  We present comprehensive measurement data on the pump and collection beam
parameters necessary to achieve high collection efficiency ($89.0 \pm 1.7 \%$)
together with high brightness when a single $\beta$-Barium Borate crystal is
operated in the thick-crystal regime and pumped with a narrow linewidth laser
source. Spectral analysis of the collinear, non-degenerate photons suggest that
the effective interaction length within the crystal is dominated by the
collection beam mode and the use of longer crystals with increased spatial
walk-off does not necessarily lead to a reduced collection efficiency. This
result is an important consideration for optical designers who seek to develop
practical photon pair sources.
