Acoustic Detectors

    The first proposal on the detection of high energy neutrino interactions by acoustic methods dates back to 1957. Ultra-high-energy (E > 100 PeV = 1017 eV) neutrinos produce particle cascades which heat the interaction medium and generate a bipolar acoustic pulse. The signal strength is determined by the cascade energy, the expansion coefficient and the sound velocity of the medium. The detection threshold depends strongly on the ambient noise and the internal noise of the receiver.

    Moving to higher energies, the expected fluxes of neutrinos become so small, that even a cubic kilometre detector is not able to detect them. Optical neutrino telescopes are limited however in size, because light is attenuated after (100m) propagation of water or ice by about a factor three. Larger volumes can be achieved by measuring the acoustic or radio signals also emitted from high energy neutrino interactions. The reason is that acoustic and radio waves can propagate over distances of several kilometres in certain media allowing therefore larger detector volumes to be instrumented at reasonable cost.

    Research and development programs on acoustic particle detection are carried out by several groups at various places in ice, water and salt. Recently permafrost has been suggested to be an additional medium for such an experimental program [PE08].

    The DESY group started in 2002 with the development of piezo-electric sensors, transmitters and signal filters. After lab studies of the signal transmission over short distances in artificial ice, measurements have been performed at the accelerator of the PLS Laboratory at Uppsala. An intensive 180 MeV proton beam simulates the energy deposition of neutrino interactions in ice in the EeV (1018 eV) energy region. The results indicated that a large ice or water array with 100 PeV threshold may be feasible [PO05].

    A corresponding simulation of a hybrid optical-radio-acoustic detector of ~100 km3 size around the presently installed IceCube experiment demonstrated, that about 20 interactions from neutrinos produced in collisions of highest energy charged cosmic rays with the microwave background photons (GZK neutrinos) could be measured per year in Antarctic ice, assuming a theoretically predicted acoustic attenuation length of more than a kilometer [BP06].

    Within the IceCube collaboration groups from the universities of Berkeley, Gent, Stockholm and Uppsala joined the DESY initiative during the last years. Together they build the South Pole Acoustic Test Setup – SPATS – to measure the acoustic properties of the Antarctic ice shield at the South Pole [ST12]. Aim of the test is to get in-situ information about acoustic signal attenuation, the velocity of sound at different depth and the acoustic background noise.

    SPATS consists of four strings, 400 m to 500 m long, carrying seven acoustic stations with transmitters and sensors. All components were tested in a lake in North of Sweden and in Zeuthen and found to work fine [PO06].

    The strings were deployed in January and December 2007 in the upper part of IceCube holes. Data taking started immediately after deployment. All sensors and transmitters were found to work well. First SPATS results [PO07] were presented at the International Cosmic Ray Conference ICRC-2007 (pdf d, pdf v). One could hear acoustic pulses through 125m ice in 320m depth. (wav1; wav2, slowed by factor 10; wav3, slowed by factor 100).

    In April 2007 the Acoustic Neutrino Detection Working Group has been established within the IceCube collaboration. This initiative was joined by the five SPATS institutions, the universities of Aachen and Wuppertal, the EPFL Lausanne and several individuals from US collaborating institutes. The European groups created a common test facility for acoustic R&D in Aachen – The Aachen Acoustic Laboratory.

    During the Antarctic summer 2007/08 a removable transmitter – the “Pinger” – was used to emit sound in six water filled IceCube holes before deployment of optical strings. The corresponding data taken with the SPATS receiver array allowed a precise measurement of the sound speed versus depth for pressure and shear waves [SP09]. Due to several unexpected systematic problems it was however impossible to reasonably restrict the sound attenuation length [PO08].

    With an improved “Pinger” design the problems could be solved in the following season. The calculated acoustic attenuation length of 300+/-100 m [AT09] is however an order of magnitude smaller than theoretical predictions expected. The reason for this difference is still unclear. Data from a second “Pinger” campaign in 2009/10 done at different frequencies didn’t show a strong frequency dependence of the sound attenuation. This excludes scattering as the main reason for the lower as expected sound attenuation length.

    During the years 2010 and 2011 most effort was spent to understand the acoustic background in South Pole ice. Using results of different laboratory and in-situ measurements an upper limit of 14 mPa could be derived for the noise level which shows up very stable on the time scale of several years [NL12]. Furthermore, using data from transient noise measurements at the Pole, a neutrino flux limit has been calculated, even if this was not the original intention of the SPATS measurements. The limit is therefore not competitive with those from much larger radio detectors but the best until now using the acoustic technology (see figure below).
    Acoustic noise monitoring with SPATS is still going on at the South Pole, but nearly all problems to be studied with SPATS have been solved in the meantime. The ice at the South Pole is a favorable target material for acoustic neutrino detection with respect to constant high sound speed and low background noise. The sound attenuation length is unexpected low but still much larger than that for emitted Cherenkov light. Further simulation studies have to show, if and how a realistic hybrid detector for GZK neutrino studies could be designed under these circumstances.

    The DESY team is in close contact to the groups in Europe, the USA and Asia which are working on alternative neutrino detection methods. For this purpose also the “Workshop on Acoustic and Radio EeV Neutrino detection Activities – ARENA-2005 has been organized in Zeuthen and proceedings have been published. Further conferences of this type happened 2006 in Newcasle [AR06, AR06P], 2008 in Rome [AR08, AR08P] and 2010 in Nantes [AR10, AR10P]. The next event of this series will take place in summer 2012 in Erlangen [ER12].