MEASUREMENT OF THE THERMAL EXPANSION COEFFICIENT OF AN Al-Mg ALLOY AT ULTRA-LOW TEMPERATURES

We describe a result coming from an experiment based on an Al-Mg alloy (~5% Mg) suspended bar hit by an electron beam and operated above and below the temperature of transition from superconducting to normal state of the material. The amplitude of the bar first longitudinal mode of oscillation, excited by the beam interacting with the bulk, and the energy deposited by the beam in the bar are the quantities measured by the experiment. These quantities, inserted in the equations describing the mechanism of the mode excitation and complemented by an independent measurement of the specific heat, allow us to determine the linear expansion coefficient α of the material. We obtain α = [(10.9±0.4)T+(1.3±0.1)T3]×10-10K-1 for the normal state of conduction in the temperature interval 0.9 < T < 2 K and α = [(-2.45±0.60)+(-10.68±1.24)T +(0.13±0.01)T3]×10-9K-1 for the superconducting state in the interval 0.3 < T< 0.8 K.