Pulsed Fourier-transform ultrasonic spectroscopy for ultralow temperature applications

Pulsed Fourier-Transform (FT) ultrasonic spectroscopy has been developed fo r the purpose of studying He-3 at ultralow temperatures. Our method is anal ogous to pulsed FT NMR and possesses several advantages over conventional t ime-of-flight acoustic techniques. For use at low temperatures, the most si gnificant advantage is the ability to observe the frequency spectrum while the temperature, pressure, and magnetic field may be independently tuned. M ore specifically, using longitudinal LiNbO3 transducers operating both on- and off-resonance, we are able to study several broadband windows of freque ncy, namely 16-25 MHz, 60-70 Mhz, and 105-111 MHz. The determination of the energy gap of He-3-B is presented as an effective applications, and other novel uses are discussed.