Vibrational properties of Pu and Ga in a Pu-Ga alloy from neutron-resonance Doppler spectroscopy

We have determined some of the phonon spectral parameters for Pu and Ga in the ``stabilized'' {δ} phase (fcc) of the Pu-3.6 at. % Ga alloy by measuring the Doppler broadening of neutron resonances as a function of temperature. We find the mean phonon energy (first moment of the phonon spectrum) of Pu to be 〈hν〉=8.22±0.12meV and the second moment to be 〈(hν)²〉=(7.2±0.3)×10^-5eV². These moments and the value of the (-1)st moment determined from Debye-Waller factors from neutron powder diffraction are consistent with a Debye model with a Debye temperature of 127.2±1.7K. For Ga in the Pu-Ga alloy, 〈hν〉=16.3±1.4meV, which is slightly larger than the m1/2-weighted value of 15.3 meV expected from the value for Pu (however, the measured value cannot be distinguished from the latter value within experimental error). These results show that Pu-3.6 at. % Ga behaves very much like a Debye solid at ambient pressures and low temperature and that the Ga impurity experiences approximately the same (or a slightly stiffer) force field compared with the Pu it replaces.