Potassium - 37 decays by positron emission to form the unstable argon - 37. And then another positron emission finally yields chlorine -37 which is stable.
K-37 + e- --> Ar-37
Ar-37 + e- --> Cl-37
Positron emission is a process where a proton is converted to a neutron, a positron and a neutrino. The emission of a positron can be considered to be the same as electron capture as a positron is the anti-matter form of an electron. A positron emission reduces the atomic number of the resultant atom by one though the atomic mass remains the same.