A diffractometer using perfect crystals in symmetric Bragg arrangement, hence being useful for both neutron and X-ray radiation, has been built. The angular resolution of that instrument is better than 1 microradian, which corresponds to a scattering vector resolution of about 10-6 Å-1. This spectrometer was equipped with a laser light adjustment system which is sensitive to the relative angular position of the two perfect crystals. The spectrometer was used for the measurement of neutron scattering at very low angles of hydrated niobium and vanadium samples. The experimental results show an anomalously low attenuation of the neutrons when the metal-H sample is within the solubility a region. This property is explained by a lattice gas model of atomic H in the α-phase of the system. In the two phase region (α - β) a stronger attenuation of the neutron beam occurs due to the sample now being in an ordered interstitial solution phase.