Friday, 23 November 2012

Radioactivity half-life is not a constant

Radioactivity is defined as the spontaneous disintegration of certain atomic nuclei (e.g. carbon-14) into more stable forms (e.g. nitrogen-14). Radioactivity half-life is a measure of how quickly this disintegration occurs, but not for an individual atom.

 It is only measurable and meaningful when considering a large number of radioactive atoms. Then one can talk about how long it takes for half the atoms to decay (i.e. for their nuclei to disintegrate); but looking at one single atom it is absolutely impossible to know whether its nucleus will have decayed in 1 second or 1000 years. One can only talk about the most probable lifetime.

For instance, radium 226 decays into polonium, lead, bismuth and other elements with a half-life of 1601 years. Carbon-14 decays into nitrogen with a half-life of 5,740 years. Yet these figures say nothing about any one atom of Ra-226 or C-14. They are statistical in nature.

Until recently it was assumed that the half-life of samples of elements were constants of nature. Nothing could change them. They reflected the quantum phenomena that go on inside the nucleus of an atom.

It is now looking as though this 80 year old assumption is wrong. There is growing evidence that the radioactivity half life varies with processes occurring inside the sun and the position of the earth in its orbit. This has been found independently in at least three different labs:

 Purdue University, Indiana (manganese-54)
 Brookhaven National Laboratory, NY State (silicon-32)
 German national metrology Institute, Braunschweig (radon-226).

The solar effects are very small – typically 0.1% .Also, they are confined to decay by beta-ray emission - no such effect is observed for decay by alpha- or gamma-ray emission. But they are definitely synchronised to the time when the earth is closest to and furthest from the sun’s northern hemisphere, which is February and August. The radioactive decay rate is higher in February and lower in August than at other times of the year. There is also a correlation with solar flares which could be the basis of a means of forecasting them.

Experiments need to be carried out on carbon-14 because this is used extensively in radiocarbon dating. Variations of a fraction of a percent would not seriously affect existing carbon dating data on archaeological or palaeontological samples. Yet we know that the sun’s activity has not been constant. It is thought to be 20 – 25 % more active now than at the time of the earth’s formation. If miniscule variations in solar activity affect radioactivity very slightly, what would be the effect of large variations? Perhaps none. But if not, the implications for history could be enormous.

Even the small effects discovered to date are inexplicable in terms of existing elementary particle physics. E.g. a new kind of neutrino, the neutrello, has been proposed to explain them but there is no evidence that such a particle exists. It may even be necessary to hypothesise a fifth force in addition to the four already known: electromagnetic, weak nuclear, strong nuclear and gravitational.

Another case of interesting times ahead for physics and possibly for history.

Author 2077 AD
See, for example,

proton size puzzle