte Carlo
simulation, predict a reduction of materials background ($\sim$17%) with
respect to its predecessor XENON1T. Through radon emanation measurements, the
expected $^{222}$Rn activity concentration in XENONnT is determined to be
4.2$\,(^{+0.5}_{-0.7})\,\mu$Bq/kg, a factor three lower with respect to
XENON1T. This radon concentration will be further suppressed by means of the
novel radon distillation system.
