  Cryogenic liquids, particularly liquid xenon and argon, are of interest as
detector media for experiments in nuclear and particle physics. Here we present
a new detector diagnostic technique using piezoelectric sensors to detect
bubbling of the liquid. Bubbling can indicate locations of excess heat
dissipation e.g., in immersed electronics. They can also interfere with normal
event evolution by scattering of light or by interrupting the drift of
ionization charge. In our test apparatus, four sensors are placed in the vacuum
space of a double-walled dewar of liquid nitrogen and used to detect and locate
a source of bubbling inside the liquid volume. Utilizing the differences in
transmitted frequencies through the different media present in the experiment,
we find that sound traveling in a direct path from the source to the sensor can
be isolated with appropriate filtering. The location of the source is then
reconstructed using the time difference of arrivals (TDOA) information. The
reconstruction algorithm is shown to have a 95.8% convergence rate and
reconstructed positions are self-consistent to an average +/-0.5cm around the
mean in x, y, and z. Systematic effects are observed to cause errors in
reconstruction when bubbles occur very close to the surfaces of the liquid
volume.
