A man is screened with a backscatter X-ray machine as travelers go through a security checkpoint at Los Angeles International Airport in 2011. Danny Moloshok/Reuters/Corbis hide caption

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Farmer Magoichi Shigihara checks on his cucumber farm in Nihonmatsu in Fukushima prefecture, about 31 miles west of the Fukushima nuclear power plant, in May 2011. Testing shows radiation in foods grown and raised in Fukushima is back to pre-accident levels. Yoshikazu Tsuno/AFP/Getty Images hide caption

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Emily Whittington, a radiation therapist at Vanderbilt-Ingram Cancer Center, pulls down a mask intended to guide radiation beams into patients with head and neck cancer. Emily Siner/NPR hide caption

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Japan Aerospace Exploration Agency astronaut Aki Hoshide makes a space walk outside the International Space Station in 2012. NASA/Getty Images hide caption

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Radiation therapist Jean Etienne holds a range compensator, which shapes the depth to which the proton beam enters a patient's body to target a tumor. Rebecca Davis/NPR hide caption

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This illustration shows a device made by MammoSite used to deliver targeted doses of radiation as part of brachytherapy. Courtesy Radiological Society of North America hide caption

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