Research shows radiometric dating still reliable (again)

Sep 15, 2010

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.

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 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 . 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.

Explore further: Cold Atom Laboratory creates atomic dance

More information: * R.M. Lindstrom, E. Fischbach, J.B. Buncher, G.L. Greene, J.H. Jenkins, D.E. Krause, J.J. Mattes and A. Yue. Study of the dependence of 198Au half-life on source geometry. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. doi:10.1016/j.nima.2010.06.270

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eachus
4.2 / 5 (5) Sep 15, 2010
Sigh! A clever experiment to disprove something not observed being used to cast doubt on real observations.

The first thing to ask in a situation like this is "What anomaly are we investigating?" Answer: The decay rate of Silicon-32 with a half-life of ~100 years. The experiment with Gold-198 is also a beta decay, check, is only twice as energetic, 411 keV vs. 221 keV, okay...and half-life of two days. Oops.

So this neat experiment might have found another case of an element whose half-life was non-constant. Unfortunately they chose Au-198 which is widely used medically for cancer treatments. Total dosage is important, down to the measured minute important. Any variability in the half-life of Au-198 would likely have been noticed before now.

So mildly interesting negative result, but real results will require long duration observation of Si-32. I have to wonder if special properties of the Si-32 nucleus make it a good neutrino detector.
MaxwellsDemon
5 / 5 (2) Sep 15, 2010
It's more likely that the claims of Fischbach's group were the result of unknown factors influencing the measurements, as Lindstrom said.

The burden of proof is on Fischbach et al to prove that the effect is physical, and not an artifact of mundane anomalies influencing the data. With this experiment the chance that they’d found actual changes in decay rates plummets from ‘unlikely but provocative’ to ‘fully dismissible barring some new and compelling data.’

It’s great to see NIST and Purdue responding so quickly and decisively on this, with a clever and sensible experiment.
thermodynamics
5 / 5 (1) Sep 16, 2010
MaxwellsDemon: You make a very good point. However, what if there is something other than neutrinos affecting the decay rate. The assumption here is that it can only be neutrinos. However, we know that most of the matter in the Universe is unknown to us and we also know that it is gravitationally active. For that reason it can be concentrated in the sun and it could be distributed by the violent eruptions of solar flares (ejecting mass that is the equivalent of a fraction of the mass of the moon off the solar surface). What if it is tossing dark matter at us and that has an unknown and important affect on decay rate? In other words, eachus could be correct in that it might be a particular experiment under particular circumstances that shows the effect. I would be more comfortable if NIST had tried to duplicate the original experiment.
kevinrtrs
1 / 5 (6) Sep 16, 2010
The researchers specifically did not mention stripping the source material of its electrons. This has been known to influence decay rates. Whether that influence applies to C14 needs to be seen.
As an aside, I wish that ALL dinosaur fossils were subjected to double/triple blind C14 dating. Currently the practise of simply assuming 65 million years plus and therefore skipping a simple test is, to put it mildly, negligent.
barakn
4.5 / 5 (8) Sep 16, 2010
The researchers specifically did not mention stripping the source material of its electrons. This has been known to influence decay rates.
Yes, they did mention it. The fact that they didn't turn their spheres and foils into plasmas at millions of degrees indicates they did not strip electrons.
As an aside, I wish that ALL dinosaur fossils were subjected to double/triple blind C14 dating. Currently the practise of simply assuming 65 million years plus and therefore skipping a simple test is, to put it mildly, negligent.
Most dinosaur fossils are of bones, bones don't have a lot of carbon to start with, and most of that carbon disappears as the bone undergoes the fossilization process. So basically you're asking that they perform a test on what's likely to be a newer contaminant of the fossil. Making unreasonable demands on scientists because of your profound lack of knowledge is negligence on your part.
MaxwellsDemon
5 / 5 (4) Sep 16, 2010
@thermodynamics
Fischbach et al didn't conduct their own experiments, they were just data mining. But their findings seemed potentially important, so:

"Eric Norman of the Lawrence Berkeley National Laboratory in California reanalyzed data from experiments on radioactive americium, barium, silver, titanium and tin, and found no seasonal variations, he says."
Source: http://dinosaurc1...ecay.htm

And Peter Cooper of Fermilab analyzed Cassini's Pu-238 nuclear battery data and found that solar proximity had no effect on decay rate (to a precision 350X finer than the deviations reported in Jenkins' and Fischbach's 2008 paper http://arxiv.org/...1.pdf.).
Source: http://arxiv.org/...48v1.pdf

So this new test is just the latest refutation.

If Jenkins and Fischbach remain convinced that they haven't mistaken an error for a physical phenomenon, they're going to need something new. Like experimental data, preferably.
Skeptic_Heretic
5 / 5 (5) Sep 17, 2010
The researchers specifically did not mention stripping the source material of its electrons. This has been known to influence decay rates. Whether that influence applies to C14 needs to be seen.
As an aside, I wish that ALL dinosaur fossils were subjected to double/triple blind C14 dating. Currently the practise of simply assuming 65 million years plus and therefore skipping a simple test is, to put it mildly, negligent.

You can't use c14, you've been told this before.

All fossils dates are difficult to dispute as not one but many, sometimes upwards of ten, different methods are used and come to an agreement on the date. Multiple paths to the same answer means that answer is probably correct.

Keep asking for silly things, kev. Especially when you know better.
Parsec
5 / 5 (2) Sep 20, 2010
I await confirmation or refutation based on actual reproduction of the same data for the same isotopes mentioned in Fishcbach's original paper. It is well known that different nuclei have different cross sections to different kinds of particles. This article simply proves that neutrino's do not effect the decay rates of Au-198. That's an important result, but doesn't refute in any way the original data. In fact, proving that the effect isn't ever caused by neutrino's would be very interesting but still wouldn't negate the original data. You actually have to measure the decay rates for a lot of isotopes for long periods of time to be able to detect any variations due to the sun. As someone else said, neutrino effects are only one (very implausible) explanation, not the only one.
eachus
1 / 5 (1) Sep 21, 2010
It is not a waste of time to perform experiments on other radioisotopes. Finding a second isotope with changing decay rates would be a very interesting confirmation. But finding other half-lives are constant only makes silicon-32 a more interesting subject of study.

Realize that variation in measured rates of Si-32 decay are nothing new, Go to any references on isotopes, and you will find wildly different half-lives. The first four websites found by searching Google for "silicon-32" had half-lives of ~100, 140, and 170 years.

The same experiment for phosphorus-32 yields half-lives of 14, 14.262, and 14.3 days. It is probable that these are all the same number rounded to different precisions. How do you collect P-32 for testing? Chemical separation from Si-32 which decays into P-32. So the half-life for Si-32 should be known better than that for P-32.

So there is something strange about Si-32. Could it have an extremely long lived excited state? Sure.