237Np nuclear relaxation rate in heavy fermion superconductor NpPd ₅Al₂

27Al nuclear spin-lattice relaxation rate 1/27T₁ has been measured in heavy fermion superconductor NpPd₅Al₂ for a wide range of external fields between 0.7 and 11 T. We found that 1/27T ₁ exhibits a strong, field-dependent behavior at low fields below H∼3 T, which is attributed to unlike-spin cross relaxation between 237Np and 27Al. An analysis based on the cross relaxation process allows us to make the first quantitative estimates of the Np nuclear relaxation rate 1/237τ in an itinerant 5f electron system. The estimated values of 1/237τ∼6×107 s^-1, with a weak temperature variation, are below the resolution of NMR relaxation times, indicating that a direct observation of the 237Np NMR signal would be very difficult, at least in the paramagnetic state. It is proposed that a direct observation of 237Np NMR might be possible in the superconducting state at T∼1 K.