  A spectral decomposition method has been implemented to identify and quantify
isotopic source terms in high-resolution gamma-ray spectroscopy in static
geometry and shielding scenarios. Monte-Carlo simulations were used to build
the response matrix of a shielded high purity germanium detector monitoring an
effluent stream with a Marinelli configuration. The decomposition technique was
applied to a series of calibration spectra taken with the detector using a
multi-nuclide standard. These results are compared to decay corrected values
from the calibration certificate. For most nuclei in the standard ($^{241}$Am,
$^{109}$Cd, $^{137}$Cs, and $^{60}$Co) the deviations from the certificate
values were generally no more than $6$\% with a few outliers as high as $10$\%.
For $^{57}$Co, the radionuclide with the lowest activity, the deviations from
the standard reached as high as $25$\%, driven by the meager statistics in the
calibration spectra. Additionally, a complete treatment of error propagation
for the technique is presented.
