National Geographic

HOW IT WORKSIMAGING WITH X-RAYSX-rayphoton``````Electron backscatter``````CARBONIRON``````LEAD``````0``````3ALUMINUM``````(QHUJ\IURPQXFOHDUƄXRUHVFHQFH6 Electron volts``````Zinc and heavierArgon to copper``````Hydrogen to oxygen``````Fluorine to chlorine``````ǭǪǫDzǓǬdz``````ǫǓDz``````dzǓǫDZ``````Density of material``````Atomicnumber``````HIGHHIGH``````LOW``````LOW``````Suspectedcontraband``````Suspectedcontraband``````SALT COCAINE``````NaCl C 17 H 21 NO 4``````Seaports handle roughly 80 percentof worldwide trade by volume andplay a vital role in border security. Ina pilot program at the Port of Boston,scientists and engineers have designedan advanced scanner that can identifythe molecular makeup of substancesZLWKIDUPRUHVSHFLƃFLW\WKDQHYHUEHIRUHTXLFNO\GLƂHUHQWLDWLQJIRUexample, between salt and cocaine.``````7UDQVPLVVLRQ[UD\LPDJLQJX-ray photons are sentthrough an object. Its densityis determined by measuringthe number of photons thatsuccessfully pass through it.``````+LJKHQHUJ\EDFNVFDWWHULPDJLQJHigher energy x-ray photons areGHƄHFWHGRUEDFNVFDWWHUHGE\WKHobject’s electrons and nuclei. Theresults can be used to determine theobject’s density and its atomic number.``````&UHDWLQJD'PRGHOA scanner A bombards a truck andits cargo with high-energy x-rays.Sensors B then measure for back-scatter radiation (see “Imaging WithX-Rays,” below) to create a 3-Dmodel. The truck and its contentsDUHLGHQWLƃHGE\IRXUFDWHJRULHV(right) based on average atomicnumber (the number of protons).12+XQWLQJIRUFRQWUDEDQGAlgorithms then analyze the 3-Dmodel for atomic numbers and densi-ties that could indicate contraband,such as explosives and drugs. Suspi-cious material is further scanned bya detector C WKDWPHDVXUHVVSHFLƃFcombinations of elements based onhow much energy they release.``````CARGOWATCHING