Cryogenic acoustic microscopy

Abstract
Resolution in the scanning acoustic microscope is determined by wavelength which is in turn limited by attenuation in the acoustic medium. In order to make use of the low attenuation found in cryogenic liquids we have developed an acoustic microscope suited for use at low temperatures. In this paper we present images taken in liquid argon held at 85°K and in superfluid helium at 1.95°K. In liquid argon, wavelengths as short as 0.43 μm were achieved while in preliminary work with superfluid helium a wavelength of 0.36 μm was used. In order to operate in liquid helium, it was necessary to improve the power transfer from the acoustic lens to liquid helium by using double quarter-wave matching layers. Techniques of fabricating and testing these layers are described. Finally, prospects for operating at still shorter wavelengths in superfluid helium held at temperatures below 0.5°K are discussed.