In the subsurface environment, muon capture by 40Ca becomes more important. The production rates are about 4200 atoms 36Cl/yr/mole 39K and 3000 atoms 36Cl/yr/mole 40Ca, due to spallation in rocks at sea level.
The significance of muon capture by 40Ca increases in the subsurface environment due to spallation in rocks at sea level, leading to higher production rates of 36Cl and 40Ca.