Positron trapping rates in metal single crystals at low temperatures

For a number of single-crystal metal samples with small defect density, we have observed 'anomalous' low-temperature behaviour of positron trapping, previously seen in certain cases of metals prepared with various contents of defects. In general, the trapping rate is found to be a linear function of T -1 2/. It is suggested that an explanation for this effect can be given on the basis of the quantum mechanical cross section between positrons at low velocity, and point defects.