Which isotope decays via electron capture?
Electron capture occurs in proton-rich nuclei where an orbital electron, usually K-shell, is captured by nucleus converting proton to neutron emitting neutrino. Atomic number decreases by one while mass number stays constant. Iodine-125 decays predominantly by this route, creating excited tellurium-125 which releases low-energy gamma and X-rays around 35 keV, ideal for radioimmunoassay labeling because it avoids heavy molecular damage and provides detectable photons. Phosphorus-32, tritium, and sulfur-35 emit beta-minus electrons via neutron conversion, not electron capture, making iodine-125 classic electron capture example widely used.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.