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Banning What Bomb?

by Tac Leung

These days, there is more to detecting nuclear blasts than finding flaming mile-high mushrooms clouds. So, to enforce the U.N. nuclear test bomb treaty, scientists are rewriting the book on bomb spotting.

Ground zero.

      With the first nuclear bomb, it was easy to find. Wait for the double flash of blinding light and searing heat to pass, then watch for the rising fireball. With a pair of binoculars, start at the crown of the expanding mushroom cloud, then follow the stalk down to where it touches the ground. Bingo. That's where the nuclear bomb was detonated.

      These days, though, there is more to detection than spotting mile-high mushroom clouds. Despite their power, bombs can be evasively muffled in underground caves, or masked by synchronizing them with scheduled mining blasts. Rogue nations who get hold of bombs can test them at sea in international waters, miles from the nearest detector. Sensors have to be sensitive enough to detect the blasts from these distances, through air, water, rock, and soil, yet distinguish them from a constant background of earthquakes, volcano eruptions, meteor crashes, and mining activity. And they have to do it now.

     Last September over 130 countries signed the Critical Test Ban Treaty (CTBT) completely prohibiting all explosions nuclear, anywhere on the planet. But for the treaty to be effective, its watchdogs need to be so good that any country considering a test on the sly will hold back out of fear of being caught. That means that an accurate-and absolutely dependable-network of monitors.

     And so Thorne Lay, a seismologist at the University of California, Santa Cruz, and other CTBT scientists around the world are rewriting the book on bomb-spotting. Lay is chairing a US National Research Council panel appraising further research needed to properly enforce the ban, while international teams of scientists interlock stations into a sentinel that will stand guard over the planet, watching for covert blasts.

     The scientists need to build the International Monitoring System quickly and affordably, while governments are still excited and are willing to pay for it. The scientists need four types of off-the-shelf sensors, which they've begun borrowing, bartering, or buying used. They are tying together earthquake detecting seismographs; radioactivity testers; and microphones, both underwater and on land. And already the backbone of a prototype monitoring system is in place.

     Fifty seismic stations around the globe--previously used only to feel the ground for natural ground-shaking events-have been rewired to also sense vibrations from underground blasts. Over 120 other earthquake stations can be called on to clarify unclear readings. "Seismic systems," says Lay, "will be the workhorse to detect [nuclear] events."

      The ears of the system are under construction. Sixty listening stations will dot the continents to hear for telltale rumbles in the air. They will sense the bass kicks from blasts called infrasound, so low they are beneath the human range of hearing. Undersea listening posts have begun to feel for shock waves from nuclear explosions in the world's oceans. Four new hydroacoustic stations will join the seven already being used by the US Navy to track ocean impacts of missiles.

      The nose of the sentinel is also under construction--80 refrigerator-sized boxes that will sniff the air for radioactive particles and gasses. At the slightest whiff of the unmistakable nuclear odor, these radionuclide detectors will send the alarm to the nerve center.

     Up to 10 gigabytes-ten thousand floppy disks-of data from all 201 detectors will pass through the nerve center of the monitoring system each day. This International Data Centre will have to constantly and efficiently digest the raw feeds, says Steve Bratt, director of the prototype center, and send its readout to each country's scientists for interpretation. "We'll be producing a geophysical 'day-in-and-day-out' of the planet," he says.

      "To get that data and interpret it in a timely and accurate fashion is a huge challenge," says Jay Zucca, leader of the Critical Test Ban Treaty research program at Lawrence Livermore National Laboratory. But a bigger challenge will be detecting covert explosions-in the atmosphere, at sea, or underground.

     Within the first microsecond of a subterranean blast, a fiery bubble of vaporized rock forms at pressures of several million atmospheres. The expanding gas forces open a cavity, while the shock wave pulverizes rock as it expands into the surrounding earth, then travel for hundreds of miles through the planet. Seismographic stations feel the waves as they pass beneath.

      The breadbox-sized seismographs are stationed in groups of three, each detecting one type of motion: up-down, north-south or east-west. From the readings, scientists estimate an event's size and location-which they can now do for any magnitude 4.0 event, the seismic equivalent of an unconcealed one kiloton (kT) blast. (The bomb dropped on Hiroshima was 15 kT.) Ideally, says Peter Marshall, a seismologist with Britain's Atomic Weapons Establishment, they would like to be able to do the same for covert 1 kT blasts.

     A rogue nation trying to cover up an explosion could detonate a 1 kT bomb in an underground cave and muffle the blast down to a whisper of magnitude 2.5, the size of a barely-felt quake. Earthquakes, bombs, and volcanoes each have a slightly different seismic wiggle, and usually the three can be distinguished by their seismic fingerprints-but below magnitude 4.0, the vibrations weaken, the fingerprints smear, and ╩seismographs can't tell them apart.

      While an average of 20,000 quakes above magnitude 4.0 are reported annually, seismologists figure that at magnitude 2.5, that number is 120,000-a formidable string of ambiguous rumblings to sift through. In addition, another hundred thousand chemical explosions are set off by mining operations. While these are often less than magnitude 3.0, determined testers can hide nuclear explosions by synchronizing the blasts.

     Researchers Scott Phillips and Craig Pearson of the Los Alamos National Laboratories are compiling more detail of the seismic signatures of all possible events into a library. They hope to profile a unique "nuclear fingerprint" for even very small explosions. Similar work with underwater blasts is also underway.

     Underwater explosions create an enormous expanding bubble filled with scorching gases. The growth eventually loses momentum, and the water pressure squeezes the gassy globule back down. The internal pressure rebuilds and the gas and water pressure continue this tango, cyclically expanding and contracting the walls of the gas pocket as it rises to the surface.

      According to Lay, a few microphones stationed in the major bodies of water are all that are needed to listen for this "bubble pulse." Hydrophones, as they are called, consist of a pressure sensitive floating two-foot long cylindrical bump of on a hose that can feel sound waves. "Sound travels extremely well in the ocean," says Lay. "Even small chemical explosions are extremely loud. An underwater nuclear explosion is likely to bust everyone's eardrums." But that does not mean that there won't be any false alarms.

     One source of false alarms are the detonations of depth charges during naval exercises. Oil exploration ships also use arrays of air guns to ping sonar-like profiles of the sea floor, and the sound can be heard across the sea. Whales songs have well-defined signatures that are easily discriminated by spectrograms, but some species produce periodic wails that could be confused with bubble pulses.

      Scientists will face similar problems detecting atmospheric explosions. ╩Detonations in the atmosphere do not produce bubble pulses, but their low rumblings can be recognized by infrasound microphones. The pressure sensitive boxes that fit in the palm of a hand have six or eight perforated garden hoses extending out several hundred meters. Grouping of three to five of the spider-like devices into families allow scientists to determine the direction in which the bomb was exploded. Infrasound is a very young field, and researchers are still working out the best configurations for the spider families, as well as how to differentiate an assortment of atmospheric bangs and crackles such as volcanic explosions and meteor crashes.

      Infrasound, hydroacoustic, and seismic sensors perform the same function in different environment-they listen for shock waves then print out a corresponding series of crests and troughs. But radiation detectors will provide a completely different readout.

      Radionuclide detecors chart of what elements and isotopes are floating in the wind. Of all the detection methods, says Lay, "radionuclides are the least ambiguous. There is no natural process that produces certain Xenon isotopes. Where they see them, they know a nuclear reaction has taken place."

      The six-foot-by-five-foot metal boxes contain a big pump constantly sucking air, blowing it through sticky paper. "Every day, they take the paper and fold it up and put ╩it in a mass spectrometer to see if it's glowing," says Lay. "It's pretty crude."

      Yet, says Peter Marshall, "that's the method that is really the 'smoking gun'." Wisps of radioactive material eventually escape to the surface of even the most well-buried explosions. By sniffing the air then backtracking based on weather patterns, scientists can draw a footprint of where a nuclear blast could have been detonated.

      While any one of the technologies has its blind spot, simultaneous readings from all of the four--seismic, hydroacoustic, infrasound and radionuclide should allow nuclear blasts to be identified and located with a high level of certainty.

      But the question of whether the monitoring system and ban will work, in the end, is a political one. Each nation that signed the test ban treaty is now independently reviewing it against their politics and budget, deciding whether to ratify the ban and fund the IMS.

      "The idea of the treaty and the monitoring system is to provide a credible deterrent to testing," says Zucca. "If you are a potential evader, the idea of this treaty is to make you think twice, or thrice, or four, or five, or six times." So in the end, the decision to build the sentinel will be based not just on whether its technology works, but as much on how technologically menacing it appears--from potential ground zero.


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