Cool!
Okay, some background. Every radioactive isotope has a distinct
half-life. While radioactive decay of nuclei is the quintessential example of a random process, and so you can never tell when a
particular nucleus is going to decay, if you put enough atoms of a given radioactive isotope together, you can tell with
great precision how long it will take for exactly
half of them to decay. This is the "half life" of that isotope.
For the longest time, it was assumed that virtually no outside factors could influence the rate at which a radioisotope decays. Turns out, that's apparently
not true.
The Earth's
average orbital distance from the Sun is about 149,600,000 kilometers (92,960,000 miles), but it varies somewhat. Our closest approach to the Sun (
perihelion) is in January, when we're only about 147,166,000 kilometers (91,445,000 miles) from the Sun, and we're farthest from the Sun (
aphelion) in July, when we're about 152,172,000 kilometers (94,555,000 miles) from the Sun.
As it turns out, there are slight but measurable differences in the decay rates of certain isotopes when we're at aphelion and perihelion. For example, Chlorine-36 has a higher decay rate in January and February than it does in July and August.
But it gets even better. It turns out that these decay rates change measurably just prior to the eruption of a major solar flare. Since the charged particles emitted during a large solar flare can interfere with electronic equipment here on Earth -- and be potentially lethal to astronauts who don't have the Earth's atmosphere to shield them -- getting a day or so's advanced warning before the eruption of a major flare could be a very important piece of information.
The question is:
How does solar activity affect the decay rates of radioisotopes here on Earth? The best guess is "neutrinos."
The Sun, of course, is powered by nuclear fusion reactions. These and some other nuclear reactions release tiny, charge-less and nearly mass-less particles called neutrinos. Because they're tiny, charge-less, and essentially mass-less, they don't normally interact with other forms of matter.
At all.
How unreactive are they? Every second, literally
trillions of them are passing right through your body. The odds are that not even
one of them will interact with a
single atom in your body. To a neutrino, the planet Earth is a ghost -- it will pass right through the planet without slowing down, interacting with a single atom of it, or in any way noticing its presence (other than the slight effect on its trajectory of the planet's gravity, perhaps).
Neutrinos are almost completely noninteractive with normal matter. Still, the key word is "
almost." Every once in a great while, a neutrino will happen to strike an atomic nucleus dead-on and interact with it. Otherwise, we'd have no way to detect neutrinos at all.
Anyway, we know that neutrinos are involved in many forms of nuclear reactions, including some forms of radioactive decay. So it's not totally implausible that if the number of neutrinos that we're receiving from the Sun varies over the course of a year (and it does), that this could somehow affect the rate of decay of radioisotopes here on Earth. [When the Earth is closest to the Sun we will, of course, intercept a slightly greater percentage of the neutrinos that it's emitting than we will when we're at aphelion.]
Still, it's something of a surprise to learn that our proximity to the Sun can somehow influence rates of radioactive decay -- which were thought to be constant and unalterable -- here on Earth.
Right now, the leading explanation for the observed variations in decay rates is the varying numbers of solar neutrinos that we receive over the course of a year. Though
how solar neutrinos could be influencing decay rates here on Earth is far from clear.
It's conceivable that some other particle -- one that we don't yet know about -- is responsible. That's highly unlikely though.
So, it's really strange that our proximity to the Sun appears to affect the decay rates of radioisotopes here on Earth. And it's even stranger that changes in these decay rates can apparently be used to give us a few hours' warning of a solar eruption. (That's
especially strange, because the vast majority of the Sun's neutrinos are produced by the fusion reactions in its core, not by flares.)
Whatever the ultimate explanation turns out to be, it's quite a fascinating finding.
Cheers,
Michael
Solar Activity Affects Radioisotope Decay on Earth
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