Measurements of the Threshold Displacement Energy in Ta and Nb

Changes in the damage rate (residual electrical resistivity per unit electron fluence) were measured as a function of incident electron energy in the range 1.0-2.2 MeV for 0.002-in.-diam Nb and Ta wires. The threshold energies, estimated by simple extrapolation to zero production rates, were 36 eV for Nb and 32 eV for Ta. The resistivity of a Frenkel pair was determined to be about 5.3 {μ}{Ω} cm/at.% for Nb and 1.9 {μ}{Ω} cm/at.% for Ta. The damage production curves for two other bcc metals, Mo and W, are compared with those for Nb and Ta.