We present an experimental and theoretical study of atom-molecule collisions in a mixture of cold, trapped atomic nitrogen and NH molecules at a temperature of $sim 600$~mK. We measure a small N+NH trap loss rate coefficient of $k^{(mathrm{N+NH})}_ma
thrm{loss} = 8(4) times 10^{-13}$~cm$^{3}$s$^{-1}$. Accurate quantum scattering calculations based on {it ab initio} interaction potentials are in agreement with experiment and indicate the magnetic dipole interaction to be the dominant loss mechanism. Our theory further indicates the ratio of N+NH elastic to inelastic collisions remains large ($>100$) into the mK regime.
We observe magnetic trapping of atomic nitrogen (14^N) and cotrapping of ground state imidogen (14^NH, X-triplet-Sigma-). Both are loaded directly from a room temperature beam via buffer gas cooling. We trap approximately 1 * 10^11 14^N atoms at a pe
ak density of 5 * 10^11 cm^-3 at 550 mK. The 12 +5/-3 s 1/e lifetime of atomic nitrogen in the trap is limited by elastic collisions with the helium buffer gas. Cotrapping of 14^N and 14^NH is accomplished, with 10^8 NH trapped molecules at a peak density of 10^8 cm^-3. We observe no spin relaxation of nitrogen in collisions with helium.
