14. When a thin foil containing 10 atoms Of all radioactive isotope B is produced with a cross-section of 5 x 10" Cm . 11.55 min., find the activity of B after 1 hr. bombardment. 15. Calculate the activity of 140Ce in example 17, if a fission source having a yield of 40 mci of Xe per sec is operated for 10 hr. 16. 124Sb source (T = 60.2d) can be prepared by thermal neutron irradiation. The capture cross section for 123Sb is 2.5 barns. How long must a 500 mg sample of natural antimony (with 42.75% of 123Sb isotope) be irradiated in a thermal neutron flux