Answer:
Beta decay
Step-by-step explanation:
Radioactive decay changes one nucleus to another. The type of decay that a particular nucleus undergoes depends on the number of neutrons and protons present in such nucleus.
Radioactive decay tends to lead to the attainment of a stable neutron/proton ratio.
The decay mode for cesium-137 is beta decay. The beta decay of cesium-137 yields Barium-137.
Beta decay decreases the neutron - proton ratio.