Amplitude Calibration with Acoustic Transducers

1. Amplitude Calibration with Acoustic Transducers Jonathan Perkin - University of Sheffield

2. Contents • Introduction: The calibration problem • The ideal calibration tool • Single source emitter • Hydrophone modelling • Laboratory results so far • Proposed deployment in the field • Linear phased array • Understanding the environment • Summary Deep inelastic scattering, with the sea as your calorimeter…

3. neutrino Sea/Ice/Salt Hydrophone Array Acoustic Pressure Waves The problem of amplitude calibration for acoustic neutrino telescopes • In order to successfully undertake astronomy via the acoustic detection of UHE neutrinos it is essential that we can reconstruct its trajectory and its energy • Therefore one must have some facility with which to calibratehydrophones w.r.t to the energy of a given interaction • The problem posed is that there is no natural background against which calibration can be performed (c.f atmospheric muons in other particle detectors)… Far field Radiation pattern 100 Pressure (Pa) 10-3 Angle (degrees) -5 5

4. The ‘ideal’ calibrator • The ideal calibration device for an acoustic detector will produce a thermoacoustic emission identical to that emitted by the hadroniccascade resulting from a UHE neutrino interacting in the sea • We have already seen that proton induced showers produce equivalent energy deposition to neutrino induced showers - however it may be impractical to put a protonaccelerator 2km deep in the sea T.Karg Arena2005 The principle of thermoacoustic emission from lasers has been proven in the laboratory - it would be advantageous if this can be done in the field

5. Calibration with lasers? Short duration laser pulses → energy deposition at any given point is a delta function in time • PAPV counter optical system • Powerful laser pulse (1.5 J at 1060nm or 0.2J 530 nm) operates at ranges from 300 to 1,500 metres - deployable? Path length limited by reflecting element? → angular spread of the acoustic pulse mimics that of the shower Extra (de)focussing optics to control lateral spread of the energy deposit → ensures the pulse shape and frequency spectrum mimic that of a shower • Direct excitation of thermoacoustic emission may prove impractical, move to transducers instead…

6. Single source emitter • Can use electrical theory to model piezo electric effect in piezo-ceramics • Characterise the transfer function (TF) of a hydrophone using known inputs such that the required input for a given desired output can be determined [O.Veledar, S.Danaher] • Apply known signal (e.g. step) • From step response determine transfer function (increase order of model to find best fit) [RC circuit model represents transfer function] • From transfer function can determine impulse response (IR) • We want to determine the input u required to generate a bipolar output (dGaus) • u = IFFT[ FFT(dGaus) / FFT(IR) ]

7. V V c - i n = IC R I c I = IR - IC V i n V c Hydrophone modelling • 5th order RC circuit model used to characterise hydrophone via the equivalent circuit technique Omnidirectional emitter in breathing mode: Single order circuit model: 1R and 1C

8. Laboratory results so far:Sheffield tank • Limited by reflections in tank, shown here is response to 10kHz single cycle sine (measured and predicted) B&K 8106 Hydrophone (4.0 dB) 0.1 Hz to 80kHz with B&K Amplifier Data Acquisition system NI DAQ Card-6062E (for PCMCIA) 500 kS/s, 12-Bit, 16 Analogue Input

9. Laboratory results so far: University swimming pool • Much larger dimensions so notlimited by reflections, however large low freq (~3kHz) background from pool pumps (x2) • Must dejitter pulses because of large varience in trigger level Excitation pulse

10. Laboratory results so far: University swimming pool • Time averaged pulses after dejittering give much cleaner signal than in tank… • No filtering of received signal here • There still remains some features either side of pulse, currently testing 7th order model to see if an improvement can be achived

11. Laboratory results so far:Kelk Lake • Use of Kelk lake permits both tests over a large dimension (>30m separation source-emitter) depth ~10m • Only mild background due to surface conditions, and the occasional duck…

12. Laboratory results so far:Kelk Lake • Hitting hydrophone hard (>10V peak input) appears to excite non-linear modes - no longer guarantee increase in source level with corresponding increase in excitation amplitude - requires compensation? • Have satisfied ourselves we can see signal at 30m separation, and can gain a factor of 10 in signal/noise with offline filtering • Something like >90% of background is below ~2.5kHz - high pass filter improves signal/noise • Therefore should be able to see pulses from omnidirectional calibration source on at least one of the Rona hydrophones when deployed in the field…

13. Linear phased array • In analogy to the coherent emission of radiation along the hadronic cascade resulting from a UHE interaction in the sea, a linear array of hydrophones can be constructed to emit signals in phase such that an interference pattern similar to the neutrino induced acoustic pancake is formed • Theoretical modelling thus far has indicated that between 6 and 8 elements are required in order to generate the desired angular distribution of the acoustic emission….

14. Creating a bipolar pancake • How many individual bipolar sources do we need to generate a suitable pancake? • 1.2x1020eV pulse simulated 1km from source • N sources deployed over 10m with (10/N)m spacing • Study the angular profile as a function of the number of sources • Of order 6 to 10 hydrophones (minimum) are needed

15. Understanding the environment • Refraction will deform our pancake…. • Not so important at Rona (hydrophone locations uncertain, thermaly well mixed water) …Needs careful understanding of acceptance of array geometry at deeper sites

16. Proposed deployment in the field • The ACoRNE collaboration has chartered a vessel in order to facilitate the addition of calibration pulses to the water surrounding the Rona hydrophone range • Awaiting confirmation from MoD(QinetiQ) of readiness • Will also deploy an SVP profiler • It is imperative that a calibrated bipolar pulse is successfully registered by the Rona array this summer…

17. Future amplitude calibration activities • Complete linear phased array device • Deploy single + multi-element devices both at the Rona site and any other available locations - Nemo test site?… • The problem of deployment is an open issue, have not resolved: • Rigidity • Inclination, rotation • Amplitude control • Depth proofing • … • However, if we get to the stage of worrying about this we should be happy of our progress

18. Summary • The ACoRNE collaboration has characterised the response of a hydrophone such that a spherically emitted bipolar pulse can be introduced above the Rona array • Theoretical modelling has suggested that in principle 6-10 omnidirectional sources can reproduce desired angular emission • The single element calibrator will be deployed over Rona this summer • Future devices should be robust enough to deploy at >2km depth