https://en.wikipedia.org/wiki/Green_Run
The "Green Run" was a secret U.S. Government release of radioactive fission products on December 2–3, 1949, at the Hanford Site plutonium production facility, located in Eastern Washington. Radioisotopes released at that time were supposed to be detected by U.S. Air Force reconnaissance. Freedom of Information Act (FOIA) requests to the U.S. Government have revealed some of the details of the experiment