Eric Stevenson, Director of Technical Services for the Bay Area Quality Management District, checks a radiation sensing device Wednesday, March 23, 2011, in San Francisco. Federal officials had to scramble last week to make sure the first line of defense against radiation, a network of sensors perched on rooftops across the country, was up and running. Their quick work has revealed no worrisome spikes since the Japanese nuclear crisis began, but more than one-third of the stationary machines weren't working when the radiation began migrating over California last week, a finding that has raised questions about the little-known program that relies partly on volunteer help. (AP Photo/Ben Margot)