  The proton hyperfine shift K and NMR relaxation rate $1/T_1$ have been
measured as a function of temperature in the S=1/2 Heisenberg antiferromagnetic
ladder Cu2(C5H12N2)2Cl4. The presence of a spin gap $\Delta \simeq
J_\perp-J_\parallel$ in this strongly coupled ladder ($J_\parallel < J_\perp$)
is supported by the K and $1/T_1$ results. By comparing $1/T_1$ at two
different proton sites, we infer the evolution of the spectral functions
$S_z(q,\omega_n)$ and $S_\perp(q,\omega_n)$. When the gap is significantly
reduced by the magnetic field, two different channels of nuclear relaxation,
specific to gapped antiferromagnets, are identified and are in agreement with
theoretical predictions.
