The Atomic Detectives: Finding traces of a Supernova under the sea

It is in theory possible to extrapolate the whole of creation—every Galaxy, every sun, every planet, their orbits, their composition, and their economic and social history from, say, one small piece of fairy cake - Douglas Adams 

Aside from a marvellous quote from one of the most original of writers, it perhaps isn’t as fictional a concept as you would think.  What if, hiding amongst the atoms that make up the rocks around us are the signatures from cataclysmic events that happened a long time ago and far far away? Well, it can happen, but it is all about hunting for the right stuff - in this case, the right element radioisotopes.  

It all starts with atoms.  Atoms are the basic building block of everything, and we all know they come in a number of different versions - the elements.  Sitting on every science classroom wall is the periodic table of elements, charting out the 118 or so that are known to exist.  In fact, in the realms of theory, it’s thought that up to 174 elements could actually exist.  There’s a range of experiments occurring around the world to determine how long these extra elements can exist and if it is feasible to even make them.

But even your basic individual element can come in a few different guises, and it’s all to do with the amount of neutrons that sit in the nuclei.   Carbon, for instance, has three naturally occurring guises, or isotopes, one with six neutrons in the nuclei, another with seven and one more with eight.  We quote these numbers along with the numbers of protons that sit with the neutrons in the nuclei, meaning these isotopes are called ¹²C, ¹³C and ¹⁴C.  While both ¹²C and ¹³C are stable, if you leave ¹⁴C sitting out on a bench, it will eventually decay (to a nitrogen-14 atom).   If you have a group of ¹⁴C atoms sitting out, then the time for half of them to decay to ¹⁴N atoms is 5,730 years. This is our measure of the element’s ‘half-life’ and tells us that in about this time, there is a probability that half the atoms in our sample would have decayed to ¹⁴N in this timeframe.  

There are thousands of isotopes known across all the elements, though only a small subset known to be naturally occurring. Then a smaller set is known to be stable over the long life of the Universe. 

Charting the relative amounts of various isotopes is the main way we have to understand processes that occur over many many years.  Carbon dating is perhaps the most famous example of this, but many different isotopes are used to focus on specific problems - like Uranium isotopes that were used to date the oldest rocks on Earth. 

It is important to understand the natural process that creates a specific isotope - and there’s a great number of these elements that we know can only be formed in the most dramatic of astronomical events.

It takes at least a supernova event to create many of the elements and isotopes on our periodic table, in fact, to form anything with a bigger nucleus than iron.  But, of course, the energy scale doesn’t stop there and thanks to LIGO observations we have even observed neutron star collisions, which have presented heavy elements like Strontium in the light signatures of the electromagnetic counterpart signal from the event.  

One way to chart the difference in energy from these events is to determine how many elements are produced from the ‘r-process’.  This is where already heavy elements are flooded with neutrons much quicker than they can shed them - leaving them to swell to become atoms of gold, platinum or plutonium.

Many have been searching for a radioisotope (one that decays, rather than the majority that are stable) that could only come from the r-process for a while, with the aim to answer quite a few fundamental questions.  There seems to be quite a lot of (very useful) heavy elements in our Solar system - does that mean a few of these cataclysmic events happened about the time our system formed.  Or do they occur all the time?    

It is also already known where to look out for these radioisotopes, a particular sediment found in the ocean that accumulates very very slowly.  A few centimetres, carefully drilled out, can represent millions of years of geological history.   These ferromanganese crusts have already been found to contain iron-60, a radioisotope signature of a supernova that went off locally 2-3 million years ago.  But ⁶⁰Fe is formed in smaller supernovae, and not by the r-process.  

The focus was now to find Plutonium-244, a radioisotope with an 80 million year half-life.  But the difficulty was finding an instrument that could hunt down the tiny amount of atoms that would not only signify the results of the r-process on Earth but also tell us when it happened.

Radioisotopes are detected with atomic mass spectrometry, where a sample is separated into charged atoms, accelerated in a beam before their path is bent by a magnet.  Atoms of different weights will bend in slightly different paths and this method can be so precise as to separate out different isotopes of the same element (often the difference in their weights will only be one neutron!).  

But plutonium atoms, because of their sheer size (on an atom scale at least) need a particularly tuned and sensitive instrument to tease out their isotopes.  Some nifty innovation means that the only instrument that can currently do this in the world is right here in Australia, the VEGA accelerator at ANSTO.  

This success, like many big studies, hasn’t come overnight - and is down to great technical skill of the instrument scientists.  But it has paid off, with a study published last week in Science. 

Counting just 181 Plutonium-244 atoms in ocean sediments from Japan puts the yield of this isotope quite a bit lower than would be expected from the supernova that went off 2-3 million years ago. Complementing these findings in the same samples is a simultaneous signal of Iron-60, which is a known by-product of supernovae explosions. 

This points out that we do need neutron star collisions to account for many of the heavy elements we see on Earth and in our Solar system, and that such an event must have occurred relatively nearby as our planets were all formed.   

So while not fairy cake, we really can find quite an awfully important result from a tiny collection of atoms. The kind of atoms that make up the stars, the planets, the moons, the humans, even the device you are reading this on, or in the last meal you had. 

Quite the atomic detective story.   

We acknowledge the Dharawal people as the traditional landowners of the ANSTO nuclear reactor facility.