How Nuclear Isomers Give Themselves Away with Gamma Flashes express
A collision of two neutron stars is a cosmic forge where the heaviest elements of the periodic table are hammered out in seconds. In 2017, detectors picked up gravitational waves from such a merger, and telescopes saw a flash — a kilonova. Its glow is powered by the decay of unstable nuclei born in the rapid neutron capture process (r-process). But this forge has quiet helpers that had previously gone unnoticed.
These are nuclear isomers — states in which a nucleus lingers for anywhere from fractions of a second to hours before shedding energy and settling into its ground state. Standard calculations have ignored them, assuming that after beta decay the daughter nucleus instantly calms down. The authors of this study, for the first time, tracked several isomers as independent players in the reaction. They calculated how temperature affects transitions between isomeric and ground states, and how beta decay populates different nuclear levels.
It turns out that two lines could be bright enough to be spotted by gamma-ray observatories like COSI, AMEGO, or LOX — if the explosion happened in our Galaxy. These are the 743.3 keV line from the isomer niobium-97 and the 555.6 keV line from yttrium-91. So far, the calculation covers only a small fraction of known isomers, but it is already clear: without them, the picture of the gamma-ray sky after a merger would be incomplete. The cosmic forge works more subtly than we thought, and now we have a chance to hear its quiet signals.
Original abstract
as written by the authors arXiv:2606.26447
The multi-messenger observations of GW170817 have provided strong evidence that binary neutron star mergers are a site of heavy-element production through the rapid neutron capture process. The decay of unstable $$r$$-process nuclei produces a significant number of $$\gamma$$-rays, which not only power the associated kilonova, but may also be observable in current and future $$\gamma$$-ray observatories, providing a direct probe of the nuclei synthesized. Current models of the emitted $$\gamma$$-rays link a nuclear reaction network with individual decay spectra. Typically, only the population of the ground state of the nuclei is tracked in the reaction network, and rapid de-excitation of the daughter nuclei is assumed when calculating the decay spectra. This may not be accurate in nuclei where there are longer-lived, high-energy nuclear isomers. In this work, we relax these assumptions, modeling the dynamic behavior of nuclear isomers for a select list of nuclei. These isomers are included as independent states in the network, with temperature-dependent effective transition rates between ground and isomeric states and $$\beta$$-feeding probabilities calculated using the nuclear level structure. We estimate the $$\gamma$$-ray flux from isomeric transitions, finding that the 743.3 keV line of Nb-97m and the 555.6 keV line of Y-91m may be detectable in $$\gamma$$-ray observatories such as COSI, AMEGO, and LOX at galactic distances. These results, calculated for a small fraction of nuclei with isomers, highlight the need for the robust inclusion of nuclear isomers in nuclear reaction networks, as well as careful modeling of the resultant $$\gamma$$-ray spectroscopy to characterize observational signals.
Some nuclear isomers live so long that they can be stored in a lab for years — for example, the hafnium-178 isomer has a half-life of 31 years.
In Peter Watts's novel Blindsight, alien probes use gamma-ray bursts to communicate — maybe isomer lines will one day help us decode cosmic messages.