Good morning Dr. Zoomie – so I’m wondering about a few things about radioactivity. I know that being radioactive is just the way some atoms are – but is there any way we can make a radioactive atom non-radioactive? For instance, we can incinerate dangerous chemicals to make them safe – can we treat radioactive stuff to make it non-radioactive? Or can we shorten the half-life so it’s not radioactive at all? I know it if was easy we’d have done it already, but I’m wondering if maybe one day we might be able to do something. If we can, then wouldn’t it make nuclear waste easier to deal with?
You know, that’s a great question! And I’m not just saying that to be nice – it’s a question that at least one Nobel Laureate grappled with. Over a century ago, the great physicist Ernest Rutherford tried putting radioactive atoms under high pressures, high temperatures – he even tried to blow some up – to see if extreme conditions might change the rate at which radioactive materials decayed, and nothing he did had the slightest effect. The conclusion was that being radioactive and the rate at which the radioactivity was diminished (the half-life) were apparently fixed properties – just like an atom’s mass, the color of the crystals they formed, the density and hardness of the chemical compounds they formed, and so forth. In short – there’s nothing we can do to radioactive atoms that will make them non-radioactive or that will make them decay away more rapidly. Almost…but first, let’s talk about why atoms are radioactive in the first place.
Like teens, atoms want to be in the lowest energy state whenever possible. But what does that actually mean? Or to put it another way, what are the sources of energy in an atom?
In an atom’s nucleus there are a bunch of protons and a bunch of neutrons. The protons all have a positive electrical charge; two protons will push each other away. Cramming multiple protons into the small volume of an atom’s nucleus requires a lot of energy to overcome the electromagnetic force that’s trying to push the protons apart. Luckily for us (and for anything else that’s not entirely made of hydrogen) there’s another force at play – the strong nuclear force – that helps to hold the protons together. As long as these two forces are balanced the nucleus will be stable; if they’re unbalanced then the nucleus is unstable. So if the nucleus has too many or foo few neutrons for the number of protons then the nucleus will have excess energy; the way it sheds this energy is by giving off radiation.
Some forms of radioactive decay turn protons into neutrons or vice versa. A neutron, for example, can eject a negatively charged electron, turning the neutron into a proton – this is called beta decay. If the original atom had, say, 8 protons and 11 neutrons (Oxygen-19) the nucleus is being held too tightly to be stable; by ejecting an electron and turning one of those neutrons into a proton will reduce the ratio of neutrons to protons and stability is restored. On the other hand, an oxygen atom with only 7 neutrons (O-15) has too little strong nuclear force; in this case one of the protons will eject a positively charged electron (a positron) to turn into a neutron to give us a stable atom of nitrogen-15.
Another way to turn a proton into a neutron is for the proton to absorb an electron from the atom’s electron cloud – the electron’s negative charge cancels out the positive charge of the proton and turns it into a neutron with no electrical charge. This is called electron capture, and there are a number of elements that decay this way. So…sure, this is all pretty cool (well, to me anyhow), but what’s it got to do with changing a radionuclide’s half-life?
Well…it turns out that, if you can put an atom under, literally, tons of pressure then the electrons are pushed a little closer to the nucleus, making them easier to capture. So under exceptionally high pressures, some nuclides that decay via electron capture will decay just a little bit faster (less than 1%) than they do at atmospheric pressure. Changing an atom’s chemical environment can have a small effect as well because chemical reactions can affect the availability of electrons to be captured – the catch is that the atom has got to be small enough to have only a handful of electrons so that the innermost electrons are also the outermost. These are both real, but they’re also both minor. And then we get to the biggies!
One is the heavy nuclide, rhenium-187 (Re-187), which decays by beta emission with a half-life of 42 billion years. Except that it turns out that the half-life changes if all 75 electrons are stripped away, the half-life shortens considerably – to only about 14 years. This an astounding reduction – a factor of more than one billion – and, while I’ve read a paper about the reason this happens, I have to admit that this one’s going to take me a while to understand. But what I do understand is that this a real phenomenon that is not only understood, but that it also affects our understanding of the history of our galaxy. Which, to me, is just as cool as the fact that a nuclide’s half-life can change.
Heavy elements such as rhenium are forged when stars explode, and the explosion scatters those elements into the galactic neighborhood. By tracking the Re-187 and the osmium that’s formed when it decays, astronomers can learn about where stars have died. Not only that, but the stars that are massive enough to end their lives in an explosion are so massive that they burn out very quickly, which means that they tend to die very close to the place they were born. In other words, rhenium and osmium show us where stars have formed, and by looking at the relative amounts of these nuclides, astronomers can lean about where stars have formed through the history of our galaxy, and even how old our galaxy is. At least, that’s the theory, and it works great for a nuclide with a half-life of billions of years, which is the case with Re-187 unless it’s been stripped of all of its electrons – to fully ionize the atom as the physicists would say. But it takes a lot of energy to strip off 75 atoms, which is why we don’t tend to see many fully ionized atoms in the universe.
It’s usually safe to assume that an atom has most or all of its electrons. But an exploding star is a very high-energy environment and atoms that are born in such an environment can lose all of their electrons. This means that there’s a very real chance that much (maybe even most) of the Re-187 formed in supernovae has a half-life of only 14 years, not the 42 billion years that the astronomers expect. But, knowing about this effect, the astronomers can correct for it – so our understanding of galactic history seems safe!
And it turns out there’s one more way that fully ionizing a radioactive atom can affect its half-life; in this case, lengthening it considerably – this is where electron capture becomes important. Consider – how can a nuclide decay by electron capture if it has no electrons? This, too, can only happen in space – it’s only in the vacuum of space that an ionized atom can travel without collecting electrons, and with no electrons to capture an atom cannot decay this way.
So after all of this, the short answer to your question is that it is possible to change the half-life of a radioactive atom, but it takes extreme conditions to do so, and even then most of the methods only change the half-life by a fraction of 1%. The two exceptions to this both require fully ionizing atoms and keeping them that way for a long period of time – which can only happen if the atoms are in the vacuum of space. And, sadly, as interesting as this all is, it can’t help us with radioactive waste in the slightest. But it’s still pretty cool!