Your doctor and dentist are careful not to give you too many X-rays, but a new report says that Homeland Security has bought 500 mobile X-ray vans that can scan cars, trucks and homes without the drivers or residents even knowing it’s been done. The vans contain Z Backscatter X-ray devices, which aim a powerful X-ray beam that can of penetrate 14 inches of steel, and mobile X-ray machines, that can be hidden in a pocket or purse, will soon be ubiquitous among law-enforcement personnel.