  We present here a characterization of the low background NaI(Tl) crystal
NaI-33 based on a period of almost one year of data taking (891 kgxdays
exposure) in a detector configuration with no use of organic scintillator veto.
This remarkably radio-pure crystal already showed a low background in the SABRE
Proof-of-Principle (PoP) detector, in the low energy region of interest (1-6
keV) for the search of dark matter interaction via the annual modulation
signature. As the vetoable background components, such as $^{40}$K, are here
sub-dominant, we reassembled the PoP setup with a fully passive shielding. We
upgraded the selection of events based on a Boosted Decision Tree algorithm
that rejects most of the PMT-induced noise while retaining scintillation
signals with > 90% efficiency in 1-6 keV. We find an average background of 1.39
$\pm$ 0.02 counts/day/kg/keV in the region of interest and a spectrum
consistent with data previously acquired in the PoP setup, where the external
veto background suppression was in place. Our background model indicates that
the dominant background component is due to decays of $^{210}$Pb, only partly
residing in the crystal itself. The other location of $^{210}$Pb is the
reflector foil that wraps the crystal. We now proceed to design the
experimental setup for the physics phase of the SABRE North detector, based on
an array of similar crystals, using a low radioactivity PTFE reflector and
further improving the passive shielding strategy, in compliance with the new
safety and environmental requirements of Laboratori Nazionali del Gran Sasso.
