Acoustic Sensors for the ANTARES Experiment

1. Acoustic Sensors for the ANTARES Experiment supported by the BMBF Christopher Naumann, Physikalisches Institut IV Universität Erlangen-Nürnberg ARENA Workshop,Zeuthen, 2005

2. Acoustic Particle Detection – Sound Production Neutrino createshadronic shower in water • energy is deposited heats water up local expansion+relaxationpressure signal (bipolar)  sound range in water: O(km) (3.1km for 10kHz,135m for 100kHz) other marine sources of sound: wind, waves, ships, animals

3. Signal and Noise Spectrum in the Sea • noise depends on wind speed • at high frequencies dominated by thermal noise • Expected signal (green, arbitrary units):maximum between 10 and 50kHz, where noise is minimal (at sea state zero) •  look for signal in frequency band ~10 to ~50kHz  try to look for neutrinos in the deep sea !

4. The ANTARES Neutrino Telescope Antares-Design: 12 Strings x 25 Storeys x 3 optical modules for the optical detection of neutrinos from 100 GeV up to 1 PeV Additional goal: use ANTARES as test infrastructure for acoustic detection !

5. Aim: Acoustic Storeys • Equip several ANTARES storeys for acoustic particle detection… 1.) remove optical components (photomultipliers, etc.) ? 2.) replace with acoustic sensors "Eyes of ANTARES" "Ears" Unfortunately, it's not as easy as that…

6. ANTARES Acoustics: Requirements • ANTARES site 2400|m below the sea: • must be pressure resistant (>250|bar) and able to survive hostile environment (sea water at ~13°C) • expected pressure signals from neutrino events (Askaryan):~10mPa peak-to-peak for 1018eV in 400m distance • require very sensitive sensors (µPa – mPa) • high acoustic background in the sea (~2.5 mPa between 10-100kHz at no wind) • need sophisticated background suppression (filters, coincidence triggers) • future acoustic detector needs at least O(103) sensors (probably more)  cost of single sensors should be as low as possible

7. Concept One: Single Hydrophones easiest method: build an „array“ of hydrophones, which are attached to the optical module frame • hydrophones are used by the navy and for fishing • idea: buy commercial hydrophones • alternative: customisedhydrophones from ITEP • commercial hydrophone(here: HTI) • guaranteed waterproof and pressure-resistant • already calibrated • notoptimised for neutrinos ! Difficult to change specifications ! • currently sensitivity too poor ( <<1V/Pa) hydrophones electronics possible design for „acoustic storey“ with 6 hydrophones

8. Self-Made Hydrophones alternative: build our own hydrophones from scratch 5cm polyurethane coating against water and pressure piezo tube pre-amp+cable 2cm  can build to our own specifications (piezo, amplifier, coating, directional characteristics)  probably much cheaper(<200€ ?)  R&D necessary ! small hydrophone prototype for test in pressure tank

9. Second Concept: “Acoustic Modules“ • basic idea: equip glass spheres of optical modules (OM) with acoustics hardware instead of photomultipliers OM 17" (42cm) "AM" acoustic sensors and electronics (schematic) Photomultiplier

10. Sensors - Layout piezo ceramics (eg. PZT-5A or PZT-7A), glued to glass sphere pre-amplifiers (70dB), band pass filters (3.4 – 72kHz) 2.5cm use an array of several sensors per sphere for background suppression via correlation sensor array for sphere must be able to calibrate and study sensors…

11. Signal Measurement and Sensor Calibration external or internal pre-amplifier "Fish Tank" calibrated transducer (HTI) calibration signal sensor response compare sent and received signals Þcalibration

12. Comparison of Signals - Examples hydrophone / sensor response to bipolar pressure signal from calibrated transducer (vertical scale identical) commercial hydrophone self-made hydrophone sensor in glass sphere signal shapes influenced by coating, glass sphere and pre-amplifier 200µs • limited amplifier bandwidth to reduce noise => signal distorted • have to find events in background => must understand signal shape To understand signal shape, must know sensitivity spectrum...

13. Device Calibration Calibration Chain: • Cross-calibrate transducers using identical pair • Use this calibration to calculate receiver sensitivity  can get complete spectrum from onlyone measurement per sensor device Sensitivity of Piezo+Pre-Amp piezo resonances “plateau” at -120dBre(V/µPa) •  can calibrate self-made sensors and sensor arraysin our laboratory • no need for complicated calibration source ! Use calibrated transducer only  only relative calibration  need at least one absolute calibration source as reference (laser ?) or fit transducer calibration curve to values given by manufacturer amplifier cut-off 10kHz 100kHz but lab measurements alone are not enough…

14. Test Experiment: AMADEUS • important for signal analysis: good knowledge of the acoustic situation at the ANTARES site and the performance of sensors in deep sea conditions... Basic Idea: test acoustic sensors and data acquisition + study acoustic background in situ using an autonomous acoustic system on an ANTARES test line Situation: ANTARES test string "Line Zero" successfully deployed in March - together with... AMADEUS = Autonomous Modulefor the Acoustic DEtection Underthe Sea

15. AMADEUS (Architecture) hardware for data taking and storage 5 acoustic sensors on walls and top top view 60cm batteries for >75 hours continuous data taking titanium cylinder 158mm

16. Data Taking and Storage Hardware • ADC-Board (PCI Card)(500kHz, 16bit, 2.5/10V)single or multiple channels, controlled via run script • autonomous PC-Board (small, low power consumption) • Buffer and Storage: • mass storage: 80GB HDD • buffer: 1GB compact flash card • Timer to start data taking after set time (hours to weeks) • read from 1 to 5 sensors (at 500kHz total) • write to CF card (silent !) • when flash card full: • stop data taking • dump card to disk (noisy !) • clear flash card repeat until hard disk full or data taking run completed

17. AMADEUS (Operation) • Deployed in March together with ANTARES test string,stayed there until the middle of May • first Data already available in March, as string was recovered and re-deployed after about a day ! • data taking successful…  have first acoustic data (about 12 GByte) from the ANTARES site ! data analysis has just begun… and we hear already something !

18. Sound of the Sea • at the ANTARES site, have acoustic beacons for positioning • can use these... • as calibration source • to study reconstruction algorithms (“find beacons“) amplitude (V) time (s) time (s) spectral density (au) First result:can find and identify individual beacons acoustic beacons but what we really want is hidden between the lines...

19. can see depth dependence of noise level (during line recovery) preliminary Sound of the Sea (2) – noise analysis • only small portion of data analysed: • measured noise spectrum roughly in agreement with predictions... but expected to get better as full data analysed noise spectrum, only few seconds of data noise spectral density (dB re µPa2 / Hz) SS 3 sea state zero sea state 1 0 10 20 30 40 50 60 70 80 90 100kHz AMADEUS = an audible success !

20. Conclusion and Outlook • Hydrophones and acoustic modules under development – first prototypes working fine • Autonomous experiment AMADEUS very successful – both sensors and data acquisitionworking fine • have acoustic data from the ANTARES site • next data expected soon (in May, Line Zero recovered on May 12th) • piezo+amplifier only noise level of self-made hydrophones and sensors already comparable to sea state zero (~4mV RMS at –120dBre(V/µPa)  4mPa RMS) • Top sectors of ANTARES strings 10 and 12 to be fitted with acoustic hardware (see Robert's talk) Thank you !