7. Application of nuclear techniques: 7.1 (6 Points) Mercury pollution in water is of concern since fish often concentrate this element in their tissues. To measure such mercury contamination by neutron activation analysis, a fish sample is irradiated in a reactor in a thermal neutron flux = 1.5 x 10 12 cmos". The resulting "He can be detected in a fish sample at a minimum activity of 15 Bq. What irradiation time is needed to detect mercury contamination at a level of 30 jig/g (30 ppm) in a 10-g fish sample? Given: The stable mercury isotope "He has a neutron absorption cross section for thermal neutrons of 3.2 kb. The resulting 19Hg has a half-life of 2.67 d