A long time ago, in our galaxy, there was a rare, very violent explosion.

We may never know what it was: it could have been an especially large supernova (star explosion) or a collision between two neutron stars, and it was at least 100 million years ago.

Whatever it was, it was a hot enough furnace to create some of the rarest elements in the universe such as plutonium-244. These elements were strewn across the sky by the violence of the explosion, eventually reaching earth, where they settled at the bottom of the ocean some ten million years ago, to be dug up and analysed in the 21st century by Dr Dominik Koll and an international team of researchers.

“It’s mesmerising that we can measure signatures today, in a sample that is tens of millions of years old, of a process from more than 100 million years ago,” said Dr Koll, honorary lecturer at the Research School of Physics and postdoctoral researcher at the Institute of Ion Beam Physics and Materials Research at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Germany.

The team’s findings are published in Nature Astronomy and detail the detection of plutonium-244 using the VEGA AMS facility at ANSTO, which signifies an event much bigger than a standard supernova.

Supernovae are rare – we have only witnessed one standard supernova in our galaxy (in 1987) since the invention of the telescope, although we have seen them in other galaxies. The explosions that create plutonium-244 are thought to be another 1000 to 10,000 time less frequent than standard supernovae.

These rare violent reactions create heavy elements in a process whereby neutrons are captured rapidly (inspiring the name ‘r-process’), although details of the process remain mysterious.

“The r-process has been replicated on Earth in thermonuclear explosions, but where it occurs naturally is still not clear.

“We know the process works, but it is hard to model and then find the cosmic conditions in nature that can create these elements,” he said.

Establishing the timeframe of the explosion required careful detective work, based on dating techniques similar to carbon dating, but using other radioactive elements, including plutonium-244, which is radioactive with a half-life of 80 million years.

Using a sample of ferromanganese crust extracted from the Pacific Ocean floor Dr Koll and the team had previously isolated another tracer of supernovae, iron-60, using the ANU Heavy Ion Accelerator Facility. Detailed modelling of the age of the sample layers was done at the DREAMS facility at HZDR using radioactive beryllium-10, which is created in the upper atmosphere.

Combining the data they found peaks of iron-60 at two specific depths in the crust, pointing to two nearby supernovae in the last ten million years.

If plutonium-244 had been created in the same explosions as iron-60, their influx profiles should match and they should appear together in the same layers of the ferromanganese crust.

But the data did not match – instead, the plutonium-244 was evenly spread throughout the sample, suggesting the recent two supernovae were not big enough to fuel the r-process, and there must have been an older event with enough time for its debris to disperse more evenly across the interstellar medium.

To tie down this date, they analysed the ferromanganese crust layers for curium-247, which is also radioactive and created together with plutonium in these giant explosions. But the ANSTO analysis showed none at all.

Curium-247’s half-life is only 15.6 million years, less than a fifth of plutonium-244’s: its absence suggests that the event happened long enough ago for it all to decay, pushing back the date of the event to 100 million years ago, or more.

The assembly of the puzzle pieces to land on such an ancient creation date for the plutonium-244 will send theorists back to the drawing board.

“We have ruled out several scenarios with this finding, such as a proposed neutron star merger near Earth within the last 10 million years.

“Models generally need to simplify the interstellar medium to reduce their computing power, but our data shows that nature is more complicated than that. We need to invest more in models and also in experiments to get to the truth!”  Dr. Koll said.