  Searches for rare nuclear processes, such as neutrinoless double beta-decay
and the interactions of WIMP dark matter, are motivating experiments with
ever-decreasing levels of radioactive backgrounds. These background reductions
are achieved using various techniques, but amongst the most important is
minimizing radioactive contamination in the materials from which the experiment
is constructed. To this end there have been decades of advances in material
sourcing, manufacture and certification, during which researchers have
accumulated many thousands of measurements of material radiopurity. Some of
these assays are described in publications, others are in databases, but many
are still communicated informally. Until this work, there has been no standard
format for encoding assay results and no effective, central location for
storing them. The aim of this work is to address these long-standing problems
by creating a concise and flexible material assay data format and powerful
software application to manipulate it. A public installation of this software,
available at http://www.radiopurity.org, is the largest database of assay
results ever compiled and is intended as a long-term repository for the
community's data.
