Amplitude Calibration with Acoustic Transducers

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Amplitude Calibration with Acoustic Transducers

• Jonathan Perkin - University of Sheffield

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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
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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 calibrate hydrophones 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
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The ideal calibrator

• The ideal calibration device for an acoustic
  detector will produce a thermoacoustic emission
  identical to that emitted by the hadronic cascade
  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 proton accelerator 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
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Calibration with lasers?
Short duration laser pulses ? energy deposition
at any given point is a delta function in time
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

• 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?

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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)

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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
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Laboratory results so farSheffield tank

• Limited by reflections in tank, shown here is
  response to 10kHz single cycle sine (measured and
  predicted)

BK 8106 Hydrophone (?4.0 dB) 0.1 Hz to 80kHz
with BK Amplifier
Data Acquisition system NI DAQ Card-6062E (for
PCMCIA) 500 kS/s, 12-Bit, 16 Analogue Input
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Laboratory results so far University swimming
pool
Excitation pulse

• Much larger dimensions so not limited by
  reflections, however large low freq (3kHz)
  background from pool pumps (x2)
• Must dejitter pulses because of large varience in
  trigger level

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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

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Laboratory results so farKelk Lake

• Use of Kelk lake permits both tests over a large
  dimension (gt30m separation source-emitter) depth
  10m
• Only mild background due to surface conditions,
  and the occasional duck

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Laboratory results so farKelk Lake

• Hitting hydrophone hard (gt10V 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 gt90 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

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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.

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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

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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
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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

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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

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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 gt2km depth