New limit of $^{244}$Pu on Earth points to rarity of actinide nucleosynthesis

TL;DR

This study reports the detection of interstellar $^{244}$Pu in Earth's deep-sea floor, revealing a much lower abundance than expected and suggesting that actinide nucleosynthesis events are rare, possibly limited to neutron-star mergers or specific supernovae.

Contribution

It provides the first measurement of live interstellar $^{244}$Pu in Earth's geological record, constraining the frequency of actinide-producing r-process events.

Findings

Detected $^{244}$Pu at levels two orders of magnitude lower than expected.

Suggests actinide r-process nucleosynthesis sites are rare.

Supports the idea that neutron-star mergers or specific supernovae are primary sources.

Abstract

Half of the heavy elements including all actinides are produced in r-process nucleosynthesis whose sites and history still remain a mystery. If continuously produced, the Interstellar Medium (ISM) is expected to build up a quasi-steady state of abundances of short-lived nuclides (with half-lives <100My), including actinides produced in r-process nucleosynthesis. Their existence in today's ISM would serve as a radioactive clock and would establish that their production was recent. In particular $^{244}$Pu, a radioactive actinide nuclide (81 My half-life), can place strong constraints on recent r-process frequency and production yield. Here we report on the detection of live interstellar $^{244}$Pu, archived in Earth's deep-sea floor during the last 25 My, at abundances lower by about two orders of magnitude than expected from continuous production in the Galaxy. This large discrepancy may signal a rarity of actinide r-process nucleosynthesis sites, compatible with neutron-star mergers or with a small subset of actinide-producing supernovae.