Which isotope emits both positrons and gamma rays?
Certain neutron-deficient isotopes decay by positron emission where a proton converts to neutron releasing positron and neutrino. The positron quickly annihilates with an electron producing two 511 keV gamma photons. Sodium-22 is classic example used in biology and physics; it decays approximately 90 percent by positron emission followed by gamma from excited neon daughter. Phosphorus-32 and sulfur isotopes are pure beta-minus emitters, while potassium-40 predominately beta-minus. Therefore sodium isotope undergoing beta-plus decay uniquely provides coincident positron and gamma signature valuable for calibration.
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.