You've probably read or heard about scientists using radioactivity to determine the age of objects - most notably carbon-14 dating. We asked physicist Bruce Buchholz of the Lawrence Livermore National Laboratory to describe how it works.

"Carbon-14 is produced naturally in the atmosphere by cosmic ray interactions with molecules in the atmosphere. Some of the nitrogen-14 in the atmosphere gets converted to carbon-14 by nuclear reaction from the cosmic rays. That carbon-14 gets oxidized to carbon dioxide, which enters the food chain. So, everything that's alive has a level of carbon-14 naturally and that component of carbon-14 is used for traditional radiocarbon dating to measure how old things are after they die."

At the lab, Buchholz helped pioneer their Center for Accelerator Mass Spectrometry, or CAMS. It takes advantage of a dramatic increase in the atmospheric level of carbon-14 from aboveground nuclear testing in the late 50s and early 60s.