Results from an initial air sampling after North Korea's announced nuclear test showed no evidence of radioactive particles that would be expected from a successful nuclear detonation, a U.S. government intelligence official said Friday.

The test results do not necessarily mean the North Korean blast was not a nuclear explosion, the official said, speaking on condition of anonymity because he was not authorized to disclose the sampling results.

[...]

The U.S. government remains uncertain of the nature of the underground explosion Monday trumpeted by North Korea as a nuclear test. The air sampling tends to reinforce earlier doubts about whether the test blast was entirely successful, officials said. Data from seismic sensors indicated the explosion was smaller than expected.

At the White House, press secretary Tony Snow said the Bush administration's analysis of North Korea's claim was still ongoing and is covering a wide range of data in an attempt to reach a conclusion about whether it is valid.

"We still do not have a definitive statement on it," he said. "They still think the analysis that they're doing will take another day or two."

The air sample was taken Tuesday by a specialized aircraft, the WC-135, flying from Kadena air base in Okinawa, Japan. It apparently took the sample over the Sea of Japan, between the Korean mainland and Japan.

In Beijing, a government official said Friday that Chinese monitoring also has found no evidence of airborne radiation from the test-explosion. The official with the State Environmental Protection Administration said China has been monitoring air samples since Monday.

"We have conducted air monitoring and found no radiation in the air over Chinese territory so far," said the official, who requested anonymity because he was not authorized to publicly release the information. The official declined to explain how the Chinese monitoring was conducted.

The U.S. intelligence official said an initial result from testing of the U.S. air sample became available late this week. He said a final result would be available within days but the initial finding is considered conclusive.

It was not immediately clear whether the WC-135 took additional samples after the Tuesday effort.

If it was a nuclear device, but the yield was a paltry half a kiloton, that would mean that North Korea's nuclear scientists aren't up to snuff and blew their country's initial nuclear test (something that no other country has done before), but if it wasn't nuclear at all, that would mean that NK set off a half a kiloton of coventional explosives, knowing that the magnitude of the blast would be almost immediately apparent, but also that, eventually, its non-nuclear nature would be known as well.

Why?

I'm confused as to their motivation to do that -- or is it as simple as trying to attract attention to their repeatedly expressed desire for one-to-one talks with the United States? If they thought that was a way to get Bush to the table, they sadly misunderstand the psychology of this administration, and the nature of American politics as well.

More.

Update: According to a comment to this post on Arms Control Wonk:

If it is a well contained test, only isotopes of noble gases like argon and xenon are likely to surface through barometric gas transport along faults. But, then soil gas sampling in on-site inspection operations alone can detect them. And that not before 50 days after the test for a 1 kT yield. With no access to the site for such sampling, all efforts at radionuclide monitoring from the air amount to fishing expeditions. Simple radiation level measurements will tell nothing. They may serve to give the lay public the impression that something is being done. And possibly to let DPRK believe that their tricks whatever they might be will be found out.

Update (10/14 2:30am): A WaPo report in the Boston Globe now says that radioactivity has been found:

As the United States and Japan pushed for a vote today on a Security Council resolution condemning North Korea's reported nuclear test, US intelligence officials said the government had detected radioactive debris consistent with a nuclear explosion.

The radioactive finding was the only positive result among many tests conducted since North Korea announced a nuclear explosion Monday, the officials said last night.

Environmental samples collected by a US military aircraft detected signs of radiation over the Sea of Japan, possibly confirming North Korea's nuclear test, but intelligence officials stopped short of declaring that an atomic test was conducted. ``The intelligence community continues to analyze the data," said Frederick Jones, spokesman for the National Security Council.

Earlier detection attempts by the United States, China, and South Korea did not pick up any radiation. In addition to collecting more samples, analysts are taking a harder look at seismic data, satellite photos, and communications intercepts.

US intelligence analysts say they believe, though cannot prove, that the North Korean blast was the result of a partial implosion of plutonium at the core of the test device. That would mean some of the plutonium failed to implode. The blast yield was less than a kiloton, far smaller than the 20-to-23-kiloton bomb the US military dropped on Japan 61 years ago.

I guess this puts up back to thinking that the North Koreans probably botched things, and their bomb fizzled. It would be nice to get some confirmation of this from the Comprehensive Test Ban Treaty Organization's International Monitoring System, which has yet to say anything about it.

The Comprehensive Nuclear Test Ban Treaty Organization, or CTBTO, has about 200 stations worldwide designed for monitoring nuclear tests as part of what it hopes will become the world's most reliable source for such tests. But until the treaty comes into force, the data are not made public, only released to governments and vetted partners.

[...]

The test ban treaty, which bans all nuclear explosions, will not enter into force until it has been ratified by 44 states who possess either nuclear power or research reactors. So far 34 have ratified it. Holdouts include the United States, China, India, Pakistan, Israel and North Korea.

The CTBTO's stations are more extensive than those used by most countries. They monitor seismic events but also underwater data, radioactive particles in the air and radiowaves.

"Within 72 hours we will have full data. Then all this will be available to member states," said Zerbo.

While the North Korean explosion was small, potentially complicating monitoring efforts, sensors in South Korea were likely close enough to categorize it as nuclear, if that is what is was, said Friedrich Steinhaeusler, professor of physics at Salzburg University.

A nuclear blast also gives off a clear signature -- a clear graph of peaks and curves -- that differentiates it from other kinds of shocks, he added.

"We'll have the confirmation soon," he said.

[Sci-Tech Today 10/10]

Without verification coming from a presumably neutral source, we're left to depend on the word of various American officials, on- and off-the-record. Given the politicalization of many aspects of American government under Bush (especially when it has to do with disputes involving science), I'm somewhat less than confident than such people can be relied on, one way or the other. (Such is our relationship to the state after 6 years of right-wing rule.)