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Japanese planes detected a trace amount of xenon-133 during monitoring flights over Japan the day after North Korea carried out its third nuclear test last week, the science ministry said Thursday.
The radioactive material amounting to 1.9 millibecquerels per cubic metre of air was detected from samples collected at an altitude of 300 metres (990 feet) off the coast of Aichi prefecture in central Japan, Jiji Press reported.
It was the first time Japan had detected xenon-133 since it strengthened radiation monitoring in response to the test on February 12, the news agency said.
Xenon-133 is released into the air not just by an atomic explosion but also from nuclear power plants and medical institutions.
Ministry officials quoted by Jiji said they did not know whether the xenon was connected to the North's underground test.
South Korean warships and air force planes equipped with highly sensitive detection devices were deployed after the blast to try and collect any traces of radioactive fallout.
But Seoul experts said on February 14 they had been unable to detect any.
Experts are eager to discover whether the North used highly enriched uranium for its latest test rather than plutonium as in 2006 and 2009.
An enriched uranium programme would give the North a new and easier way to produce bomb-making material.