1. Compute the mean energy per nuclear disintegration, A, for ^{99m}TC if 2.0 x 10^{9} Bq of this isotope is injected into a patient. Assume that one-half of the administered dose is taken up by the thyroid and that the removal kinetics follow first-order exponential form with a biological half-life of 4 h. What is the dose to the thyroid? Assume for purposes of this problem that the specific absorbed fraction is 0.5 and the thyroid weighs 80 g. Express the dose in gray.