  We present an experimental demonstration of Additive Point Source
Localization (APSL), a sparse parametric imaging algorithm that reconstructs
the 3D positions and activities of multiple gamma-ray point sources. Using a
handheld gamma-ray detector array and up to four $8$ ${\mu}$Ci $^{137}$Cs
gamma-ray sources, we performed both source-search and source-separation
experiments in an indoor laboratory environment. In the majority of the
source-search measurements, APSL reconstructed the correct number of sources
with position accuracies of ${\sim}20$ cm and activity accuracies (unsigned) of
${\sim}20\%$, given measurement times of two to three minutes and distances of
closest approach (to any source) of ${\sim}20$ cm. In source-separation
measurements where the detector could be moved freely about the environment,
APSL was able to resolve two sources separated by $75$ cm or more given only
${\sim}60$ s of measurement time. In these source-separation measurements, APSL
produced larger total activity errors of ${\sim}40\%$, but obtained source
separation distances accurate to within $15$ cm. We also compare our APSL
results against traditional Maximum Likelihood-Expectation Maximization (ML-EM)
reconstructions, and demonstrate improved image accuracy and interpretability
using APSL over ML-EM. These results indicate that APSL is capable of
accurately reconstructing gamma-ray source positions and activities using
measurements from existing detector hardware.
