Research Shows Radiometric Dating Still Reliable (Again)

Recent puzzling observations of tiny variations in nuclear decay
rates have led some to question the science of using decay rates to
determine the relative ages of rocks and organic materials. Scientists
from the National Institute of Standards and Technology (NIST), working
with researchers from Purdue University, the University of Tennessee,
Oak Ridge National Laboratory and Wabash College, tested the hypothesis
that solar radiation might affect the rate at which radioactive elements
decay and found no detectable effect.

Radioactive elements transmute into more stable materials by shooting off particles at a steady rate. For instance, half the mass of carbon-14, an unstable isotope of carbon, will decay into nitrogen-14 over a period of 5,730 years. Archaeologists routinely use radiometric dating to determine the age of materials such as ancient campfires and mammoth teeth. ©Zoltan Pataki/courtesy Shutterstock

Atoms of radioactive isotopes are unstable and decay over time by
shooting off particles at a fixed rate, transmuting the material into a
more stable substance. For instance, half the mass of carbon-14, an
unstable isotope of carbon, will decay into nitrogen-14 over a period of
5,730 years. 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 decay of
uranium-238, which has a half-life of nearly 4.5 billion years, enabled
geologists to determine the age of the Earth.

Many scientists, including Marie and Pierre Curie, Ernest Rutherford
and George de Hevesy, have attempted to influence the rate of
radioactive decay by radically changing the pressure, temperature,
magnetic field, acceleration, or radiation environment of the source. No
experiment to date has detected any change in rates of decay.

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. Data from laboratories in New York and
Germany also have shown similarly tiny deviations over the course of a
year. This has led some to suggest that Earth’s distance from the sun,
which varies during the year and affects the planet’s exposure to solar
neutrinos, might be related to these anomalies.

Researchers from NIST and Purdue tested this by comparing radioactive
gold-198 in two shapes, spheres and thin foils, with the same mass and
activity. Gold-198 releases neutrinos as it decays. The team reasoned
that if neutrinos are affecting the decay rate, the atoms in the spheres
should decay more slowly than the atoms in the foil because the
neutrinos emitted by the atoms in the spheres would have a greater
chance of interacting with their neighboring atoms. The maximum neutrino
flux in the sample in their experiments was several times greater than
the flux of neutrinos from the sun. The researchers followed the
gamma-ray emission rate of each source for several weeks and found no
difference between the decay rate of the spheres and the corresponding
foils.

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.

Media Contact: Mark Esser, mark.esser@nist.gov, 301-975-8735