6.What is the greatest contributor to natural background radiation dose? E.g. space radiation,radon, ingestion of radioactive food, etc. Describe the relative contributions to background radiation from natural and medical sources.

7.A thyroid cancer patient receives 120 mCi of I-131. Assume 5% of the dose is deposited to the thyroid, and the remainder is cleared very quickly. The patient comes back a week post therapy for planar imaging. What is expected amount of activity in the patient? Hint: you must look up the half-life of I-131 if you dont know it. Cite your source. State any assumptions and show your work.