An international research team has been given the novel task of developing a practical means of monitoring underground stores of CO2 using none other than cosmic rays. The research hinges on the detection of the muons that occur as cosmic rays interact with the Earth's atmosphere, but which can penetrate several kilometers beneath the Earth's surface. It's thought that the approach could save significant amounts of money compared to alternative techniques.

Muons – remind me…

Muons are elementary particles, indivisible units of matter which also include quarks and bosons. Along with electrons, neutrinos and taus, muons belong to the family of elementary particles known as leptons, or those that are not subject to the


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strong interaction. Until the introduction of the Euro, a lepton was also one hundredth of a drachma, Greece's former unit of currency, but that's neither here nor there.

So muons are like electrons, then?

Yes, very much so! Except, not really, no.


Well, like an electron, muons have charge of −1 e and a spin of 12. But muons are much heavier than electrons, having about 200 times the mass. Mark that, it'll be useful later. Muons are also a bit on the unstable side, sticking around, on average, for no more than 2.2 millionths of a second.

That's pretty unstable

Yes, but spare a thought for the positively unhinged W and Z bosons, which exist for an average of about 3×10 −25 seconds. Compared to those guys muons really have their $#*! together.

That's good. And muons come from cosmic rays?

Where there are high-energy interactions with matter, muons are wont to follow. This is especially true when you bang hadrons together with a particle accelerator. But it's also the case with so-called cosmic rays, which is a somewhat antiquated term for what are really just high-energy particles (mainly protons) from space colliding with nitrogen and oxygen nuclei in the Earth's atmosphere.

…making muons?

Well the cosmic rays actually ionize the atmosphere, creating a downfall of light hadrons (pi mesons, specifically) which, in the merest blink of an eye (though really, much much faster than that), decay into a variety of leptons, including muons, and electrons too.

What makes muons so important?

Remember that muons are much more massive than electrons? That's really the same as saying that its energy content is higher (thanks, Einstein). This makes muons much more resistant to kinetic energy-sapping Bremsstrahlung, or braking radiation, the upshot of which is it's the muons that reach and penetrate the Earth rather than the electrons or anything else.

What on Earth has this to do with carbon sequestration?

Good question, though strictly speaking that's under Earth. The key to this is that, when passing through matter, the degree of scattering is determined by the size of the atomic nuclei of whatever substance the muons are passing through. If you can assess that scattering, you can assess the density profile of matter and hence identify the type of matter the muons have actually had to pass through. The idea is not to find potential CO 2 stores, but to monitor the CO 2 content of those already in use.

Is that possible?

Sure. Scientists have been noodling with muon detection to identify matter since 2003, when research Larry Schultz and colleagues at Los Alamos developed a detector with a view to scanning cargo (with the possibility of detecting illicit nuclear materials). The field of research is known as muon tomography.

Cargo's one thing…

Indeed, and this is early days for the research. But the researchers hope that this could lead to better cheap monitoring of underground CO 2 stores which could be cheaper and more effective than alternative techniques, which involve expensive technology for monitoring seismic data. Whereas that approach provides only occasional glimpses of what's happening underground, a muon detector would allow continuous monitoring. Professor Jon Gluyas of Durham University thinks this could save hundreds of millions of dollars every year.

And who's carrying out this research?

I thought you'd never ask. It's a team of particle physicists, geophysicists and engineers led by Durham University, and including (deep breath) the Universities of Sheffield and Newcastle, Bath University, Caltech, STFC Rutherford Appleton Laboratory, and the and NASA Jet Propulsion Laboratory.

Source: Durham University