The average natural background radiation received for a person in the U.S. is about 0.3 rem per year. (rem is a unit useful for radiation dosage).
Calculate how much radiation a person could receive in this lab if they held one of the 0.1 Ci Cs-137 sources in their hand for 1 hour and all of the radiation was absorbed (100 % efficiency). Assume the mass of the hand is 2 kg. The gamma radiation produced by Cs-137 is 662 keV (i.e. each decay releases a 662 keV photon). How does this compare with the average natural background radiation received in a year?