geigercounters are very sophisticated, page-11

re: geiger counters falling from the sky..sun gone "The newer detectors forgo wires and gas in favor of highly engineered crystals made from sodium iodide or cesium iodide. These crystals, the size of ice cubes, are grown slowly in ovens heated as high as 980 degrees Celsius, increasing by just one centimeter a day. They are so dense that they can absorb radiation entirely. When exposed to an isotope, the crystals flash for 50 billionths of a second. Since every isotope that spews gamma or X rays or charged particles does so in a unique way--an atomic fingerprint, of sorts--the color and intensity of the flash tells the detector which isotope is present and in what quantity."