Acoustic neutrino detection
at the South Pole:
latest results from SPATS
Delia Tosi
Acoustic Neutrino Detection working group
IceCube Collaboration
July 15th, 2009
TEV Particle Astrophysics 2009
SLAC National Accelerator Laboratory
Delia Tosi
SLAC National Accelerator Laboratory
TeVPA 2009 – July 15th
Outline
1)
Hybrid neutrino detection
2)
Acoustics at the South Pole
- IceCube & South Pole Acoustic Test Setup (SPATS)
3)
Results:
- sound speed
- noise: vs. depth and time behavior
- transients localization
- attenuation length
4)
Conclusions and open questions
Delia Tosi
SLAC National Accelerator Laboratory
TeVPA 2009 – July 15th
Hybrid neutrino detection
air
Optical Cherenkov
dense medium
v-induced
cascade
Radio Radio/Acoustic
acoustic
“pancake”
coherent
radio signal
optical
Cherenkov signal
UHE neutrino events are HYBRID:
• Optical: IceCube, Antares … HYBRID detection
• Radio: RICE, ANITA …
• Acoustic: SAUND, ONDE …
Delia Tosi
possible in ice
• extend energy range of sensitivity
 large volume (?)
• calibrate R/A with O and R & A
• reconstruct energy and direction
• reject background
SLAC National Accelerator Laboratory
TeVPA 2009 – July 15th
Simulation studies
astro-ph/0512604
10 km3
• IceCube + 13 optical strings: 60 DOMs between 1.4/2.45 km
+ 91 radio/acoustic strings :
… BUT
FIRST:
5 radio antennas (every
100m
in [200-600 m])
WHAT ARE
THE ACOUSTIC
PROPERTIES?
300 acoustic
receivers
(every 5m ICE
in [5-1500
m])
• Inputs: ESS GZK flux model (ΩΛ =0.7) + ice model + hadronic shower models + etc ..
Delia Tosi
SLAC National Accelerator Laboratory
TeVPA 2009 – July 15th
South Pole Acoustic Test Setup (SPATS)
4 strings in IceCube holes
D
C
• instrumented depth:
80 m - 500 m
per string:
A
• 7 sensors
B
• 7 transmitters
String-PC
• digitization
• time stamping
Master-PC
• data handling
• GPS timing
• data transfer via satellite
Strings A, B, C installed in 2006/07
Monitoring through daily mails
String D installed in 2007/08
Delia Tosi
SLAC National Accelerator Laboratory
TeVPA 2009 – July 15th
SPATS stage design
Transmitter:
String-D:
• HV generator
• improved sensors:
• ring shaped piezoceramic
coated in epoxy
mechanical decoupling of channels
• improved transmitters: higher power
Sensor:
• HADES:
• 3 piezoelectric ceramic tablets
• pre-amplifier
alternative sensor design with
• analog signal transmission
a piezoceramic outside the
• steel pressure housing
steel housing
HADES
String D
Strings A,B,C
Delia Tosi
SLAC National Accelerator Laboratory
TeVPA 2009 – July 15th
Acoustic Pinger
winch +
4 - wires cable
batteries &
GPS receiver
• Retrievable transmitter used in water filled holes,
before IceCube deployment as unique source for:
- calibration of the detector
- attenuation length analysis
- sound speed measurement
• 6 holes in 2007/2008; 4 holes in 2008/2009
down to 500 m depth
two stops at 190, 250, 320, 400, 500 m depth
waveform shape and
amplitude reproducible
• High quality data 2008/2009:
in-water
stage
- centralizer to avoid swinging
- higher repetition frequency
Delia Tosi
SLAC National Accelerator Laboratory
TeVPA 2009 – July 15th
Icecube, SPATS and pinger holes
SPATS
Pinger 07-08
~1 km
Pinger 08-09
Delia Tosi
SLAC National Accelerator Laboratory
TeVPA 2009 – July 15th
SPATS goals & results
SPATS: investigate feasibility of acoustic neutrino detection at the South Pole
 Goal is to gain information about:
- Sound speed:
what is the sound speed value?
is it depth dependent (= refraction?)
- Transient events:
are there transients events?
what are their features (rate, sources)?
could they be a significant source of background?
- Noise:
what is the noise level?
which neutrino energy threshold does it correspond to?
- Attenuation coefficient:
never measured up to now, only models are known
depth dependent?
frequency dependent?
Delia Tosi
SLAC National Accelerator Laboratory
TeVPA 2009 – July 15th
Sound speed profile measurement
agreement
gradient
consistent with 0
• 2 combinations ~125 m distance
from pinger data season 2007-2008
• better than 1% accuracy
• First measurement in situ for P and S
waves
vP (375m)  3878  12 m / s
vS (375m)  1975.8  8.0 m / s
Delia Tosi
SLAC National Accelerator Laboratory
TeVPA 2009 – July 15th
Transient events: sources localization
Triggered mode data taking:
• Events above threshold
recorded independently
on 3 sensors on each string
• Offline coincidence requirement
• Vertex reconstruction from arrival
times
Two kind of sources identified:
• stable: water reservoir wells
• temporary: freezing holes
Residual <1 event / day from unidentified
source
Delia Tosi
SLAC National Accelerator Laboratory
TeVPA 2009 – July 15th
Noise: Temporal evolution
String D: deployed 24 Dec 2007
• peaks correlated with IceCube drilling,
SPATS:
inter-string data taking
Gaussian 320 m
DS7-2
• Hypothesis:
500 m
IceCube drilling
freeze-in improves coupling to ice causing
in the first couple months
400 m
σ (V)
noise level to increase and then stabilize
σ increase
after deployment
firn
mean noise level
24 Dec 07
500 m
17 Jun 08
From laboratory measurements:
• Sensitivity changes from 1 to 100 bar < 30%
• Sensitivity increases by a factor 1.5 + 0.2
self noise
from 0ºC to -50ºC in air
• Noise level below firn < 10 mPa
Delia Tosi
SLAC National Accelerator Laboratory
TeVPA 2009 – July 15th
Pinger attenuation analysis -1
Pinger data from season 0809:
– single channel
– pinger stopped at same depth
– aligned holes (all 2008-2009)
 Minimized systematic uncertainty:
residual azimuthal/ polar angle variation of
sensitivity
1 example channel
• ENERGY calculated in time domain for each
channel and over all the holes, noise subtracted
from pinger-off runs
• LINEAR FIT of y = ln (√E d)
• 47 independent measurements;
45 after quality cut |α| ≥ 3 σα
• Weighted mean value and width of distribution:
α = 3.3 ± 0.7 10-3 m  λ ~ 306 ± 64 m
• Depth dependence?
• Frequency dependence?
Delia Tosi
SLAC National Accelerator Laboratory
TeVPA 2009 – July 15th
Pinger attenuation analysis -2
Cross-checks:
• Calculated spectrum of signal and noise using
time-window selection before averaging
• Noise subtraction from signal-off windows
• ENERGY calculated in frequency domain
• Integration over the whole frequency band
 Results consistent with previous
• Calculated spectrum of waveform after
averaging
• Noise subtraction from pinger-off runs
• ENERGY calculated in frequency domain
• Integration over 2 selected bandwidths
 No significant trend observed
Delia Tosi
3-17 kHz
SLAC National Accelerator Laboratory
17-30 kHz
TeVPA 2009 – July 15th
Inter-string attenuation analysis: 2 methods
Inter-string data:
- pulse with a frozen-in transmitter
- listen with all the other sensors
• single-level method:
combine a single transmitter with all sensors
at the same depth
 Systematic uncertainty:
combines unknown angular response function
of sensors
 λ ~ 320 ± 100 m
• ratio method: ratios of all the combinations
 Systematic uncertainty:
combines unknown angular response function
of sensors and transmitters
 λ ~ 210.0 ± 75.8 m
Delia Tosi
SLAC National Accelerator Laboratory
TeVPA 2009 – July 15th
Summary
SPATS: experiment designed to study the feasibility of an acoustic neutrino detector at the
South Pole.
Significant achievement in each goal set:
• Sound speed:
•1st measurement of the sound speed in deep ice both for S and P waves
• Noise level:
•stable and Gaussian, decreases with depth
• with reasonable assumptions < 10 mPa below 250 m
• Transient noise:
• transients acquisition ~60% live-time
• vertex reconstruction achieved; sources identified.
• possibility to study attenuation frequency dependence
• Attenuation length:
• pinger data allowed for measurement in the frequency range up to 20 kHz ~ 300 m
• inter-string analyses confirm the pinger result
Delia Tosi
SLAC National Accelerator Laboratory
TeVPA 2009 – July 15th
Open questions
Attenuation length smaller than expected …
• Reason for difference between expectation and measurement?
 work in progress: new models are under discussion
• Is neutrino acoustic detection in ice feasible at the South Pole?
 the detector concepts have to be re-designed
 new simulations have to answer the question
Delia Tosi
SLAC National Accelerator Laboratory
TeVPA 2009 – July 15th
Thermo acoustic model
In the lab:
• Sudden deposition of energy generates pressure wave
• Thermo-acoustic model confirmed @ Brookhaven 1979
200 MeV proton beam (LINAC), 4.5 cm diameter
energy deposited in water 10191021 eV
• Bipolar acoustic pulse proportional to  = c2  / CP
c = sound speed in medium  = expansion coefficient
CP = specific heat of the medium
From neutrinos:
• Hadronic shower formation at interaction vertex
carries (on average) ¼ E
 generates energy deposition in the ice
Delia Tosi
SLAC National Accelerator Laboratory
TeVPA 2009 – July 15th