Tracing the origins of Pu-244

Plutonium-244 could’ve come from neutron-star mergers, so knowing its abundance could reveal the rate at which such mergers happen.


The heaviest naturally occurring elements are thought to form not when a star is alive but when it begins to die. Specifically, in the explosion that results when a star weighing 8x to 20x our Sun dies, in a core-collapse supernova (cc-SNe). In this process, the star first implodes to some extent before being rebounded outward in a violent throwing-off of its outer layers. The atoms of lighter elements in these layers could capture free neutrons and transmutate into an atom of a heavier one, called the r-process.

The rebound occurs because if the star’s core weighs less than about 5x our Sun (our entire Sun!), it doesn’t collapse into a blackhole but an intermediary state called a neutron star – a small and extremely dense ball composed almost entirely of neutrons.

Anyway, the expelled elements are dispersed through the interstellar medium, the region of space between stars. Therefrom, for example, they could become part of the ingredients of a new nebula or star, get picked up by passing comets or meteors, or eventually settle down on the surface of a distant planet. For example, the isotope of one such element – plutonium (Pu) – is found scattered among the sediments on the floor of Earth’s deepest seas: plutonium-244.

Based on multiple measurements of the amount of Pu-244 on the seafloor and in the interstellar medium, scientists know how the two amounts correlate over time. And based on astronomical observations, they also know how much Pu-244 each cc-SNe may have produced. But what has caught off recent scientists is that the amount of Pu-244 on Earth over time doesn’t match up with the rate at which cc-SNe occur in the Milky Way galaxy. That is, the amount of Pu-244 on Earth is 100 times lower than there would’ve been if all of it had to have come from cc-SNe.

So where is the remaining Pu-244?

Or, a team of astrophysicists from the Hebrew University, Jerusalem, realised, was so much Pu-244 not being produced in the first place?

Read the full piece here.