Ice acoustics for detecting neutrinos

Several experiments are operating or being built to detect astrophysical neutrinos. Ranging up to about a cubic kilometer in size, those experiments are embedded in ice or in a liquid such as water, where they watch for telltale flashes of Cherenkov radiation. (See the article by Francis Halzen and Spencer Klein in Physics Today, May 2008, page 29 .) But the highest-energy neutrinos, with energies of an exaelectron volt (1EeV = 10¹⁸ eV) or higher, are so scarce that installations spanning 100 km³, along with mas sive numbers of expensive photo-multiplier tubes, would be needed to collect ade quate event statistics in a reasonable time. So other detection schemes are being explored, one of which involves acoustics: When a very–high-energy neutrino interacts with water or ice, a sudden localized thermal expansion occurs and the resulting wave propagates farther than the light flashes. To explore that method, the Aachen Acoustic Laboratory was set up in late 2007 and its first experiment made a precise measurement of the speed of sound in ice that is entirely devoid of bubbles and cracks. The Aachen physicists carefully positioned an array of sensors—six detectors and one emitter—in a 3-m³ water tank (shown here) equipped with a freeze-control unit and a degassing system. The difference in arrival times of an acoustic pulse at adjacent receivers determined the speed of sound. Between 0 °C and −17 °C, where they took measurements, the speed ranged from about 3840 m/s to 3890 m/s, agreeing well with earlier laboratory experiments. The team is also part of SPATS (the South Pole Acoustic Test Setup), which is currently obtaining complementary in situ measure ments. (C. Vogt, K. Laihem, C. Wiebusch, J. Acoust. Soc. Am. 124 , 3613, 2008 http://dx.doi.org/10.1121/1.2996304 .)