High energy neutrino acoustic detection activities in Lake Baikal: status and plans N.Budnev for the Baikal Collaboration Irkutsk State University, Russia 2008

Collaboration  Institute for Nuclear Research, Moscow, Russia.  Irkutsk State University, Russia.  Skobeltsyn Institute of Nuclear Physics MSU, Moscow, Russia.  DESY-Zeuthen, Zeuthen, Germany.  Joint Institute for Nuclear Research, Dubna, Russia.  Nizhny Novgorod State Technical University, Russia.  St.Petersburg State Marine University, Russia.  Kurchatov Institute, Moscow, Russia.

Length – 720 km, width – km, depth m 20% of world fresh water

NT200 running since strings with 192 optical modules m height, R=20m, 1070m depth, V geo =0.1Mton -  effective area: >2000 m 2 (E  >1 TeV) - Shower Eff Volume: ~1 Mton at 1 PeV NT200+ commissioned April 9, new strings, 200 m height, 36 OMs - 1 new bright Laser for time calibration imitation of 10 PeV-500 PeV cascades, >10^13 photons/pulse w/ diffuser,  SNO-Calib - 2 new 4km cables to shore - DAQ – New Underwater & Shore Station: Underwater Linux embedded PCs, Industrial Ethernet Systems The Baikal Detector NT200+ NT200+ is tailored to UHE -induced cascades - 5 Mton equipped volume - V_eff >10 Mton at 10 PeV  4fold sensitivity gain.

ExtString 1 ExtString 3 ExtString 2 NT200 Central+Outer Str. New ShoreCable 100 m Foto from March, 2005, 4km off- shore: NT200+ deployment from 1m thick ice. Ice – A perfect natural deployment platform Ice is stable for 6-8 winter- weeks/year : –Upgrades & maintenance –Test & installation of new equipment –Operation of surface detectors (EAS, acoustics,… ) –Electrical winches used for deployment operations (all connections done dry)

Hydrophone 80 m 90 m Cherenkov EAS detector Acoustic pulse NT-200 muons 2000 year – detection of a short bipolar acoustic signal in coincidence with EAS ice

experiments to search for acoustic signals from EAS pinger EAS Detectors Acoustic antennas

The acoustic antenna Hydrophones Depth of central -6 m Depth of other – 4 m

Search of acoustic signals from EAS Reconstructed pinger location

Example of bipolar pulse

The questions What is the nature of the short acoustic pulses? Are they result of an interference or they are generated by any point-like sources? How the point-like sources are distributed (near a surface and at shallow depth or at all depths)? Do up going pulses appear due to refraction and reflection from bottom?

2003 y. An autonomous recorder to study the high frequency acoustic noise 1.4 m Hydrophones

Sample of recorded noise

Ice crack

time The short bipolar pulse

Acoustic noise in frequency band kHz at end of Much 2004 y. (under ice) 1mPa 10mPa (dB/Pa) Depths m

2005 year A prototype device for stationary acoustic neutrino detection in Lake Baikal

The main requirements –3 dimensional antenna of 4 hydrophones as minimum –Sampling frequency of ADC something like 200 kHz The additional requirements –On line under water analysis (in situ) to reject data flow to shore. –Operation mode with external Trigger : from Baikal Neutrino Telescope NT200+

Schematic view of prototype device ICRC-2007

Shore Connection: via NT200+ Acoustic DAQ AVIV Modem Flex DSL Modem AVIV Modem Electric shore cable Master signal From NT Mbit/s 512 kbit/s Prototype device PC Sphere of Instrumental String

Antenna and electronic components Tetrahedral antenna 1.5m 4 hydrophones H2020C 4-ch, 195kHz, 16-bit ADC AD7722 One-plate computer NOVA-C400 2 Mbit DSL modem

Acoustic sensors H2020C Tangentionally polarized piezoceramic CTS-19 Liquid polymethyl siloxane compensator Operating depth up to 1000m Sensitivity (-185 dB re 1 V/µPa)

Sensitivity: Hydrophone + 70dB amplifier

Prototype device 150m depth NT-200

Sound absorbed hats

The working modes 1.Continuous monitoring of statistical acoustic noise parameters. 2.Search of signals of definite waveform and length.

Statistical acoustic noise parameters Every ~1 second AD makes 1 file with channels statistic. Every 20 minutes raw data (~4.1 mb) transfer to shore

Channels statistic Mean Value (Noise Level): Noise level estimation: Processing of an array with elements : 1)2) 3) Internal Dispersion (for control of electronics) External Dispersion Total Dispersion

Noise level (from to ) Ice destruction Water

Acoustic noise level in the band kHz Wind Boats (70 mPa!)

Lake Baikal noise spectral density Sea state 2 Sea state 0

Time fraction of the acoustic noise with a certain level Most of time RMS of noise is a few milli Paskal

The search of signals of definite waveform and length

In situ analysis Acoustic Time Series From Hydrophones Extraction of Statistical Information Signal Extraction And Classification Store Signals in a Dynamic Arrays Check signals for “Time Window” condition Direction and Distance Estimation Final Data Representation Send Data to the Shore Station

We accept only signals which satisfy to the following inequality: - time of propagation of a signal from one hydrophone to another - distance between two hydrophones - sound velocity at the depth where antenna placed - error in estimation of “Time window” condition

Accepted and Rejected signals

Signal Extraction and Classification Procedure Statistical Module Signal Extraction Module Dynamic Array

Parameters of waveform for classification procedure The number of tops of pulse Maximum and minimum amplitude of each top of signal The total length of signal The length of each component of signal

Direction and Distance Estimation For signals from the distances >50m acoustic front is flat. We measure time coordinates of each signal: The procedure of obtaining consists in minimization of the functional: where Start point of a signal in time series Position of center of a pyramid in the chosen system of coordinates Positions of hydrophones The moment of passage sound front through a point Time estimation mistake

Direction and Distance Estimation Using minimization procedure on we also filter signals with wrong time points. For example: electric pickups. For signals from the distances below 50 m acoustic front is expected to be a sphere. In this case we can estimate distance within 5 m uncertainty.

Bipolar pulses from 4 hydrophones

Bipolar pulses duration distribution

Bipolar pulses amplitude distribution

Angle distribution of detected bipolar pulses Downward going pulsesUpward going pulses θ

Winter depth dependence: temperature and sound speed.

Outlook and Plans

Acoustic signal amplitude in dependence on shower energy Е

Absorption of sound in water Absorption length is 10 km for 40 kHz !

Acoustic string in Lake Baikal R&D of first stationary acoustic string. It should be deployed for long time operation in 2009 year.

Summary Typical high frequency noise level in Lake Baikal is a few mPa. Main source of the noise including bipolar pulses is near surface zone of the Lake, we didn’t detect any bipolar pulses generated in deep zone of the Lake. A preferable way to search an acoustic signals from neutrino is to listen water reservoir from top to down. A prototype device for search of acoustic signals form neutrino in water was constructed and is operating since April 2006 at depth 150 m together with the Baikal Neutrino Telescope. The results of our investigations show the feasibility of neutrino detection in Lake Baikal with a threshold energy as low as eV.

Thank you!

The end

Sparse instrumentation: 91 strings with 12/16 OM = 1308 OMs  Casacde effective volume for 100 TeV: ~ km³  Muon threshold between 10 and 100 TeV Baseline schedule: - R&D +TechDesRep Funded. - Construction ≥2009. A Gigaton (km3) Detector in Lake Baikal Mini NT200+

T MD = ∙10 -2 ∙ P ∙ ∙ P 2 - ( ∙ ∙ P) ∙ S Depth dependence of maximum density temperature