High energy neutrino acoustic detection activities in Lake Baikal: Status and results

We review the status of high-energy acoustic neutrino detection activities in Lake Baikal. The Baikal collaboration constructed a hydro-acoustic device which may be regarded as a prototype subunit for a future underwater acoustic neutrino telescope. The device is capable of common operation with the Baikal neutrino telescope NT200+, and is operating at a depth of about 150 m on the “NT200+ instrumentation string”. Our measurements show that the integral noise power in the frequency band 20-40 kHz can reach levels as low as about 1 mPa, i.e. one of the lowest noise levels measured at the currently considered acoustic neutrino sites. At the same time, short acoustic pulses with different amplitudes and shapes have been observed. Low sound absorption in Baikal freshwater and absence of strong acoustic noise sources do motivate further activities towards a large-scale acoustic neutrino detector in Lake Baikal.