Energy dependence of electron-induced radiation damage in tungsten

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Abstract
The energy dependence of low dose damage production in commercial and high purity polycrystalline tungsten wires was studied near 350 K with 1.6 to 2.4 MeV electrons. From resistivity measurements at 291 K the threshold energy for the onset of observable damage was determined as 50 × 2 eV. An ‘effective’ threshold of 52 ±2 eV was also determined by directly fitting the energy dependence of the damage rates to theoretical displacement cross sections calculated from step-function displacement probabilities. A decrease of two orders of magnitude in impurity content reduced damage rates by about a factor of two but did not affect threshold. These results combined with current defect recovery models for tungsten, low temperature threshold data, and computer-calculated bcc damage theory suggest: (1) Observed damage consisted of equal concentrations of vacancies and impurity-trapped Stage I free interstitials. (2) Across Stage II (100 K to 600 K) onset threshold should be within 50 ±2 eV. (3) Minimum recoil energy required for free interstitial production near 0 K is 53 ± 5 eV. (4) Threshold has little dependence on crystal direction. An empirical method is presented for predicting threshold energies in the bcc transition metals by assuming the directional dependence of threshold is directly proportional to that of Young's modulus. By the use of one universal proportionality constant (1.2 × 10−11 eV.cm2/dyne), thresholds for a number of metals and directions are calculated and shown to have significantly better agreement with experiment than the best available theoretical estimates.