Dependence of dislocation damping on helium-3 concentration in helium-4 crystals

The mechanical properties of crystals are strongly affected by dislocation mobility. Impurities can bind to dislocations and interfere with their motion, causing changes in the crystal's shear modulus and mechanical dissipation. In He-4 crystals, the only impurities are He-3 atoms, and they can move through the crystal at arbitrarily low temperatures by quantum tunneling. When dislocations in He-4 vibrate at speeds v < 45 mu m/sec, bound He-3 impurities move with the dislocations and exert a damping force B(3)v on them. In order to characterize He-4 dislocation networks and determine the He-3 binding energy, it is usually assumed that B-3 is proportional to the concentration of He-3 bound to the dislocations. In this preliminary report, we determine B-3 in a crystal with 2.32 ppm He-3 and compare with our previous measurements of B-3 in natural purity He-4 crystals to verify the assumption of proportionality.