[Paper Review] Sensor development and calibration for acoustic neutrino detection in ice

T. Karg, M. Bissok+3 more|arXiv (Cornell University)|Jul 21, 2009

Astrophysics and Cosmic Phenomena5 references3 citations

TL;DR

This paper presents laboratory-based calibration of acoustic sensors for neutrino detection in Antarctic ice, focusing on temperature, pressure, and coupling effects. Using specialized facilities like the Aachen Acoustic Laboratory and Wuppertal Water Tank, it validates the thermoacoustic model via laser-induced signals and finds a linear increase in sensor sensitivity with decreasing temperature, enabling reliable in-situ calibration for future acoustic neutrino telescopes.

ABSTRACT

A promising approach to measure the expected low flux of cosmic neutrinos at the highest energies (E > 1 EeV) is acoustic detection. There are different in-situ test installations worldwide in water and ice to measure the acoustic properties of the medium with regard to the feasibility of acoustic neutrino detection. The parameters of interest include attenuation length, sound speed profile, background noise level and transient backgrounds. The South Pole Acoustic Test Setup (SPATS) has been deployed in the upper 500 m of drill holes for the IceCube neutrino observatory at the geographic South Pole. In-situ calibration of sensors under the combined influence of low temperature, high ambient pressure, and ice-sensor acoustic coupling is difficult. We discuss laboratory calibrations in water and ice. Two new laboratory facilities, the Aachen Acoustic Laboratory (AAL) and the Wuppertal Water Tank Test Facility, have been set up. They offer large volumes of bubble free ice (3 m^3) and water (11 m^3) for the development, testing, and calibration of acoustic sensors. Furthermore, these facilities allow for verification of the thermoacoustic model of sound generation through energy deposition in the ice by a pulsed laser. Results from laboratory measurements to disentangle the effects of the different environmental influences and to test the thermoacoustic model are presented.

Motivation & Objective

• To address the challenge of calibrating acoustic sensors in deep Antarctic ice, where in-situ calibration is impractical due to extreme conditions.
• To isolate and quantify the effects of low temperature, high pressure, and acoustic coupling on sensor response in ice.
• To validate the thermoacoustic model of sound generation in ice using pulsed lasers as a controlled calibration source.
• To develop laboratory facilities capable of producing large volumes of bubble-free ice and water for sensor testing under realistic conditions.
• To enable accurate energy and direction reconstruction in future acoustic neutrino detectors by characterizing sensor sensitivity and directivity

Proposed method

• Conducted laboratory calibrations in water and ice using the comparison method and reciprocity principle for sensor sensitivity and directionality.
• Established two dedicated test facilities: the Aachen Acoustic Laboratory (AAL) and the Wuppertal Water Tank Test Facility, each with large volumes of bubble-free ice (3 m³) and water (11 m³).
• Used pulsed Nd:YAG lasers at 1064 nm, 532 nm, and 355 nm to generate thermoacoustic signals in ice, with beam diameter and absorption length controlled via optics.
• Measured sensor response to laser-induced pulses using an array of 18 sensors to study spatial distribution, frequency content, and pulse shape.
• Analyzed signal waveforms and applied Fourier transforms to determine central frequency (≈100 kHz) and assess sensor transfer functions.
• Systematically varied temperature (from 20 °C to −25 °C), pressure, and coupling conditions to isolate their effects on sensor sensitivity.

Experimental results

Research questions

• RQ1 How does sensor sensitivity in ice vary with decreasing temperature, particularly at −50 °C as found at the South Pole?
• RQ2 What is the impact of high ambient pressure on acoustic sensor response in ice?
• RQ3 How does acoustic coupling between the sensor and ice affect signal amplitude and reliability?
• RQ4 Can laser-induced thermoacoustic signals accurately emulate the acoustic signals from hadronic cascades in neutrino interactions?
• RQ5 To what extent can laboratory measurements in water and ice be extrapolated to in-situ conditions in deep Antarctic ice?

Key findings

• A linear increase in sensor sensitivity was observed with decreasing temperature, indicating that cold conditions enhance signal amplitude.
• No significant change in sensor response was found with increasing ambient pressure, suggesting pressure has minimal impact on calibration.
• Laser-induced thermoacoustic signals were successfully generated and detected in a 3 m³ ice block using a 55 mJ pulse at 1064 nm wavelength.
• The central frequency of the detected pulse was approximately 100 kHz, consistent with expectations for a small beam diameter (~1 μm).
• The spatial distribution and frequency content of the thermoacoustic signal were measurable using a sensor array, enabling validation of the thermoacoustic model.
• The 355 nm laser line, with an absorption length of ~1 m, was identified as most suitable for emulating the 10 m-long energy deposition profile of a hadronic cascade.