  In low activity gamma spectrometric measurements of large, dense samples, the
bulk sample material shields the HPGe crystal from external background sources.
If not accounted for in studies that utilise background-subtraction methods,
this effect may result in systematic errors in the sample activity and
detection limit estimation. We introduce a Monte Carlo based method to minimise
the impact of this effect on sample gamma spectra. It is validated using
simulated detector backgrounds and applied to a measurement of low-activity
Gd$_2$(SO$_4$)$_3 \cdot$8H$_2$O. One main prerequisite for the correct
application of this method is to know in advance the nuclides which contribute
to the detector background spectrum and their spatial distribution. With a
thorough understanding of the detector backgrounds, the method improves the
accuracy of sensitive low-background measurements of low-activity samples. Even
without knowing the background sources and their distribution, conservative
results may still be presented that account for the potential systematic errors
introduced by this background shielding effect.
