A long-awaited view of unstable nuclei

For probing the internal structures of atomic nuclei, the gold standard is electron scattering. There are other ways to glean information about nuclear structure, such as by looking at the products of nuclear reactions or measuring the energies of spectroscopic transitions. But the electron—a point-like, structureless fundamental particle—is unrivaled in its ability to bore into a nucleus to reveal how the protons and neutrons are arranged in space.

Electron-scattering experiments need a lot of identical nuclei. Typically their targets are bulk samples, such as solid metal foils. That demanding requirement has mostly limited the technique to the study of stable isotopes, plus a few long-lived, naturally occurring radioisotopes, such as carbon-14. Short-lived unstable nuclei, on the other hand, have been relegated to study by less direct techniques. But those barely bound nuclei are precisely the ones with the most surprising structures that challenge theorists’ understanding of nuclear forces. (See the article by Filomena Nunes, Physics Today, May 2021, page 34 .)

A growing number of facilities around the world are producing purified beams of rare and radioactive isotopes. (See Physics Today, June 2023, page 21 .) Now Kyo Tsukada and colleagues, working at RIKEN’s Nishina Center for Accelerator-Based Science in Wako, Japan, have performed the first-ever electron-scattering experiment on a target produced from one of those beams. The researchers used a technique called SCRIT (self-confining radioisotope ion target) to pull singly charged ions straight from a radioisotope beam and trap them using the device shown in the photo, which leverages the electric potential of the electron beam itself.

It was an achievement nearly two decades in the making: The idea for SCRIT was proposed by three RIKEN researchers in 2004. The years since then have been spent building, refining, and testing all the necessary instrumentation.

In the new experiment, Tsukada and colleagues used cesium-137, whose 30-year half-life is long enough that the isotope could have been made into a more conventional solid bulk target. The way the researchers made it instead—by photofissioning uranium, separating the ¹³⁷Cs products into a beam, and collecting them into a SCRIT trap—is a powerful test of the technique. And the whole journey of each ¹³⁷Cs atom, from production to trapping to electron scattering, lasts only a few seconds, so the researchers have high hopes that shorter-lived isotope studies will be coming soon. (K. Tsukada et al., Phys. Rev. Lett. 131, 092502, 2023 .)