Which isotope is the result of activation of water?

• A. Ar-41
• B. H-3
• C. N-16
• D. Na-24

Answer: (C)

When water is exposed to a neutron field, the abundant oxygen-16 in the water undergoes a quick neutron-induced reaction to become nitrogen-16. This happens mainly through an (n,p) reaction: oxygen-16 absorbs a neutron and emits a proton, turning into nitrogen-16. Nitrogen-16 then decays with a short half-life of about 7 seconds, emitting high-energy gamma rays as it returns to oxygen-16. That rapid production and decay makes nitrogen-16 the classic activation product of reactor coolant water.

Ar-41 comes from activation of argon in the surrounding air, not from the water itself. Tritium (H-3) can be formed if deuterium in water is activated, but deuterium is only a tiny fraction of natural water, so the immediate, characteristic activation product of water in a reactor context is nitrogen-16.