While a crystalline grain such as quartz – found in desert sand – is buried and tucked away from sunlight, natural radiation from surrounding soil and rocks knocks electrons in the crystal out of position.
A few of these electrons become snagged in defects in the crystalline structure and build up over time – and i To date a buried grain, scientists heat the crystal or stimulate it with light, releasing energy from the accumulated trapped charges.
The University of Chicago professor developed radiocarbon dating in the late 1940s and won the 1960 Nobel Prize in chemistry for it.
How do you find out if it’s the remains of an ancient animal that stomped the land tens of thousands of years ago or a discarded scrap from a cooking fire only a few hundred years back?
Improved techniques now date the earliest stone structures at Stonehenge to about 2600 B. Sir Joseph Norman Lockyer, who co-discovered helium and founded the journal, Nature, wrote in 1901 that the Heel Stone section of Stonehenge "had been originally aligned with the summer solstice" and calculated that it was built in 1800 B. Further investigations have suggested that Stonehenge was an astronomical observatory, a place of worship and healing or perhaps a cemetery.
Whatever its exact history, origins or age, thousands each year flock to Stonehenge to welcome the sun on the summer solstice.
Then, only exceptionally well-preserved, pristine samples can provide reliable dates.
At Warratyi rock shelter in the Flinders Ranges, South Australia, which shows signs of the oldest human occupation of the country’s arid interior, the oldest sample – a fragment of emu eggshell – has been radiocarbon dated to 49,000 years with reasonable confidence.
So along with radiocarbon dating, they use a technique known as optically stimulated luminescence (OSL) dating.