Today I want to introduce you to the wonderfully unstable element of 37 protons: Rubidium. This element is extremely rare in the universe and on our planet. Its creation is prohibitively difficult. It can only be forged in the fires of huge explosions. And I mean huge as in a supernova explosion, or even the Big Bang kind of huge.
First of all, what do physicists mean by unstable? Imagine a coin on a table, just chilling flat. You know that, if undisturbed, the coin will remain in this position forever. This is what real stability means. If you pull up the coin a little bit from one side, you’d be creating instability, because you’re positive the coin will eventually fall back to its stable position if you let it go.
But what if you put the coin sideways, vertically placed on its edge? You know it’s not really stable. It only takes a little push, a small breeze or someone nudging the table a little bit, for the spell to be broken. It’s a matter of time before some random event forces the coin out of its “unstable” state and into its final, truly stable form.
Rubidium is an atom that lives in a similar state as our edged coin. It is unstable, and it only takes a minuscule amount of effort to push it out of its current state and into a much more stable form. All that’s needed is to change one proton for one neutron. The nucleus achieves this by spitting a positron out of the atom. When that happens, a proton is flipped into a neutron, and this transformation causes this atom of 37 protons to have only 36. So we can’t name it Rubidium anymore, it’s Strontium. One of the most stable atoms out there. An atom that won’t easily change its state or configuration, probably ever.
That shot will eventually happen. In the same way the coin would eventually fall flat on the table. The only question is when. You could say it’s a random process. We’re positive it will eventually happen, but we can’t predict how long it will take. So we do the next best thing: We use statistics. We have studied the evolution of a gazillion atoms of Rubidium and found that the average time for Rubidium to decay into Strontium is around 70 billion years.
You may have guessed this already, but 70 billion years is a really long time. It’s far more than the age of the universe. Most of Rubidium atoms that were created in the Big Bang still remain unperturbed today. But we see it happening in real time. You can pick up rocks on our planet that contain Rubidium and if you look closely enough you’ll see some of them popping out positrons and transforming into Strontium. This process and the pace at which it happens is so accurately calculated that we use it as a technique to measure the age of ancient minerals.
But wait, how can such an improbable event be spotted routinely? Because within these rocks you can find up to septillions of Rubidium atoms. And even though the disintegration of one of them is almost impossible, you should know that improbable things happen a lot, if you have enough observations.
Rubidium doesn’t enjoy the kind of mass media attention other elements do. Partly because we don’t make great use of it. Not because it lacks interesting properties, but because some other more common and cheaper elements share the same electron configuration — such as Potassium.
The chemical behaviour of Rubidium and Potassium is so similar that your body sometimes misplaces Rubidium atoms as if they were Potassium, if you eat some accidentally. In fact, you didn’t know it but you already have plenty of Rubidium flowing through your veins, travelling from one cell to another, interacting with organelles and — of course — firing positrons around like a wild rich Texan. All of it being mistaken for Potassium. You’ve been doing this your entire life and you didn’t even notice. It’s completely harmless, so don’t worry. But if you put a positron detector near you, you’ll see them flowing out.
The human body is an extremely complex machine that can repair itself in a great variety of situations outstandingly well. Despite that, sometimes it engages in weird patterns. One especially rare behaviour is when some cells short-circuit and get stuck in a loop of perpetual reproduction. They stop doing their usual job and just spend all their time replicating themselves into more cells that will commit the same aimless behaviour. This is what we usually call cancer.
Among other particularities, one curious characteristic of cancer cells is that they tend to absorb a lot of Potassium, in order to fuel their maniac growth. Far more than healthy cells. And as we’ve seen, wherever Potassium goes, some Rubidium is going too by mistake. Hence, cancer cells happen to be disproportionately filled with Rubidium atoms that are shooting positrons in all directions. And if you put the positron detector near you, you can map spots in your body that are emitting more positrons than the rest. Because that spot is full of cancer cells.
This technique is what we call Positron Emission Tomography. And most modern hospitals have a machine that does it. Saving many, many lives.
So take a moment to picture this: There is an atom of Rubidium that was created billions of years ago when the universe was formed, probably in the core of a supernova. It has travelled indescribable distances over a mind-boggingly long time. Unperturbed, unchanged. It has eventually landed on planet Earth, accidentally swallowed by a human body that misplaced it as Potassium and smuggled inside some random cell that was faulty and turned cancerous. And this single Rubidium atom just so happens to pop a positron for its first, and last, time ever - ceasing to be Rubidium and becoming Strontium - at the exact same time a tomography was taking place. The machine sees the positron flying off and uses it to identify cancerous cells, possibly saving that patient’s life.
Isn’t that wonderful?