Helium detection using acoustic metamaterials

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Picture Courtesy: The Hindu

Context:

Researchers developed sound based sensors to detect helium leaks.

Helium (He):

Helium is a noble gas with atomic number 2 and symbol He, known for being colourless, odourless, non-toxic, and extremely chemically inert. It is the second lightest element after hydrogen and has an exceptionally low boiling point of about –268.9°C, allowing it to remain liquid near absolute zero and making it indispensable for ultra-low-temperature applications. Because helium atoms are very small and light, they escape easily from containers and even from Earth’s atmosphere, which makes long-term storage difficult.

Origin:

Helium was first identified in the Sun’s spectrum before it was discovered on Earth, and most of the helium in the universe was formed during the Big Bang, with additional helium continuously produced inside stars through nuclear fusion. On Earth, helium is mainly generated through the radioactive decay of heavy elements such as uranium and thorium, which release alpha particles that become helium atoms over geological timescales. This helium accumulates in certain natural gas reservoirs underground, from where it is commercially extracted as a by-product during natural gas processing, since atmospheric helium escapes into space and is not a practical source.

Applications:

• Cryogenics: Helium is primarily used in cryogenics, where liquid helium cools materials to extremely low temperatures and enables the operation of superconducting magnets in medical imaging systems such as MRI scanners as well as in advanced scientific research facilities.

• Aerospace and Defence: Helium plays a crucial role in aerospace and defence by being used to pressurize and purge rocket fuel tanks, cool sensitive instruments in satellites, and help detect leaks in high-precision space and military systems.

• Electronics and semiconductor industry: In the electronics and semiconductor sector, helium provides an inert atmosphere and efficient cooling during the manufacturing of microchips, optical fibers, and other precision components.

• Leak detection: Helium is widely used for industrial leak detection because its tiny atoms can pass through very small cracks, allowing engineers to identify leaks in sealed systems with very high accuracy.

• Lifting gas: Because helium is lighter than air and non-flammable, it is safely used as a lifting gas in balloons and airships, unlike hydrogen which is highly flammable.

• Scientific research: Helium is indispensable in scientific research, especially in low-temperature physics, nuclear magnetic resonance experiments, cryostats, and emerging areas such as quantum computing.

How Helium changes the sound?

Speed of sound is higher in Helium: Sound travels as vibrations through gas molecules, and its speed depends largely on the gas’s density and molecular mass. Helium atoms are much lighter than the nitrogen and oxygen molecules that make up air, so sound waves move significantly faster through helium than through normal air.

Effect on voice pitch: When a person inhales helium, the vocal cords vibrate at the same rate as usual, but the sound waves produced travel faster through the helium-filled vocal tract. This increases the resonant frequencies of the throat, mouth, and nasal passages, which amplifies higher-frequency components of the voice and makes it sound squeaky or high-pitched.

Change in resonance, not vocal cord vibration: The key change happens in the resonance of the vocal tract rather than in the actual vibration of the vocal cords. The faster speed of sound shifts the natural resonant frequencies upward, similar to how a smaller musical instrument produces higher notes than a larger one.

Application in gas detection: This same principle is used in advanced helium leak sensors, where sound is passed through a chamber and the resonant frequency is monitored. When helium enters and mixes with air, the speed of sound in the chamber increases, causing a measurable shift in resonance frequency that reveals both the presence and concentration of helium.

Technological significance of sound-based Helium sensing:

Works for a chemically inert gas: Helium is a noble gas that barely reacts with other substances, which makes traditional chemical gas sensors ineffective. By relying on changes in the speed of sound rather than chemical reactions, this device can detect helium reliably where many existing sensors fail.

Faster and more practical leak detection: Helium atoms are extremely small and escape through tiny cracks, causing costly and hard-to-find leaks in industrial systems. This acoustic sensor can detect leaks quickly because gas can enter the structure easily without damaging its sound-trapping ability, allowing rapid monitoring in real time.

Rugged and low maintenance: Unlike chemical sensors that use delicate coatings that degrade over time or react to humidity, this sensor is based on a solid physical structure. That makes it more durable, less sensitive to environmental changes, and less dependent on frequent recalibration, which lowers maintenance costs.

Helps in locating the leaks: Because the device has multiple sound-trapping corners, it can compare how the acoustic signal changes at different points. This allows it not only to detect helium but also to estimate the direction of the leak, which saves time during inspection and repair.

Conclusion:

The development of an acoustic, topological helium sensor marks a significant shift from chemistry-based gas detection to physics-based sensing. By using changes in the speed and resonance of sound, the device enables fast, reliable, and low-maintenance detection of helium leaks, even in harsh environments. This innovation is especially important for industries like healthcare, semiconductors, and aerospace, where helium is both essential and scarce, making efficient monitoring crucial for cost savings and resource conservation.

Source: The Hindu