Neutron Doppler broadening studies of tantalum and tungsten metal

Abstract With the aim of providing experimental verification of the validity of the neutron resonance spectroscopy method for measuring the moments of the phonon frequency spectra, we have determined some of the phonon spectral parameters for W and Ta in the bcc phase of these metals by measuring the Doppler broadening of neutron resonances as a function of temperature. We find the mean phonon energy (first moment of the phonon frequency spectrum) of W to be 〈 hν 〉=20.2±0.4 meV and the second moment to be 〈( hν ) 2 〉=(4.9±0.2)×10 −4 eV 2 . The Boltzmann-weighted moments from all the measurable resonances of tungsten are quite consistent with a Debye model with a Debye temperature of 315±4 K. For Ta, 〈 hν 〉=14.3±0.1 meV, 〈( hν ) 2 〉=(2.2±0.2)×10 −4 eV 2 . The fitted Debye model temperature for the tantalum data is 217±2 K. In both cases the first and second moments agree well with those determined from neutron coherent inelastic scattering. Results with reasonable accuracy were extracted from resonances up to a few tens of eV. These results give high confidence that neutron resonance Doppler broadening spectroscopy is a viable and useful method for determining the characteristics of phonon frequency spectra.