The low yield of the test initially raised questions as to whether it was a nuclear explosion but detection of airborne radioactive isotopes by a United States military aircraft confirmed that it was a nuclear explosion. Radioactive isotopes of the element xenon are produced by the atom splitting that takes place in nuclear explosions and readily seeps out even from underground tests. The advance warning of the test sent to the Chinese government reportedly said that the planned test yield was to be equivalent to approximately four kilotons in strength, but most outside estimates, based largely on seismic readings, put the yield at much less.