Dating the earth with SHRIMP

Giant shrimp are coming! Giant shrimp are coming! In this case, the shrimp is not a crustacean, but it is giant. The $2.5 million, 12-ton Sensitive High Resolution Ion MicroProbe (SHRIMP) arrived at Stanford University this spring. This device will determine the age of rocks and the origins of the solar system by analyzing grains of earth or interstellar dust for differences in atomic mass. The SHRIMP fires high-energy oxygen ions at a sample at speeds of 350-km/sec or nearly 800,000 mph. The oxygen ions focus into a fine beam about the width of a single strand of human hair. The ions have a negative electrical charge. When they hit the sample, positively charged ions are "kicked" off. The impact leaves craters on the sample surface. The liberated ions travel down a tube into a curved magnet about 1m long. The magnet separates the ions according to their mass and energy. The lighter and slower ions hug the inside lane, while the heavier and faster ones accelerate to the outer lanes. The ions excite the magnet in a broad beam. They enter an electrostatic compensator, which reorganizes them according to mass only, removing the effects of energy difference between ions of the same mass. Scientists use these masses for radiometric dating and isotopic fingerprinting. FAX: (415) 725-0247.