  The Compton Spectrometer and Imager (COSI) is a balloon-borne compact Compton
telescope designed to survey the 0.2-5 MeV sky. COSI's energy resolution of
$\sim$0.2% at 1.8 MeV, single-photon reconstruction, and wide field of view
make it capable of studying astrophysical nuclear lines, particularly the 1809
keV $\gamma$-ray line from decaying Galactic $^{26}$Al. Most $^{26}$Al
originates in massive stars and core-collapse supernova nucleosynthesis, but
the path from stellar evolution models to Galaxy-wide emission remains
unconstrained. In 2016, COSI had a successful 46-day flight on a NASA
superpressure balloon. Here, we detail the first search for the 1809 keV
$^{26}$Al line in the COSI 2016 balloon flight using a maximum likelihood
analysis. We find a Galactic $^{26}$Al flux of $(8.6 \pm 2.5) \times 10^{-4}$
ph cm$^{-2}$ s$^{-1}$ within the Inner Galaxy ($|\ell| \leq 30^{\circ}$, $|b|
\leq 10^{\circ}$) with 3.7$\sigma$ significance above background. Within
uncertainties, this flux is consistent with expectations from previous
measurements by SPI and COMPTEL. This analysis demonstrates COSI's powerful
capabilities for studies of $\gamma$-ray lines and underscores the scientific
potential of future compact Compton telescopes. In particular, the next
iteration of COSI as a NASA Small Explorer satellite has recently been approved
for launch in 2025.
