Measuring acoustic pressure pulses in huge underwater acoustic arrays is a promising approach for the detection of cosmogenic neutrinos with energies exceeding 100 PeV. The pressure signals are produced by the particle cascades that evolve when neutrinos interact with nuclei in water.
The resulting energy deposition in a cylindrical volume of a few centimetres in radius and several metres in length leads to a local heating of the medium which is instantaneous with respect to the hydrodynamic time scales. This temperature change induces an expansion or contraction of the water depending on its volume expansion coefficient. According to the thermo-acoustic model, the accelerated motion of the heated volume – a micro‑explosion – forms a pressure pulse of bipolar shape which propagates in the surrounding medium.
Coherent superposition of the elementary sound waves, produced over the volume of the energy deposition, leads to a propagation within a flat disk-like volume (often referred to as pancake) in the direction perpendicular to the axis of the particle cascade.
Two major advantages over an optical neutrino telescope motivate studying acoustic detection:
- First, the attenuation length in sea water is about 5 km (1 km) for 10 kHz (20 kHz) signals. This is one to two orders of magnitude larger than for visible light with a maximum attenuation length of about 60 m.
- The second advantage is the more compact sensor design and simpler readout electronics for acoustic measurements.