The unswerving regularity of this decay allows scientists to determine the age of extremely old organic materials -- such as remains of Paleolithic campfires -- with a fair degree of precision.
The oldest known rocks on Earth, estimated at 4.28 billion years old, are the faux amphibolite volcanic deposits of the Nuvvuagittuq greenstone belt in Quebec, Canada.
According to NIST scientist emeritus Richard Lindstrom, the variations observed in other experiments may have been due to environmental conditions interfering with the instruments themselves.
"There are always more unknowns in your measurements than you can think of," Lindstrom says.
Bertram Boltwood suggested that lead is one of the disintegration products of uranium, in which case the older a uranium-bearing mineral the greater should be its proportional part of lead.
Analyzing specimens whose relative geologic ages were known, Boltwood found that the ratio of lead to uranium did indeed increase with age. Wasserburg applied the mass spectrometer to the study of geochronology.
Recently, however, researchers at Purdue University observed a small (a fraction of a percent), transitory deviation in radioactive decay at the time of a huge solar flare.