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Calcium-40 is an even-even nucleus with the closed-shell values $Z=N=20$. Its extra stability illustrates nuclear shell corrections beyond a smooth semi-empirical mass formula.

Potassium-40 is a long-lived radioactive isotope with 19 protons and 21 neutrons. It can decay to calcium-40 through beta-minus decay, or to argon-40 through electron capture and a rare beta-plus decay. The argon-producing branch supports potassium-argon dating.

TIPP 2021 presentation: indico.cern.ch/event/981823/contributions/4295324/attachments/2250469/3818551/KDK_TIPP_2021_pub.pdf

Its potassium-40 decay scheme gives the approximate atomic mass-energy differences of $1.311$ MeV to calcium-40 and $1.505$ MeV to argon-40, and distinguishes capture to an excited argon level from the rare positron branch. The positron kinetic-energy endpoint additionally subtracts $2m_e$ from the atomic mass difference.