Natural sodium consists entirely of the isotope 23Na whose cross section for thermal neutron capture is 0.53 b. When 23Na absorbs a neutron, it becomes 24Na, which is beta-radioactive with a half-life of 15.0 h. A sample of a material that contains sodium is placed in a thermal neutron beam whose flux is 2.0 × 1018 neutronsm2 . s for 1.00 h. The activity of the sample is then 5.0 µCi. How much sodium was present in the sample? (This is an example of neutron activation analysis, a very sensitive technique.)