6:04 - V The fission reaction as well as the nuclear decay of uranium and other fission products produced an assortment of radioactive and stable isotopes. One of the radioactive isotopes was iodine-131. The human thyroid gland uses iodine to produce thyroid hormones. The body cannot distinguish stable iodine-127 from radioactive iodine. After the Chernobyl accident, radioactive iodine-131 contaminated food that people ate. When officials brought the residents iodine pills, they were attempting to flood their thyroids with non-radioactive iodine so that the iodine-131 would not be absorbed. Unfortunately, the government distributed the pills much too late. Thyroid cancer rates skyrocketed in residents who were children or adolescents at the time of the meltdown. Within 20 years, there were 6,000 cases. Iodine-131 has a half-life of only 8 days. This means that after 8 days, only 12 of the mass of iodine-131 still exists. The rest has undergone beta decay to become stable xenon-131. The amount of radioactive iodine in the area would have effectively disappeared just a few months after the explosion. (2pt@) 1. Calculate the number of days after the explosion for a sample of iodine-131 to go from 100% I-131 to less that 1% I-131. Even though the iodine-131 decayed quickly, other isotopes like cesium-137 and strontium-90 still remain in the environment. These have much longer half-lives - around 30 years. 2. How many years will it take for cesium-137 with a half-life of 30 years to go from 100% cesium-137 to less than 1% cesium-137? 3. Plutonium-241 has a half-life of 14 years but decays to Americium-241, which has a half-life of 430 years. How many years will it take for Am-241 to go from 100% to less than 1%? 4. Iodine-131 decays quickly but the other components in the radioactive fallout (Plutonium-239 and Uranium-235) have very long half-lives. Look at Table 2: Common Radioactive Isotopes. No one is exactly sure how long it will be before the area around Chernobyl is safe to live in. Estimates range from a few hundred years to 20,000 years. Why do you think it will take this long to be safe (use information from Table 2)? Both short and long half-life isotopes can be dangerous. Radioisotopes with short half-lives like iodine-131 release a lot of energy in a short period of time. These are the main cause of radiation poisoning after a nuclear weapons explosion. Radioisotopes with middle to long half-lives, like plutonium, emit smaller amounts of energy in a short period of time. But they do so for much longer. Some radioisotopes, like bismuth-209, have such long half-lives that it is difficult to even know they are radioactive. Bismuth-209 has a half-life of 20 billion years. Case Analysis: (2pts@) 5. What are advantages and a disadvantages of using a radioactive substance with a short half-life in a crime such as this? 6. What are advantages and a disadvantages of using a radioactive substance with a long half-life in a crime such as this? 7. Look at Table 2: Common Radioactive Isotopes. Are there any sources of radiation you can now definitely rule out? Explain your thinking. SB Elements with mass numbers 43, 61, and 93 through 118 are synthetic elements; they do not exist on Earth. All are radioactive. BSA It is possible that these elements were present on our planet at one time. Scientists theorize that all of the heavier elements were made in supernova explosions millions of years before the Earth's formation. Elements with "short" half-lives would have long since decayed into something else. For example, technetium-98 has a half-life of 4.2 million years. After 4.5-billion-years (the age of the Earth), no technetium-98 would be left. The synthetic elements are made by nuclear reactions in laboratories. Some, like plutonium, have half-lives long enough to be useful. Others are gone within minutes (or seconds) of their creation. Meitnerium-278, the longest lived isotope of Meitnerium, has a half-life of only 0.72 seconds. (1pt@) 8. Why are some elements no longer found on Earth? SB