A mix of two isotopes, one of them from Table 38.1, is observed over a period of 15 days, and the total radioactivity is tabulated below. Determine a quantity that, when plotted against time, should yield one or more straight lines. Make your plot and use it to determine the half-lives of the isotopes. Identify the isotope from Table 38.1.

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Table 38.1 Selected Radioisotopes Decay Mode Isotope Carbon-14 (¿C) Half-life Comments 5730 years Used in radiocarbon dating Iodine-131 ('1) 8.04 days Fission product abundant in fallout from nuclear weapons and reactor accidents; damages thyroid gland Охудеn-15 ({0) 2.03 minutes B* Short-lived oxygen isotope used for PET scans 1.25x 10° years Comprises 0.012% of natural potassium; dominant radiation source within the normal human body; used in radioisotope dating Potassium-40 (fK) Plutonium-239 (Pu) 24,110 years Fissile isotope used in nuclear weapons a Radium-226 (*ÝRa) 1600 years Highly radioactive isotope discovered by Marie and Pierre Curie; results from decay of HU Radioactive gas formed naturally in decay of Ra; seeps into buildings, where it may cause serious radiation exposure Fission product that behaves chemically like calcium; readily Radon-222 (Rn) 3.82 days Strontium-90 (Sr) 29 years absorbed into bones Technetium-99m ("HTc) 6.006 hours Metastable excited state of Te-99, widely used in medical diagnostics. Hydrogen isotope used in biological studies and to enhance yields of nuclear weapons Fissile isotope comprising 0.72% of natural uranium; used as reactor fuel and in crude nuclear weapons Tritium (}H) 12.3 years Uranium-235 (U) 7.04× 10* years Uranium-238 (U) 4.46× 10º years Predominant uranium isotope; cannot sustain a chain reaction