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Acoustic detection of ultra-high energetic neutrinos - a snap shot - RolfNahnhauer DESY,Platanenallee6,D-15738Zeuthen,Germany 2 1 Abstract 0 2 Alreadymorethan 30yearsagothe acousticparticle detectionmethodhasbeenconsideredto be onepossibility n to measure signals from ultra-high energetic neutrinos. The present status and problems of corresponding model a predictions are discussed in comparison with existing experimentalmeasurements. Available acoustic sensors and J transmittersaredescribedandnewideasforcorrespondingapplicationsarementioned. Differentmethodsforin-situ 4 calibrations are discussed. Results of measurements of acoustic test arrays at different sites are presented in some detail. Futureactivitiesforapplicationsofthetechnologyinlargesizedetectorsareevaluated. ] M Keywords: acousticneutrinodetection,cosmogenicneutrinos, I acoustictransducers . h PACS:43.58,43.60Fg,95.85Ry p - o r 1. Introduction [11, 12], and first ideas about a 100 km3 acoustic de- t 1 27 s tector were discussed seriously [13]. In the following a 28 [ 2 Thedetectionand study of ultra-highenergeticneu- 29 sectionsitwillbeshown,howfarthepredictionsofthe 1 3 trinos above 1017 eV created in cosmic sources or by 30 Thermo-acousticModelcouldbeconfirmedexperimen- v 4 interaction or decay of even higher energetic particles 31 tallyandwhatquestionshavestilltobeanswered. 8 5 became of increasing interest during the last decade. 32 Duringthelast10yearsdifferentgroupstriedtouse 0 6 Severalinteresting questionsof particle physics, astro- 33 acoustictestarraystoextractbasicinformationneeded 9 7 physicsandcosmology,couldbestudiedmeasuringin- 34 to build large scale detectorsin differentmaterialsand .0 8 teractions of such neutrinos on Earth with reasonable 35 environmentalconditions.Theirresultswillbesumma- 1 9 statistics[1]. 36 rized in the second part of this paper. Finally future 0 steps for the improvement of the acoustic technology Correspondingneutrinofluxpredictionswerealready 37 2 10 anditsapplicationwillbementioned. small ten years ago [2] but had to be decreased fur- 38 1 11 : 12 ther by new bounds from cosmic ray [3] and high en- v ergygammaraymeasurements[4]. Presentdayexper- i 13 39 2. TheThermo-AcousticModel X iments could derive therefore until now only flux lim- 14 r 15 its [5, 6, 7, 8]. A first detection of such neutrinos is 40 Ideas about the Thermo-acoustic Model of the cre- a expectedwith detectorsof ∼100 km3 size. To get rea- ation and propagationof sound in particle interactions 16 41 sonable statistics will need probablyaboutan order of weredisplayedforthefirsttimeatthe1976DUMAND 17 42 magnitudelargerdetectorvolumes.Itseemsimpossible meeting[14]. Theywere formulatedindependentlyby 18 43 todaytoinstrumentsuchexperimentswithconventional Bowen[12] andAskaryanandDolgoshein[11]. More 19 44 opticaldetectorswithin reasonablecostlimits. Acous- detailed descriptions of both concepts were published 20 45 tic particle detection may be one option among others three years later [15, 16]. Recently a new approach 21 46 toovercomethisproblem[9]. based on [16] has been published including signal at- 22 47 The possibility to detect charged particles by the tenuationeffects[17]. 23 48 soundtheyproducepassingthroughmatterwasthefirst In neutrino interactions a charged or neutral lepton 24 49 time mentioned in 1957 by G. Askaryan [10]. About and a hadronic particle cascade is produced. The cas- 25 50 20 years later a corresponding model was formulated cade gives rise to a large energy deposition in a small 26 51 PreprintsubmittedtoNuclearInstrumentsandMethodsA January5,2012 Pomeranchuk-Migdal-effect and photo- and electro- 79 nuclear reactions [20]. The LPM-effect predicts de- 80 creasingcrosssectionforbremsstrahlungandpairpro- 81 duction,elongatinghadroniccascadesaboveabout1018 82 eVwhereasthesecondeffectactinthe oppositedirec- 83 tionatenergiesabove1020eV. 84 2.2. targetmaterialeffects 85 Onecouldrewriteeq. 1intheform 86 p=γ(E/R)M′ (2) 87 withγ=v2(k/c )andM′ =(1/2)(1/d2)(sinx/x). 88 s p The Grueneisencoefficientγ isa stronglymaterialde- 89 Figure1:Signalstrengthfromthreedifferentacousticmodelparame- pendent quantity. In tab. 1 this is displayed for three 90 terizations[18]andinclusionofattenuationeffects[17](from[19]) materials under discussion for acoustic detector appli- 91 cations. At the same incoming energy signals in ice 92 shouldbethereforenearlyanorderofmagnitudelarger volumeinaveryshorttime. Thevolumeisoverheated 93 52 than those in water. In salt even larger signals are ex- whatgivesrisetoapressurewavewhichdevelopesor- 94 53 pected. Alsopermafrostwasrecentlysuggestedtogive thogonaltothecascadeandthereforetheincidentneu- 95 54 risetoquitelargesignals[22]. trinodirection. 96 55 56 2.1. modelpredictions Table 1: Thermo-acoustic model parameters and boundary conditionsforthreedifferentmaterials(adaptedfrom[21]) An illustrative way to describe the dependencies of 57 theimportantquantitiesandvariablesof the processis water SouthPoleice salt 58 59 givenin[15]. c(m/s) 1530 3880 4560 (k/10−5)[K−1] 25.5 12.5 11.6 p = (k/c )(E/R)M p c [J(Kkg)−1] 3900 1720 839 M = (f2/2)(sinx/x) p γ 0.153 1.12 2.87 f = vs/(2d) fmax[kHz] 7.7 20 42 x = (πL/2d)(sinδ) (1) refraction moderate verysmall small? λ >1000m ∼300m >100m att with p : pressure amplitude, E : cascade energy, R : 60 noise variable stable,<14mPa small? distancetoreceiver, f : frequency,v : speedofsound, 61 s d: cascadediameter,k: volumeexpansioncoefficient, 62 For water things are even more complicated, be- c : specificheat, L: cascadelength,δ: anglebetween 97 63 p cause the volume expansion coefficient for water de- normaltocascadedirectionandreceiver. 98 64 pends strongly on the water temperature. At 4 degree Themodelpredictsa linear dependenceofthe pres- 99 65 Celsius it is equal to zero having different signs be- sure amplitude on the particle cascade energy, which 100 66 lowandabovethistemperature. Thesignalstrengthfor is related to the incoming neutrino energy and allows 101 67 acoustic pulses in water dependstherefore strongly on todeterminetheneutrinodirectionfrommeasuringthe 102 68 thespecificlocationoftheneutrinointeractionandmay pressurewavepropagationthroughthemedium. 103 69 varyalsobyafactor10(see[23,24]). Unfortunatelyabsolute signal predictionsare uncer- 104 70 tainmainlydueto the notwellknownparticleinterac- 71 72 tionandenergylossprocessatultra-highenergies.Dif- 105 3. Experimentalverifications ferentassumptionsaboutwhatvaluesshouldbeusedfor 73 thewidthandthelengthofacascadeofacertainenergy Alreadyattheendoftheseventiesofthelastcentury 74 106 giverise tosignalpredictionswhichdifferbyaboutan severalexperimentswere performedto test the predic- 75 107 orderofmagnitude(seefig.1). tions of the Thermo-acousticModel using accelerator- 76 108 In the considered energy range two other phenom- or laser beams [24, 25]. The basic experimental ar- 77 109 ena have also to be taken into account: the Landau- rangementisshowninfig.2. 78 110 2 Figure2:Experimentalsetupandresultsforsignaldependenceonde- Figure3:Acousticsignalsfromalaserbeamshotinatankfilledwith positedenergyforanearlyThermo-acousticmodelcheck(from[24]) water(top)andice(bottom)(from[31]) 111 An intense laser or low energy proton beam is differentvelocityofsoundinbothmaterials. Thesignal 142 stoppedin a water tank. Varyingthe beam parameters 112 strengthiniceisaboutafactorsixlargerthaninwater 143 allowedseveralmodelcheckslikethefollowing: 113 consistent with the theoretical expectation. The exact 144 • varyingintensity (E-dependence) 145 comparisonhashowevertobedoneforthecorrespond- 114 ingpressureamplitudes. Moredetailedresultsofthese • varyingbeamdiameter (f-dependence) 146 115 measurementswillbeavailablesoon. 147 • varyingdistance (R-dependence) 116 • varyingliquids (γ-dependence) 4. Acoustictransducers 117 148 118 Duringthelastdecadeseveralsimilartestshavebeen 149 Acoustictransducersforultra-soundhavebeendevel- 119 madebydifferentgroupsforwater[26,27,28],ice[29, 150 opedsincelongforapplicationsinwater. Theirquality 120 30]andpermafrost[22]. Thegeneralconclusionsfrom 151 profitedfromtheiruseinmilitaryprojects. Todaycom- 121 allthesetestsarethefollowing: 152 mercialproductsareavailablefromseveralcompanies. Special requirementslike stability at high pressure for • many predictions of the Thermo-acoustic Model 153 122 useatlargewaterdepthleadtoconsiderablyhighprices couldbeconfirmed 154 123 perpiece. Becauselargenumbersofcorrespondingde- 155 124 • the dominant mechanism for acoustic signal pro- 156 vicesarenecessarytobuildlargedetectorarrays,several 125 ductionisthermalexpansion 157 attemptswerestartedtobuildownsensitivesensorsand 126 • othercontributions,e.g.frommicro-bubbleforma- 158 transmitters.Forapplicationsinice,saltandpermafrost 127 tioncouldnotcompletelybeexcluded 159 thiswasunavoidableanyway. • absolute signal values could not be checked, be- 128 4.1. piezoceramicsensors cause the energydepositin the differenttests was 160 129 Nearlyallacousticsensorsinuseintodaystestarrays difficulttocalculatepreciselyandwasdifferentto 161 130 are based on piezo-ceramic elements [32]. Within the theultra-highenergeticparticlecase. 162 131 AMADEUSprojectanicecomparisonofdifferentcom- 163 Anunsolvedproblemwasuntilrecentlyalsotherel- mercialandself-madedeviceshasbeenperformed[33]. 132 164 ative signal strength in different target materials. It Infig.4theresponseofthreeoftheirsensorstothesame 133 165 is therefore very valuable that an experiment is per- transmittersignalisshown.Aninterestingdevelopment 134 166 formed this year at the Aachen-Acoustic-Laboratory is their “Acoustic Module”, where piezo-ceramic ele- 135 167 aimingto answerthisquestion[31]. A wellcontrolled ments are integrated in a glass pressure sphere other- 136 168 laserbeamisstoppedin alargetankplacedinaneven wiseusedtocontainphotomultipliersforlightdetection 137 169 largerfreezer. Thisallowsmeasurementsinatempera- intheANTARESexperiment[34]. 138 170 turerangefrom+20to-25degreeCelsiusinwaterand The SPATS group connected to the IceCube exper- 139 171 ice. Infig.3acousticsignalsforbothconfigurationsare iment started their own developments for ice applica- 140 172 shown. Thetimedifferenceofthesignalsisduetothe tionswith a similar concept[29, 35] butchangedtheir 141 173 3 surepulsesleadstomodulationsofthelaserwavelength 192 detectablebyinterferometry[38]. Theconcepthasbeen 193 proventoworkatastaticpressureof35MPa,i.e. 3500 194 m depth with higher sensitivity and better resolution 195 than piezo-ceramicbased devises. An application in a 196 realopenwatertestarrayishoweverstillmissing. 197 Theapplicationofcoupledwaveguideintensitymod- 198 ulatedhydrophonesis underdiscussionfor an upgrade 199 oftheBAIKALacoustictestsetup[23]. 200 4.3. transmitters 201 Acoustic transmitters are normally used in acoustic 202 arrays to get position and calibration information. An 203 exceptional idea was realized for this purpose in the 204 Figure4:Comparisonoftheresponseofthreedifferentacousticsen- SAUND-1 experiment, where light bulbes of different sortypestoabipolarpulse(from[33]) 205 sizeweredeployedontopofsevenhydrophonesatthe 206 AUTECmilitaryarrayneartheBahamas[39]. 207 The SPATS group published results from different 208 typesofpingersfixediniceorusedinwaterfilledholes 209 tomeasuretheacousticattenuationlengthinSouthPole 210 ice [40]. A byproductof these measurements was the 211 observationofshearwavesintheice[41]. 212 Positionmonitoringisalsoaproblemofalldeepwa- 213 teropticalneutrinotelescopes. Itisnormallysolvedby 214 fixingstrongacoustictransmittersattheseabedandby 215 addingsomehydrophonestotheopticalstrings. Pulses 216 emitted with short time differences allow then precise 217 position monitoring at the 10 cm scale [42]. For the 218 planned KM3Net detector a corresponding system is 219 underdevelopment,whichwillallowtoobservepulses 220 Figure5:SPATSsecondgenerationsensormodule(from[36]) fromparticlecascadeswithhighsensitivity[43]. 221 Totestthesensitivityofacousticarraysin-situisstill 222 anunsolvedproblem.Twogroupstrytodevelopacous- design finally to a cylindrical steel pressure housing 223 174 tic pulser systems which mimic pulses from ultra-high where three piezo-ceramicsare pressed against the in- 224 175 energetic neutrino interaction in strength and shape. nerwall (see fig. 5). Typicalsensitivities of sensorsin 225 176 Successfultestshavebeenmadeusingarrayswithmore useareintherange-190to-110dBre1V/µPa. 226 177 than5transmitterstakingintoaccountthetransferfunc- Recently the use of piezo-foils for the design of 227 178 tion between emitted and receivedsignals[44]. In an- acoustic sensors has been reported [31]. This would 228 179 other approach a parametric acoustic source is used. have the advantage that one would be more flexible in 229 180 The overlay of two high frequency signals fed to an sensor shape and size. Results from forthcomingtests 230 181 emitter leads to the production of a bipolar pulse at willbehopefullypublishedsoon. 231 182 lowerfrequencywhereastheremaininghighfrequency 232 componentsarequicklyabsorbedinthemedium[45]. 4.2. othersensorconcepts 233 183 A “new” concept for acoustic sensors under discus- 184 185 sioninparticledetectionapplicationssinceafewyears 234 5. Sensorcalibration istheuseofacousto-optichydrophones.Infactthecor- 186 responding working principles using either frequency Fortheapplicationoftheacoustictechnologyforpar- 187 235 or intensity modulationsin opticalfibers has been dis- ticledetectiontheuseofcarefullycalibratedsensorsis 188 236 cussedalreadymorethan30yearsago[37]. mandatory.Severalmethodsareappliedforthispurpose 189 237 Recently severalgroupspublishedresults for a fiber inthelaboratory[46],e.g.comparisonwithacalibrated 190 238 laser, where the modulationof the cavity size by pres- referencehydrophoneandreciprocitycalibrationoruse 191 239 4 waterice Vergleich Wasser Eis 1 10-1 TL2_SPATS_Ch2_Rec TL2_SPATS_Ch4_Rec 0 10000 20000 30000 40000 50000 60000 70000 80000 frequency [Hz] Figure7: Averageambientnoiselevel measuredbySAUND(from Figure 6: Preliminary result for the ratio of sensitivities of a third [50]) generationSPATSsensordeployedinwaterandice(from[49]) detectorscouldbetosignalsfromneutrinointeractions 272 240 ofcalibratedemitters. Withthelastmethodallsensors 273 at given environmental conditions like acoustic noise, 241 usedfortheAMADEUSprojectwerecalibratedinde- 274 signalattenuationand refraction. A few prominentre- 242 pendenceof the azimuthal and zenith angle in a water 275 sultsofthedifferentgroupswillbediscussedbelow. It 243 tank [33]. One has to keep in mind, however, that the 276 ishoweveroutofthescopeofthispresentationtogive 244 sensorsensitivitydependsonthespecificenvironmental 277 a detailed overview about presently available data. To 245 conditionsatthe deploymentlocationas e.g. tempera- 278 getmoreinformationthereaderispointedtotherefer- 246 ture and pressure. Together with a NATO institute the 279 encesgiveninthispaperandto theproceedingsof the 247 ONDE-group has developed a standard procedure for 280 ARENAconferencessince2005[9]. under pressure calibration. They reported a sensitivity 248 249 changeofabout2dB/1000mwaterdepth[47]. 281 6.1. SAUND In-situ calibration in ice is even more complicated The “Study of Acoustic Ultra-high Neutrino Detec- 250 282 thaninwater. Besidepressureeffects,deeptemperature tion-SAUND”startedin2003. Sevenhydrophonesof 251 283 andimpedancechangesbetweenice andwater haveto themilitaryAUTECarrayneartheBahamaswereused 252 284 betakenintoaccount. TheSPATSgrouphascalibrated tosearchforacousticsignals.In196days65×106trig- 253 285 theirsensorsinwaterandstudiedseparatelythedepen- gersweretakeninasensitivevolumeofabout15km3. 254 286 denceonpressure(inawater-oilmixture),temperature The data were used to calculate the first acoustic neu- 255 287 (in air) and impedance(measuringnoise levelchanges trinofluxlimit[39]. 256 288 duringsensorfreeze-in).Assumingthatthecorrespond- Inthesecondphaseofthisexperimentthenumberof 257 289 ingsensitivitychangescanjustbemultiplied,theygota used hydrophonesincreased to 49 and the volume un- 258 290 finalresultwithabout40percenterror[48]. derstudyto1500km3. 130daysofdatatakingallowed 259 291 A direct comparison of calibration results in water detailedstudiesofacousticnoisebehaviour(seefig.7) 260 292 and ice at normal pressure was done for a third gen- [50]. In a complex data reduction and signal process- 261 293 eration SPATS sensor in the Aachen-Acoustic Labora- ing procedure two events were found compatible with 262 294 tory [49]. Preliminary results of a reciprocity calibra- showersfromneutrinointeractionsabove1022 eV.The 263 295 tion, shown in fig. 6 are consistentwith the estimation resultwasusedtoderiveanimprovedneutrinofluxlimit 264 296 describedabove.Thefinalresultofthesemeasurements (seefig.12)[51]. 265 297 isexpectedtobepublishedsoon. 266 6.2. Acorne 298 The “Acoustic COsmic Ray Neutrino Experiment - 6. Acoustictestarrays 299 267 ACORNE” is an activity of different groups in the 300 Duringthelastdecaderesultswerereportedfromsix UK. Eight hydrophones of the RONA military array 268 301 testsiteseitherusingpartofmilitaryhydrophonearrays in North-West Scotland are used for acoustic signal 269 302 or installing own acoustic sensors. The main purpose searches. Between2006and2008nearly30Tbofraw 270 303 of these studies was to clarify how sensitive acoustic data have been collected. Advanced signal processing 271 304 5 Figure8:Acousticsignalsfromaspermwhale(from[47]) andfilteringtechnologiesweredevelopedbythegroup 305 and applied to the data [52, 53]. Corresponding data 306 reductionand efficiency calculations allowed to derive 307 a neutrino flux limit in the same energy range as the 308 Figure9:Acousticeventreconstructedasupwardgoingneutrino-like SAUND result (see fig. 12). Furthermore information 309 signal(from[23]) aboutsignalattenuationandlocalizationwascollected. 310 The examination of the data for black hole signatures 311 lead to the calculation of a corresponding upper limit 312 [54]. 313 6.3. ONDE 314 Within the “Ocean Noise Detection Experiment - 315 ONDE” four hydrophonesforming a tetrahedronwere 316 deployedin2005toabout2000mdepthintheMediter- 317 raneanSee25kmoffshoreofCatania/Italy. Noisewas 318 monitoredaboutfive minutesevery hour. The average 319 noiselevelinthe20-43kHzfrequencyregionwasfound 320 to be 5.4 ± 2.2 ± 0.3 mPa [55]. The noise level was 321 strongly correlated with the actual environmentalcon- 322 ditionsattheseasurface. 323 Signalsobservedinthenoisedatamoreoftenthanex- 324 Figure10:Angulardistributionoftransienteventsobservedwiththe 325 pectedwereproducedbyapopulationofspermwhales. AMADEUSdetector(from[58]) Analyzingthesesignalindetail(foranexampleseefig. 326 8)evenallowedtoderiveageandgenderoftheanimal 327 328 emittingit[47]. Correspondingstudieswillcontinuein 344 andfirstresultsareexpectedtobeshownatnextyears 329 collaborationwithmarinebiologists. 345 conferences. 330 6.4. Baikal 346 6.5. ANTARES A subgroup of the collaboration which constructed Adetaileddescriptionofthedesignandperformance 331 347 andoperatesthefirst opticalneutrinotelescopein lake of the “ANTARES Modulefor the AcousticDEtection 332 348 Baikal[56]hasdeployedadigitalhydro-acousticmod- UndertheSea-AMADEUS”canbefoundin[33]. 36 333 349 ulewithfourhydrophonesataregulartetrahedronof1.5 sensors are located at three storeys of both string 12 334 350 medgelengthin150mdepthatoneoftheouterstrings and the instrumentation line of the ANTARES optical 335 351 of the opticalarray. With thisdevice andits predeces- neutrino telescope [34] located at a depth of ∼ 2500 336 352 sors extensive environmental studies have been done. m in the Mediterranean sea about 40 km distant to 337 353 The noise level was measured most of the time below Toulon/France. Data taking started in 2007. Exten- 338 354 5mPa. Acceptingonlysignalsfromthe deeplake one sivenoisestudieshaveshownstrongcorrelationstoac- 339 355 interesting neutrino-like event has been observed (see tualweatherconditions. Ahighstatisticsfrequencyde- 340 356 fig. 9). pendentnoise measurementreflectssimilar resultslike 341 357 InMarch2011anacousticstringwiththreeacoustic thosefoundwithONDEinthedeepseanearSicily(see 342 358 moduleshasbeendeployed[23].Datatakingisongoing section6.3). 343 359 6 The arrival direction of transient acoustic signals 360 could be determined (see fig. 10) [57]. It was found 361 107 thatthesesignalshavemostlyantroprogenicoriginbe- 362 106 ingduetoheavyshiptrafficinthisregionofthesea. 363 105 364 6.6. SPATS 104 365 The “South Pole Acoustic Test Setup - SPATS” has −1 ]sr 103 beendeployedin the upperemptypartof holesdrilled 1 336676 ftiocrtshtaetiIocnesCuwbiethneturatrnisnmoiottbesresrvaantdoryre[c5e]i.veSresveanreacpooussi-- −2− yrm 11002 368 k 369 tionedbetween80mand500mintheiceattheSouth E) [ 1 370 Pole,withamaximumdistanceofabout520mbetween Φ(10−1 SPATS12,70mPa,measured 371 strings [36]. Data taking started in early 2007. Since E 10−2 ASANUITNAD I III thenresultshavebeenpublishedforthespeedofsound ACoRNE 372 10−3 Proton Model, Auger ofpressureandshearwavesandtheirrefractionversus Proton Model, Hires 373 depth[41],theacousticattenuationlength[40]andthe 10−4 Mixed Comp. Model, Hires 374 ESS model ambientnoiselevel[48]. 10−5 375 107 108 109 1010101110121013101410151016 log (E/GeV) 10 ν 600 Legend: Figure 12: Available neutrino flux limits from acoustic test arrays Acoustic Event comparedtosomemodelpredictionsandresultsfromsomeradiode- Receiver tectorssensitiveathighestenergies(from[48]) 400 IceCube RW Amanda RW IceCube Hole thecorrespondingresultsareshownandcomparedwith 386 200 predictions of one frequently quoted cosmogenic neu- 387 trino flux model and its recent modifications [2]. The 388 m] presentlybestlimitintheconsideredenergyregionhas y [ 0 389 beenpublishedrecentlybytheANITAcollaborationus- 390 ing radio antennas as payload of a balloon circling in 391 −200 a hight of 35 km around Antarctica [8]. The detec- 392 torwassearchingforpulsesofcoherentradioemission 393 −400 394 from neutrino induced cascades in the ice below in a volumeofabout1.6×106km3. 395 Theacousticneutrinofluxlimitsaretodaystillmore 396 −600 −400 −200 0 200 400 600 than four orders of magnitude less sensitive than the 397 x [m] bestradiolimits which is partlyexplainedby the huge 398 differenceinthecorrespondingdetectionvolumesused. 399 Figure11: Distributionoftransientsignalsourcelocationsinthex- Whatmakesalllimitsdifficulttointerpretarethepartly yplanecomparedwiththepositionofIceCubeholesandRod-wells 400 unknownsystematicerrorsofthemeasurementsandthe (from[48]) 401 reliabilityofassumptionsmadeforefficiencyandsensi- 402 Transienteventswereobservedfromre-freezingIce- tivitycalculations.Thishasbeendiscussede.g.in[48]. 376 403 Cube holes and from water reservoirs used for hole 377 drilling (see fig. 11). From the non-observation of 378 8. Futureactivities 404 acoustic signals in a region outside the IceCube con- 379 380 struction area a neutrino flux limit has been estimated 405 The experiments mentioned in section 6 have in 381 (seefig.12). 406 the meantime either finished or achievedtheir primary goals. Follow up programsare underdiscussion or al- 407 readyinaplanningstage. 7. Acousticneutrino-fluxlimits 408 382 383 Neutrino flux limits have been derived until now in 409 8.1. acousticdetectioninwater threeacoustictestexperiments[51,53,48],allnotop- With the present facilities “fake neutrino sources” 384 410 timized or not designed for this purpose. In fig. 12 (see section 4.3) will be used to get better knowledge 385 411 7 412 aboutin-situ detector efficiencies and trigger schemes. 461 (2009);P.Latridouetal.,(eds.)ProceedingsofARENA2010,in 413 Onanintermediatetimescaledesignstudiestowardsa 462 Press km3-sized hybridacousto-opticdetectorwillhopefully 463 [10] G.A.Askaryan,AtomnajaEnergiyaV3(1957)152 414 464 [11] G.A.Askaryan,B.Dolgoshein,JETPLett.25(1977)213 be followed by its construction in the Mediterranean 415 465 [12] T. Bowen, Proceedings of the 15th ICRC, Plowdiv, vol.6 416 KM3Net project [47]. 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Aynutdinov et al., NIM A (2010) sucha detectorarenowmostlyknownatthatlocation. 424 478 doi:10.1016/j.nima.2010.11.153 425 Corresponding Monte Carlo design studies are under 479 [24] L.Sulaketal.,NIM161(1979)203 way. Theyshow,thatalreadya100km3 scaledetector 480 [25] R.Golubnichyetal.,ProceedingsofDUMAND1979,p.148 426 wouldneeda largenumberof holesto be drilled. The 481 [26] V.I.Abduletal.,Instr.Exp.Techn.44(2001)327 427 482 [27] K.Grafetal.,Int.J.Mod.Phys.A21(2006)127 428 developmentof robotic drilling and deploymentmeth- 483 [28] G.DeBonisetal.,NIMA604(209)199 429 ods is mandatory for such a project. 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