Title: Naturally occurring $^{32}$Si and low-background silicon dark matter detectors,
Abstract: The naturally occurring radioisotope $^{32}$Si represents a potentially
limiting background in future dark matter direct-detection experiments. We
investigate sources of $^{32}$Si and the vectors by which it comes to reside in
silicon crystals used for fabrication of radiation detectors. We infer that the
$^{32}$Si concentration in commercial single-crystal silicon is likely
variable, dependent upon the specific geologic and hydrologic history of the
source (or sources) of silicon "ore" and the details of the silicon-refinement
process. The silicon production industry is large, highly segmented by refining
step, and multifaceted in terms of final product type, from which we conclude
that production of $^{32}$Si-mitigated crystals requires both targeted silicon
material selection and a dedicated refinement-through-crystal-production
process. We review options for source material selection, including quartz from
an underground source and silicon isotopically reduced in $^{32}$Si. To
quantitatively evaluate the $^{32}$Si content in silicon metal and precursor
materials, we propose analytic methods employing chemical processing and
radiometric measurements. Ultimately, it appears feasible to produce silicon
detectors with low levels of $^{32}$Si, though significant assay method
development is required to validate this claim and thereby enable a quality
assurance program during an actual controlled silicon-detector production
cycle.