Single crystals of copper were deformed at 4.2°K and 77.3°K. At 4.2°K, after a large strain produced by normal slip, jerky flow (discontinuous slip) occurs. From the study of reactor irradiated crystals, it was deduced that a packet of 30 slip lines, each containing 104 dislocations, was released to form each jerk of the discontinuous flow. For samples of certain orientation the region of discontinuous flow was followed by a region of deformation twinning. The fact that deformation twinning was taking place was verified by x-ray methods. The twinning elements were determined to be the (111) plane and the [112] direction. At 77.3°K, discontinuous slip was not observed. In a limited range of orientations, twinning has been observed at 77.3°K. The effects of reactor irradiations on discontinuous slip and on twinning were also studied and it was determined that the occurrence of twinning was unaffected by the irradiations.

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By relocating the fuel in the vicinity of a liquid helium cryostat located in the Oak Ridge Graphite Reactor it has been possible to separate the thermal and fast components of the reactor neutron flux. Studies of the radiation damage effects arising from each type of flux have been made. It has been found that an appreciable fraction of the reactor damage in several metals arises from thermal neutrons. The effect results from the recoil of an atom from the (n,γ) reaction at the time of thermal neutron capture. The low temperature recovery of thermal neutron damage is greater and shows more annealing peak structure than the recovery of fast neutron damage. Thermal neutron damage concentration studies have been made on cadmium, and pronounced suppression of the annealing is found as the concentration is increased. The mean primary recoil energy from a thermal neutron capture event has been calculated for several elements. Values range from about 50 eV for the heavier elements to several hundred eV for the lighter elements. The relative effects of atomic recoils from decay events and decay beta-atom collisions are estimated and found to be small compared to (n,γ) recoil effects. Damage from fission neutrons has been studied in copper, and it was found that the recovery of this damage is suppressed compared to that obtained from reactor fast neutrons.

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