Deuterium abundance in the interstellar gas of the galactic anticenter from the 327 MHz line

The deuterium abundance in the primordial gas formed during the big bang is dependent on the baryon-to-photon ratio during nucleosynthesis. We report a measurement of the deuterium-to-hydrogen (D/H) ratio from a 6 sigma detection of the hyperfine ground-state transition of deuterium at 327 MHz (92 cm) in emission from the interstellar gas in the region of the Galactic anticenter. We find the maximum signal at l = 183 degrees, where the deuterium line opacity is expected to be a maximum as a result of velocity crowding. Using these deuterium and existing hydrogen observations, we model the effects of beam dilution, spin temperature, and absorption of continuum to derive a D/H ratio of 2.3(-1.3)(+1.5) 10(-5) by number. The errors are +/- 3 sigma plus our estimate of the systematic uncertainty in the conversion of the observed line amplitude to the D/ H ratio.