Iodine-131 or Cesium-137 Iodine-131 or Cesium-137 Iodine-131 or Cesium-137 biological half-life of 70 days http://www.geigercounter.org/radioactivity/isotopes.htm http://www.geigercounter.org/radioactivity/isotopes.htm Iodine-131 or Cesium-137 Iodine-131 or Cesium-137 70 days