Dear Dr. Z – I was at our local HPS Chapter meeting and overheard a few health physicists mention “primordial radionuclides.” I’ve got to admit I’m not quite sure what this means – can you tell me what primordial radionuclides are, where they come from, and why they’re important?
You know, when I say this it’s really going to increase my geek quotient…but primordial radionuclides are some of my favorites…so what a great question! And I guess that saying “good question” is a bad answer, so let me elaborate.
Primordial radionuclides are radionuclides that have been a part of the Earth since it first formed. This means that they have to have a half-life long enough to have survived for over 4.5 billion years; it also means that these radionuclides were created in the heart of an exploding star and were seeded in the solar nebula (the cloud of gas and dust that condensed to form our Sun and planets) at least five billion years ago. For practical purposes, this means that, to still have detectable levels of radioactivity today, the primordial radionuclides need to have a half-life of at least 100 million years – which have undergone about 45 half-lives of decay since the Earth first formed (with a concomitant reduction in activity by a factor of about 35 billion).
Uranium-238 is one of these primordial radionuclides – with a half-life of about 4.5 billion years, it was part of the solar nebula and, was a part of the small whorl that eventually condensed to form our Earth. Other major primordial radionuclides include U-235, Th-232, and potassium-40 (K-40), all of which are easily detectable in soil and many rocks and minerals, and K-40 is also found in many minerals as well as in biological materials. Uranium-238 has a half-life of about 4.5 billion years (about the age of the Earth) and the half-life of Th-232 is 14 billion years, which is about the age of the universe, and K-40 has a billion-year-plus half-life. But, believe it or not, these are two of the shortest-lived primordial radionuclides! Lutetium-176 (which is used to help calibrate some types of radiation detectors) has a half-life of nearly 38 billion years; Rb-87 (which is used to calculate the age of some rocks and minerals as well as to tease out the origins of different igneous rock formations) has a half-life of nearly 50 billion years, and samarium-147 (used in yet another geologic dating technique) has a half-life of more than 100 billion years. The longest-lived primordial radionuclide, tellurium-128, has a half-life of more than 2 trillion trillion years – more than 160 trillion times the age of the universe.
OK – from a radiation safety standpoint, Rb-87, Sm-147, Te-128, and every other primordial radionuclide with a half-life longer than Th-232 is insignificant when it comes to potential health effects or regulatory issues – these are radionuclides that we can detect, but there’s just not enough of them on Earth to be of anything more than academic interest. But, having said that, from an academic standpoint, they are pretty interesting! But let’s talk about the others first.
Potassium-40 has a half-life of about a billion and a quarter years. Every living organism on Earth uses potassium as part of its metabolism, so every living thing on Earth receives some level of radiation exposure from K-40 – about 35 mrem annually for humans and more or less for other organisms depending on the amount of potassium they have in their tissues. The thing is, without potassium we’d die pretty quickly since our muscles (including our hearts) wouldn’t function properly – that would seem to offset the very minor risk of getting radiation-induced cancer in our 70s or 80s (less than one quarter of one percent risk).
Getting back (briefly) to the longest-lived primordial radionuclides, some of these are used for various geologic studies, mostly used to figure out if, say, magma that’s erupting from a volcano has all come from the same magma chamber or if it reflects mixing from multiple magma source. Some of these are also used in simple geologic dating, to figure out how old a rock formation is. And many of them, quite frankly, are simply of little interest to science – at least, until we learn enough about them to put them to use. But none of these (with the exception of K-40, U-235, U-238, and Th-232) expose us to measurable amounts of radiation.
Something else that bears mention – and that I should have noted up front – is that there are three primary types of natural radionuclides on Earth. There are the primordial radionuclides that we’ve been talking about, and there are also cosmogenic radionuclides formed when cosmic rays slam into the Earth, turning stable atoms radioactive (and, hmmm, maybe I should write something about these cosmogenic radionuclides one of these days). And then there are what are called “decay series” radionuclides – these are radioactive atoms formed by the decay of a handful of the heaviest primordial radionuclides; specifically, from the decay of uranium and thorium. It turns out that these decay series nuclides give us most of our background radiation exposure every year.
Take U-238, for example. U-238 decays by emitting an alpha particle – a helium nucleus with two protons and two neutrons (so a mass of 4 atomic mass units). The way this is written is ; the way to translate this is “Uranium-238 emits an alpha particle to form Thorium-234. It turns out that Th-234 is radioactive and it decays by emitting a beta particle to form protactinium-234. The protactinium is also radioactive, decaying to form U-234, which undergoes alpha decay…and so forth until, nearly a dozen decays later, we finally end up with stable lead (Pb-206). These decay series nuclides include radium (Ra-226) that used to be used for painting glow-in-the-dark watch dials, radon (Rn-222) that bedevils homeowners who are stuck doing radon testing to sell their houses), polonium (Po-210) such as was used to poison former Russian spy Alexander Litvenenko in 2006, and others. These decay series nuclides are found in rocks and mineral around the world, contributing to our background radiation exposure.
And this is why your question is such a good one – you asked about primordial radionuclides, but to give a good answer I get to think about exploding stars, cosmic rays, and radioactive decay series, in addition to musing about the age of the universe! And maybe that’s why these are my favorites – it’s hard to think about them without thinking about so many other fascinating things.