Could this novel spectroscopy revolutionise radiation, groundwater and gas sensing?

Helium-3 (³He) may be a rare isotope, but it can play an outsized role in environmental monitoring and radiation detection technologies.  

Much recent attention has focused on high-precision spectroscopy studies of the ³He nucleus, offering fundamental insights into nuclear structure and quantum electrodynamics.  

But helium-3’s most direct relevance to environmental monitoring lies in its unique physical and nuclear properties that make it indispensable for advanced sensing applications.

Neutron detection in radiation sensing

³He is best known in applied science circles as the gold standard for neutron detection. When a thermal neutron interacts with a helium-3 nucleus, it triggers a reaction that produces a proton and a triton, releasing 764 keV of energy.  

This energy release is easily detectable using gas-filled proportional counters. As a result, ³He detectors are widely deployed for:

• Environmental radiation monitoring near nuclear facilities
• Occupational safety in nuclear medicine and power generation

The isotope’s effectiveness, combined with its low background noise and high sensitivity, has made it the preferred medium for many years, though supply shortages have led to increased interest in boron-10 and lithium-6 alternatives.

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Atmospheric, subsurface and groundwater tracing

³He also plays a critical role in tracing the movement of gases and fluids through the environment.  

The ³He/⁴He ratio, for instance, is a well-established tracer in:

• Groundwater and aquifer studies, helping map the age and movement of water
• Volcanic gas monitoring, where elevated ³He/⁴He ratios can signal magma movement and potential eruptions
• Oceanographic circulation models, particularly in deep ocean waters, through tritium-³He dating techniques
• Geological leak detection, such as verifying containment integrity at carbon capture and storage sites

These applications leverage helium’s inert nature and stable isotope behaviour, allowing scientists to track natural processes without introducing reactive or contaminating agents.

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Cryogenics and advanced sensing

Helium-3’s exceptionally low boiling point (~3.2 K) makes it a critical coolant in ultra-low-temperature environments. This property supports:

• Cryogenic sensors, including superconducting quantum interference devices (SQUIDs), used in magnetometry and geophysical monitoring
• Bolometers and calorimeters for high-resolution radiation detection
• Quantum sensing platforms for precision environmental and atmospheric measurements

While these applications are more common in research or specialised monitoring contexts, their role is growing as the push for ultra-sensitive environmental sensing expands.

From atomic physics to field instruments

Although the spectroscopy work conducted on muonic helium-3 ions may seem far removed from field instruments, this type of fundamental research has indirect but real implications for environmental monitoring. 

High-precision measurements of the ³He nuclear charge radius refine the constants and theoretical models that underpin many optical and quantum sensing technologies.

Improved models can lead to more accurate laser-based instruments, such as cavity ring-down spectrometers and LIDAR systems, by fine-tuning their atomic reference data.

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New insights from muonic helium-3

Beyond its applied uses, helium-3 is now at the centre of ground-breaking research with potential long-term benefits for precision instrumentation.  

A team from the PRISMA+ Cluster of Excellence and the Paul Scherrer Institute recently measured the charge radius of the ³He nucleus with unprecedented precision, using laser spectroscopy on muonic helium-3, an exotic form of helium where electrons are replaced by heavier muons.

Because muons orbit much closer to the nucleus than electrons, they are far more sensitive to nuclear size and structure.  

The new measurement – 1.97007 ± 0.00097 femtometers – is 15 times more precise than previous values obtained via traditional scattering methods.

Importantly, the researchers also determined the precise difference in nuclear radius between helium-3 and helium-4, refining our understanding of nuclear interactions in light atoms.  

The findings align with parallel studies using electronic helium-3 spectroscopy conducted in Amsterdam and confirm the validity of current nuclear models.

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Theoretical teams at Mainz contributed calculations on how muons interact with protons and neutrons, providing a tight coupling between theory and experiment.

For environmental instrumentation professionals, this may seem far removed from day-to-day fieldwork, but the implications are real:

Refined nuclear models and constants, such as the Rydberg constant, directly influence calibration of high-resolution spectrometers and quantum sensors.

The same theoretical improvements that benefit atomic spectroscopy and metrology support the development of more accurate environmental sensors.

Advances in modelling light nuclei inform radiation transport codes and detector designs, all of which is relevant to safety, compliance and innovation in monitoring.

To read the full paper, click here. 

By Jed Thomas