The interaction between hydrogen and defects in neutron irradiated n-type FZ silicon has been studied with deep level transient spectroscopy, transient capacitance at constant temperature and infrared absorption spectroscopy. It has been observed that the absorption peaks of infrared spectroscopy corresponding to the interstitial hydrogen reduce after neutron irradiation. In the unannealed neutron irradiated n-type FZ silicon grown in hydrogen, a new defect-Z center has been observed for the first time. Because the energy level of the Z center (Ec-0.20 eV) is very close to the energy level of the A center (Ec-0.16 eV) and the concentration of the former is much lower than that of the latter. In order to detect the Z center, neutron dose has been selected so that the Fermi level locates at several kT below the energy level of A center. In all probability the Z center is a hydrogen-vacancy complex. The possible electron structure of this center is discussed.