Nitrogen doping and the performance of superconducting radio-frequency niobium cavities: insights from neutron diffraction and neutron Compton scattering

Neutron Compton scattering and neutron diffraction have been applied to investigate the influence of nitrogen doping of niobium on the performance of superconducting radio-frequency niobium cavities. To this end, a comparative study of the neutronic response of two samples has been performed. An electro-polished and nitrogen-doped niobium sample was compared with a standard, a niobium sample that has only undergone the electro-polishing procedure. The first piece of information, provided by neutron diffraction, is that additional conditioning of the electro-polished cavity material, through doping with nitrogen, leads to a systematically larger niobium lattice expansion, which provides an upper conservative limit of nitrogen concentration consistent with values reported in the literature. Furthermore, neutron Compton scattering shows a broadening of the niobium momentum distribution in the nitrogen-doped sample, as compared to the standard, thus indicating an increased degree of ordering and binding of niobium in the metal lattice. On the whole, these observations suggest that nitrogen-doping leads to some degree of lattice ordering, most likely due to increased hydrogen trapping, in agreement with previous results using surface spectroscopy.