Elastic after-effect studies of lattice defects in Mo after fast neutron irradiation at 5 K

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Abstract
The elastic after-effect of Mo single crystals with various orientations were measured after fast neutron irradiation at about 5 K. Two prominent relaxations were observed at 8 K and 26 K (at relaxation time ≈2 min). These two relaxations correspond, respectively, with the 12 K and 39 K peaks in previous high frequency experiments (at vibrational frequencies ≈500 Hz). The 26 K relaxation was found to be about 2.5 times larger in strength than that expected from the unstable 39 K peak observed in the high frequency measurements. The 8 K and 26 K relaxations annealed out in the recovery stages I (∼28 K) and IIa (∼42 K), respectively. Their dependence on irradiation fluence and orientation and their independence on pre-irradiation deformation and/or doping indicate that these relaxations are caused by intrinsic point defects, in good agreement with observations in the high frequency measurements. The origins of these two relaxations are considered to be a stress-induced rotation of free (110) split-type self-interstitials (8 K relaxation) and, possibly, of di-interstitials (26 K relaxation), respectively. Anisotropy ratios and activation parameters of these defects are determined more accurately than in the previous experiments. Both activation energies for rotation and for recovery show a somewhat broad distribution for the 8 K relaxation, but not for the 26 K relaxation. In addition to these relaxations, many minor relaxations are also observed in the temperature range studied (below 50 K). Properties of these relaxations suggest that these are attributable to the dislocation-interstitial interactions.