From potassium 40 to argon 40
The electron capture which causes potassium 40 to transform into argon 40 in its ground state takes place in only 0.04% of cases. Far more frequently (10.68% of the time), an indirect capture leads to an excited argon atom which needs to return to its ground state by emitting a gamma ray at an energy of 1.46 MeV. Without this characteristic gamma ray, it would be impossible to detect and identify the decay of potassium 40. The neutrinos emitted in these captures defy detection. The beta electrons of the decay into calcium 40 (89.3% of the time) are not accompanied by gamma rays, and are generally absorbed into the medium they find themselves in.
IN2P3