The low-temperature dependence of positron annihilation momentum spectra in Al

The positron annihilation momentum spectrum was measured in annealed and plastically deformed Al samples as a function of temperature between 9 and 298K by a recently developed two-detector Doppler broadening technique. There is no indication of a low-temperature positron trap to 9K, implying that this effect may be strongly sample-dependent. The specific trapping rate of dislocations and the accompanying substructures shows no detectable temperature dependence at low momentum, whereas the high-momentum contributions exhibit a small temperature dependence in agreement with theoretical predictions. The change as a function of temperature of the high-momentum contributions for annealed Al is 3.4 times smaller than the change predicted by recent calculations.