To find out, they turned to thehigh-power X-rays generated at the Stanford Synchrotron Radiation Lightsourcein California, this time measuring how water scatters rays arrivingfrom various angles.

At one extreme, the scientists estimated fly ash radiation in individuals' bones at around 18 millirems (thousandths of a rem, a unit for measuring doses of ionizing radiation) a year.

Lowering radioactive material into a borehole and measuring how well the radiation passes through the surrounding rock provides more information about what's down there.