Effects of thermal-neutron irradiation on the elastic constants of copper

The three independent elastic constants of copper irradiated with thermal neutrons below 4 K have been measured. Simultaneous measurements of attenuation and resistivity were used to monitor the defect density. The elastic-constant changes during irradiation per unit concentration $\gamma$ of defects, $\frac{d\ln C}{d\gamma }$, are -2, -15.8, and -18.1 for the bulk modulus, $C_{44}$, and $C^{\prime }=\frac{(C_{11}-C_{12})}{2}$, respectively. The percentage elastic-constant changes measured at 3.6 K after pulse annealing at temperatures up to 60 K were different for the different elastic constants in the same annealing range. Also the elastic-constant changes were strongly temperature dependent. The results show that there are five different types of elastic-constant effects, each of which depends upon the type of defect involved. This analysis also helps to account for the wide variation of results found previously for measurements using different types of irradiation at different temperatures.