14 C gets into living organisms, because it is produced by neutron bombardment of 14 N, the most common, stable isotope of nitrogren in the atmosphere. We must therefore assume that neutron bombardment has remained constant. The neutrons come from the Sun. Creationists believe that this shield was a canopy of water above the atmosphere. This canopy collapsed at the time of the flood.
Water molecules contain hydrogen atoms.
A measurable proportion of hydrogen atoms are of the isotope 2 H, sometimes known as deuterium, or heavy hydrogen. This isotope is radioactive, and decays to become an isotope of helium, namely 3 He. Normal helium is 4 He. Now this is the point. We can calculate how much 3 He would be caused in the atmosphere as a result of neutron bombardment on normal, atmospheric moisture.