We have performed molecular dynamics computer simulation studies of 25 keV displacement cascades in Cu at low temperature. At this energy we observe the initial splitting of a cascade into subcascades and show that at low temperatures in metals displacement cascades can lead to the formation of both vacancy and interstitial dislocations loops. We discuss a new mechanism of defect production based on the observation of interstitial prismatic dislocation loop punching from cascades at 10 K. We also show that below the subcascade threshold, atomic mixing in the cascade is recoil-energy dependent. We obtain a mixing efficiency that is proportional to the square root of the recoil energy.