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LUX-ZEPLIN (LZ) is a second-generation direct dark matter experiment with spin-independent WIMP-nucleon scattering sensitivity above $${1.4 \times 10^{-48}}\, {\hbox {cm}}^{2}$$ 1.4 × 10 - 48 cm 2 for a WIMP mass of $${40}\, \hbox {GeV}/{\hbox {c}}^{2}$$ 40 GeV / c 2 and a $${1000}\, \hbox {days}$$ 1000 days exposure. LZ achieves this sensitivity through a combination of a large $${5.6}\, \hbox {t}$$ 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.

We report a comprehensive study of the energy response to low-energy recoils in dual-phase xenon-based dark matter experiments. A recombination model is developed to explain the recombination probability as a function of recoil energy at zero field and non-zero field. The role of e-ion recombination is discussed for both parent recombination and volume recombination. We find that the volume recombination under non-zero field is constrained by a plasma effect, which is caused by a high density of charge carriers along the ionization track forming a plasma-like cloud of charge that shields the interior from the influence of the external electric field. Subsequently, the plasma time that determines the volume recombination probability at non-zero field is demonstrated to be different between electronic recoils and nuclear recoils due to the difference of ionization density between two processes. We show a weak field-dependence of the plasma time for nuclear recoils and a stronger field-dependence of the plasma time for electronic recoils. As a result, the timedependent recombination is implemented in the determination of charge and light yield with a generic model. Our model agrees well with the available experimental data from xenon-based dark matter experiments.

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