Isotopic Composition of Cosmic-Ray Nitrogen at 1.5 GeV/amu

We have determined the cosmic-ray ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$/(${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ + ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$) ratio with a detector sensitive to velocities of particles in the penumbra of Earth's magnetic field. Our instrument was flown twice on high-altitude balloons from Palestine, Texas, obtaining an exposure factor of 20 ${\mathrm{m}}^2$ sr h. We measure the fraction of ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$ to be 0.45 ± 0.07 at 1.5 GeV/amu consistent with an origin for nitrogen primarily due to spallation in the interstellar medium.