Thinking about nuclear terrorism. The realistic threats settle into two broad categories. (46) The less likely but far more ruinous is an actual nuclear explosion, a great hole blown in the heart of New York or Washington, followed by a toxic fog of radiation. This could be produced by a black-market nuclear warhead procured from an existing arsenal(军工厂), which might be in Russia, Pakistan or other countries or areas. Or the explosive could be a homemade device, lower in yield than a factory nuke(核武器) but still creating great suffering. (47) The second category is a radiological attack, contaminating a public place with radioactive material by packing it with conventional explosives in a "dirty bomb" by dispersing it into the air or water or by destroying a nuclear facility. By comparison with the task of creating nuclear fission, some of these schemes would be almost childishly simple, although the consequences would be less horrifying. Nothing is really new about these perils. The means to inflict nuclear harm on America have been available to rascals for a long time. Serious studies of the threat of nuclear terror dated back to the 1970"s. (48) American programs to keep Russian nuclear ingredients from falling into murderous hands were hatched soon after the Soviet Union disintegrated a decade ago. When terrorists get around to trying their first nuclear assault, as you can be sure they will, there will be plenty of people entitled to say I told you so. (49) All Sept. 11 did was to turn a theoretical possibility into a felt danger. All it did was to supply a credible east of characters who hate us so much that they would thrill to the prospect of actually doing it, and, most important in rethinking the probabilities, would be happy to die in the effort. All it did was to give our nightmares legs. And of the many nightmares animated by the attacks, this is the one with pride of place in our experience and literature—and, we know from his own lips, in Osama bin Laden"s(奥萨马.本.拉登)aspirations. In February, Tom Ridge, the Bush administration"s homeland security chief, visited The Times for a conversation, and at the end someone asked, given all the things he had to worry about—hijacked airliners, anthrax(炭疽热)in the mail, smallpox, germs in crop-dusters—what did he worry about most He cupped his hands prayerfully and pressed his fingertips to his lips. "Nuclear", he said simply. My assignment here was to stare at that fear and the inventory of the possibilities. How afraid should we be, and what of, exactly I"ll tell you at the outset, this was not one of those exercises in which weighing the fears and assigning them probabilities laid them to rest. I"m not evacuating Manhattan, but neither am I sleeping quite as soundly. (50) As I was writing this one Saturday in April, the floor began to rumble and my desk lamp shook precariously(不稳定的,充满危险的). Although I grew up on the San Andreas Fault, the fact that New York was experiencing an earthquake was only my second thought.