  We report a combined experimental and theoretical study of collision-induced
dipolar relaxation in a cold spin-polarized gas of atomic nitrogen (N). We use
buffer gas cooling to create trapped samples of 14N and 15N atoms with
densities 5+/-2 x 10^{12} cm-3 and measure their magnetic relaxation rates at
milli-Kelvin temperatures. Rigorous quantum scattering calculations based on
accurate ab initio interaction potentials for the 7Sigma_u electronic state of
N2 demonstrate that dipolar relaxation in N + N collisions occurs at a slow
rate of ~10^{-13} cm3/s over a wide range of temperatures (1 mK to 1 K) and
magnetic fields (10 mT to 2 T). The calculated dipolar relaxation rates are
insensitive to small variations of the interaction potential and to the
magnitude of the spin-exchange interaction, enabling the accurate calibration
of the measured N atom density. We find consistency between the calculated and
experimentally determined rates. Our results suggest that N atoms are promising
candidates for future experiments on sympathetic cooling of molecules.
