A vessel for rare-gas filling is provided, where almost complete light 
circular polarization is realized, which has a light incident window of a 
single crystal material whose thickness and 
crystal axis orientation are optimized, and a polarization method of 
rare gas nuclei in the vessel is also provided. In addition, embodiments of the vessel is provided, which is impervious to 
alkali metal, and sustains 
high pressure, and shows no permeability for 3He gas, and has negligibly small 
neutron absorption so as to be suitable for application to basic science, for example, 
neutron scattering, and the polarization method in the vessel is also provided.The vessel for rare-gas filling of the present invention comprises a vessel body 3, and a 
pipe 4, which is connected to the vessel body 3 and introduces a rare-gas containing gas and an 
alkali metal into the vessel body 3. A light incident window 2 made of a single-
crystal material, which has proper thickness and proper 
crystal axis orientation, is attached to the vessel body 3. The vessel for rare-gas filling is preferably made of 
sapphire or the like.