I have lately been researching methods of "cold fusion", or fusion reactions which do not generate much waste heat, for use as a spacecraft power system (my propulsion system idea is separate, but requires large amounts of power). While doing so, I came across the idea of muon-catalyzed fusion. More details can be found in this Wikipedia link, but briefly: apparently replacing electrons in an atom with muons, a more massive subatomic particle, can catalyze fusion reactions by attracting the atomic nuclei with a greater force. I then looked up possible muon sources and found that they can be produced with relative ease by a beam of protons aimed at a sheet of metal.

However, I wonder if it is possible to conceive of a naturally occurring atom which contains muons instead of electrons (I have already been considering a superheavy element as a possible McGuffin). If so, it seems logical to suspect that heavier elements would contain more muons, based on probability. Could this be possible based on current scientific knowledge? I would appreciate anyone's input on this topic.

  • $\begingroup$ Sorry... but first off, the title card and the question you posed are different. Are you asking about a real periodic element that can be used to make a muon beam? Or are you asking about a fictional atom that has muons instead of electrons? $\endgroup$ Jan 25 at 15:13
  • 1
    $\begingroup$ As I read it, the OP is asking for a naturally occurring muon emitter. $\endgroup$
    – Zeiss Ikon
    Jan 25 at 15:16
  • $\begingroup$ @ZeissIkon "it is possible to conceive of a naturally occurring atom which contains muons instead of electrons" $\endgroup$ Jan 25 at 15:17
  • $\begingroup$ Sorry, I should have made that more clear; I am asking about a fictional atom which contains muons, and can therefore be used as a direct muon source. $\endgroup$
    – Lelu
    Jan 25 at 15:21

3 Answers 3


To your first query

No. There is no such element (as far as we know) that could have muons instead of electrons. As I read it, you have the misconception that muons are rarer forms of electrons (I might be wrong but I got that impression).

Muons are heavier particles that are related to electrons, except that they're unstable. This lets them pull atoms closer together, thus effectively reducing the repulsive energy needed for the nuclei to fuse. But the problem is that muons aren't exactly stable, which is bad. But they are exceedingly common as part of a pion's decay chain, which is great!

To your second query

Muons arriving on the Earth's surface are created indirectly as decay products of collisions of cosmic rays with particles of the Earth's atmosphere.

About 10,000 muons reach every square meter of the earth's surface a minute; these charged particles form as by-products of cosmic rays colliding with molecules in the upper atmosphere. Traveling at relativistic speeds, muons can penetrate tens of meters into rocks and other matter before attenuating as a result of absorption or deflection by other atoms.

When a cosmic ray proton impacts atomic nuclei in the upper atmosphere, pions are created. These decay within a relatively short distance (meters) into muons (their preferred decay product), and muon neutrinos. The muons from these high-energy cosmic rays generally continue in about the same direction as the original proton, at a velocity near the speed of light. Although their lifetime without relativistic effects would allow a half-survival distance of only about 456 meters ( 2.197 µs × ln(2) × 0.9997 × c ) at most (as seen from Earth) the time dilation effect of special relativity (from the viewpoint of the Earth) allows cosmic ray secondary muons to survive the flight to the Earth's surface, since in the Earth frame the muons have a longer half-life due to their velocity.

-Wikipedia Article

If you are looking at a muon-catalyzed reactor (probably an aneutronic type reactor if this is for a spaceship) or catalyzed direct fusion drive (like the Epstein drive in the expanse (best guess as to what that drive even is)), then you'd want a sort of particle collider.

Muons are unstable subatomic particles which are similar to electrons but 207 times more massive. If a muon replaces one of the electrons in a hydrogen molecule, the nuclei are consequently drawn 196 times closer than in a normal molecule, due to the reduced mass being 196 times the mass of an electron. When the nuclei move closer together, the fusion probability increases, to the point where a significant number of fusion events can happen at room temperature.

Methods for obtaining muons, however, require far more energy than can be produced by the resulting fusion reactions. Muons decay rapidly due to their unstable nature and cannot be usefully stored.

To create useful room-temperature muon-catalyzed fusion, reactors would need a cheap, efficient muon source and/or a way for each individual muon to catalyze many more fusion reactions. Laser-driven muon sources are one possible approach.

-Wikipedia Article

There are three (relatively easy) ways to make muons. Firstly, natural cosmic rays slamming into the nucleons (protons and neutrons) inside the atoms in the outer atmosphere generate plenty of pions. These decay extremely quickly (measured in meters traveled at relativistic speed) into (mostly) muons.

Secondly, proton and antiproton annihilation (antimatter) generates lots of deadly gamma rays and a few pions, which turn into muons. This is where most of the thrust of a beamed-core antimatter engine comes from, as pions and muons are charged.

And lastly, slamming particles (like protons) into other atoms with enough energy will trow off pions, and thus muons, in the general direction that the initial components were, in this case, roughly a beam.

They can then be channeled in the same way as electrons (with magnets) into the reactor or engine plasma where it can catalyze the fusion reactions.

There are two issues you have to deal with to make this work. An efficient source of muons, and getting those muons to jump between atoms fast enough to catalyze more fusion (and give off more energy) than it took to make them.

The second issue can be hand-waved off, but for making muons, you'd need some sort of laser assembly that either uses protons or photons to throw off muons, which you would set up around the reactor or engine and have them throw muons into the reactor plasma.

These would have impactor plates that are expended, possibly some artificial meta-material that is the story's McMuffin. Or the laser could be expendable and highly valuable. What if pirates blow up the engines of massive cargo ships and raid them for the laser assemblies, to sell off on the black market?

What if some extreme form of matter like a special variant of diamonds, or crystal (metallic) hydrogen, or some other exotic material serves as the expendable meta material, maybe mined deep inside gas giants.

Or alternatively, you could use supercritical electron-positron pairs in supercooled suspension (when both particles orbit each other in their quantum ground state, never touching) to generate a sort of gamma-ray laser, which hits your meta material and throws off the muons.

Since we don't know how to build an efficient muon laser to catalyze our fusion reactors, its up to you to pick the handwavium that powers such a device.

  • $\begingroup$ Maybe the meta-material used in these lasers are tiny crystals of metallic hydrogen (in a hydrogen-lithium lattice), crushed by a diamond anvil under thousands of tonnes of force, used as the optical centerpiece for the massive catalyzer assembly. It only runs for so long until it is "burnt out" or "spent", as enough of the hydrogen gets its proton knocked out and thus collapses a little, until there isn't anything left to hit. $\endgroup$ Jan 25 at 16:19

You want muonium.


Supposedly it is short lived but under certain cirumstances it can be trapped in crystals.


The behavior of triplet muonium in several substances when temperature and other conditions are varied is investigated by observing its (spin) precession in a transverse magnetic field. The presence of atomic muonium in crystal and fused quartz, in solid carbon dioxide, and in ice is demonstrated. The results are discussed from the viewpoint of the physical and chemical interactions of muonium.

For a fiction a cool place to find muonium would be trapped in diamonds, which your people harvest from certain gas giants.

Bond-centered hydrogen or muonium in diamond: The explanation for anomalous muonium and an example of metastability

Maybe these diamonds have a certain look to them which connotes high muon content. You could make this cool or edgy as appropriate for your fiction. If I were writing it I would have the gruff engineer state the best ones "look like frozen cat piss", without further exposition.


So, no. A muon is like an electron, only 207x as massive. If an atom has a muon instead of an electron, its shell is much smaller than that of an electron shell. You initiate fusion by shoving atoms close together, and the smaller shell makes the effort to push atoms into fusion significantly lower.

The problem is that muons take a lot of energy to generate. Their half-life is only a couple of microseconds, so you don't just "find" them.

Replacing the one electron of hydrogen (or deuterium or tritium) with a muon makes the shell smaller, but if you then go up to helium, you now have two electrons that you have to replace with muons. If you don't replace both of them, then the electron will defeat the purpose of the muon by having a full-sized shell.

Thus, looking for a muon in heavier elements is self-defeating.


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