[shelanachium]

Muons are a major component of cosmic rays at sea-level (the result of the decay of pions produced by the collision of primary cosmic rays with atmospheric nuclei). On reaching the Earth's surface, those that do not decay in flight are mostly captured by heavy nuclei in the rocks.

Because muon orbitals are 206 times smaller than electron orbitals, captured muons spend much more time close to or even within the atomic nucleus. This, combined with the great amount of energy available to drive the reaction, means that such reactions as

muon + Fe-56 --> Mn-56 + muon-neutrino

occur very rapidly, and with heavier elements are much faster than simple muon decay. The process is analagous to the well-known EC decay of neutron-poor nuclei such as Be-7.

In most cases muon-capture yields a nucleus which decays back to the original one, as in the example above. So no long-term trace of the capture occurs.

However, Sr-84 on capturing a muon becomes Rb-84, most of which decays to Kr-84.

Similarly, muon capture by Ba-130 and Ba-132 leads mostly to isotopes of xenon.

These noble gases are unlikely to have been originally present in Sr and Ba minerals. Although the Sr and Ba isotopes concerned are very rare (<1%); modern technology must by now be capable of detecting the traces of Kr-84 in Sr minerals, and of Xe-130 and Xe-132 in Ba minerals, that should have been produced by muon capture. Has anyone looked for these?