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Iodine-131 is a radioactive isotope of the element iodine.
The thyroid gland is often imaged using a salt of iodine-131 as this gland has a high uptake of iodine.
Fission product xenon-131
Iodine 131 has a half-life of 8.0197 days.
Barium has no half-life.
So no, Iodine-131 is not more stable than barium-137.
Iodine-131 (not iodone) is a radioactive isotope of iodine: this isotope has important applications in the treatment of thyroid diseases.