  The MAJORANA DEMONSTRATOR is a neutrinoless double-beta decay
($0\nu\beta\beta$) experiment containing $\sim$30 kg of p-type point contact
germanium detectors enriched to 88% in 76Ge and $\sim$14 kg of natural
germanium detectors. The detectors are housed in two electroformed copper
cryostats and surrounded by a graded passive shield with active muon veto. An
extensive radioassay campaign was performed prior to installation to insure the
use of ultra-clean materials. The DEMONSTRATOR achieved one of the lowest
background rates in the region of the $0\nu\beta\beta$ Q-value, 15.7 $\pm$ 1.4
cts/(FWHM t y) from the low-background configuration spanning most of the 64.5
kg-yr active exposure. Nevertheless this background rate is a factor of five
higher than the projected background rate. This discrepancy arises from an
excess of events from the 232Th decay chain. Background model fits aim to
understand this deviation from assay-based projections, potentially determine
the source(s) of observed backgrounds, and allow a precision measurement of the
two-neutrino double-beta decay half-life. The fits agree with earlier
simulation studies, which indicate the origin of the 232Th excess is not from a
near-detector component and have informed design decisions for the
next-generation LEGEND experiment. Recent findings have narrowed the suspected
locations for the excess activity, motivating a final simulation and assay
campaign to complete the background model.
