Three weeks into the nuclear crisis in Japan, minute traces of radioactive dust have circled the globe, even arriving in Maryland and Virginia.

Fallout from the Fukushima Daiichi nuclear plant has landed on 30 exquisitely sensitive detectors on desolate Arctic islands, on the tops of tall buildings and in other windy locales across the Northern Hemisphere, according to the Comprehensive Test Ban Treaty Organization, which maintains those sensors. Sniffing the air like silent sentinels, the 63 shack-like stations (with 17 more planned) are capturing tiny radioactive particles in filters much like those on a home furnace.

Analysis of that dust is a key step in an intricate process of nuclear sleuthing: The dust’s distinctive chemical signature can show scientists whether the particles blew into the air from a bomb, a damaged nuclear reactor or used uranium fuel. It can even point to the extent of damage suffered by a fission reactor. Tracing global wind patterns back then pinpoints where the emissions originated.

“It’s nuclear forensics,” said Kai Vetter, a professor of nuclear engineering at the University of California at Berkeley, who built his own radiation detector atop a campus building after the Fukushima crisis began.

“You can learn quite a lot from the pattern of radioactive isotopes,” said Hamish Robertson, a physicist at the University of Washington in Seattle.

In the United States, another network of more than 100 stations maintained by the Environmental Protection Agency is also gathering radioactivity from Japan. State health departments maintain their own monitoring systems, which is how Maryland detected tiny traces in the air and water March 24.

Super-sensitive detection

After the March 11 earthquake and tsunami damaged Fukushima, the global sensor network began to light up for the first time since nuclear detonations in North Korea in 2006 and 2009.

On March 14, a station on the Kamchatka Peninsula in Russia sniffed out unusual radioactive elements. That cloud then split, drifting southward and eastward, with one arm arriving two days later in Sacramento, and three days after that in Charlottesville. On March 20, the ultra-thin broth of radioactive particles blew over Iceland. And it reached all the way to Kuwait City on March 25, two weeks after the first emissions from Fukushima.

The reporting of even these minuscule amounts of unusual radiation has caused alarm, driving a run on potassium iodide pills and Geiger counters. But officials at the EPA and the Centers for Disease Control and Prevention and independent experts have repeatedly stressed that the amount of radioactivity detected outside Japan is far too low to affect human health.

Modern radiation detection systems are simply astoundingly sensitive, they explain, designed to pick up traces of nuclear explosions anywhere in the world. The detector atop the CTBTO’s headquarters in Vienna, Austria, still catches vestiges of the Chernobyl disaster that occurred 25 years ago, said Lassina Zerbo, director of the group’s international data center.

Vetter’s detector at Berkeley even catches radioactivity on the wind from treatments received by thyroid cancer patients passing by six stories below.

Taking radioactive fingerprints

Natural radiation — mostly from airborne radon — “drowns out” the radiation from Fukushima spotted in the United States, said Michael Miller, a University of Washington physicist who, along with Robertson, helped construct a radiation detector in a campus building’s air intake duct.

So a sensor that simply measured the total amount of radiation from airbone particles would be useless in nuclear forensics. Modern detectors do much more. They outline the dust’s distinctive radioactive fingerprint by measuring precise concentrations of five or more radioactive elements, or isotopes.

Each atom of these isotopes is unstable, shedding excess energy — via radiation — in a process called decay. By measuring the form and intensity of this energy, the radiation detectives can identify the isotopes in play and deduce from them what might have happened.

Radioactive iodine-131 and cesium-137 are key to this process. They don’t exist in nature, so their appearance signals a nuclear event — either a bomb or a reactor in trouble. Both can cause health problems in large amounts. But iodine-131 decays relatively rapidly: After eight days, half the original amount is gone. Its presence means that the event that created it occurred just weeks beforehand. Cesium-137 takes much longer to decay, with a half-life of 30 years. Traces of cesium-137 from Chernobyl still waft on Earth’s great jetstreams.

Clues in the air

It was detective work of this kind that alerted the world to the world’s worst nuclear disaster 25 years ago. In April 1986, nuclear power plant workers in Sweden detected a spike in iodine-131 and cesium-137, which — after a check of wind patterns — revealed the unfolding disaster at Chernobyl, which the Soviet Union had not disclosed.

Because both isotopes can come from a bomb or a reactor, nuclear sleuths also search for another isotope that originates only in reactors: cesium-134. It is produced during the slow-boil nuclear fission inside reactors, but not the flash-bang of a nuclear explosion. The ad-hoc sensors built by academics on the West Coast have picked up cesium-134 from Fukushima, as have the permanent CTBTO stations.

Nuclear detectives can dive deeper still, sorting out whether radioactive emissions emanate from a dangerously active and still-fissioning reactor core, from burning fuel rods, or from used fuel sitting in pools.

When the active core of Chernobyl exploded, it sent dozens of different radioactive elements into the atmosphere, including isotopes of strontium, yttrium, and rhodium — all produced only by active reactor cores or burning fuel rods.

The University of Washington team say they have not seen any of these isotopes, indicating that the fuel rods at Fukushima have not caught on fire.

And the lack of other short-lived isotopes — especially iodine-133 — indicates that the primary safety systems at Fukushima kicked in as planned during the earthquake and shut down fission in the reactors. Halting fission starts an atomic clock of sorts in which quickly decaying isotopes vanish within hours or days. Andreas Knecht of the University of Washington team said their Seattle detector can sense iodine-133 up to seven days after a reactor produces it, but has found none.

The detection of still another isotope, tellurium-132, offers a further clue about the source of radioactive emissions from Fukushima. Fuel rods sit in pools for months or years, releasing fewer and fewer different isotopes over time, and — unlike hotter fuel rods — they don’t produce tellurium-132. Spotting that isotope, as the detectors have, points to a damaged reactor core.

By putting these radioactive puzzle pieces together, the Seattle team was able to conclude in a paper published March 28 that fission in Fukushima’s three active cores was halted during the earthquake, and that the reactor cores launched radioactive debris into the air on clouds of steam shortly thereafter.