Researchers Obtain the First High-Precision Mass Measurement of Aluminum-22

The Science

Newswise — Researchers at the Facility for Rare Isotope Beams (FRIB) have made a high-precision mass measurement of aluminum-22. The measurement reached all the way to the “proton dripline” of the nuclear chart, which defines one of the limits of where nuclei can exist. This drip line is the edge of where protons and neutrons can form atomic nuclei. Not all combinations of protons and neutrons work. On one side of the boundary, protons are energetically bound in a nucleus. However, if the boundary is crossed and you try to add another proton to the nucleus, the proton will become unbound and will quickly be ejected from the nucleus. The location of the dripline and the physics that occur near it challenge our understanding of nuclear structure. One element of these physics is nuclear halos, in which a core nucleus is surrounded by a “halo” of orbiting protons or neutrons. High-precision mass measurements such as the one of aluminum-22 help scientists determine how tightly bound atomic nuclei are as they get closer to the dripline.

The Impact

FRIB has delivered 270 rare-isotope beams to experiments since the start of user operation in May 2022. As FRIB enhances capability based on scientific needs, it provides rare isotopes not available at any other facility. Measurements of very rare isotopes are key to testing nuclear theory. The best test cases exhibit exotic characteristics that challenge a theory’s predictive capabilities; nuclear halos are one of these test cases. Researchers used this mass measurement of aliuminum-22 to determine the energy required to remove the outermost proton in the isotope. For a nucleus to form a proton halo, the last proton added must be very loosely bound to that nucleus. The research found this to be the case for aluminum-22.

Summary

Researchers used the Advanced Rare Isotope Separator at FRIB, a Department of Energy Office of Science user facility, to produce, separate, and identify a beam of aluminum-22 at relativistic energies. The researchers then sent the beam to the Beam Stopping Facility, where the beam was stopped and extracted at low energy using the Advanced Cryogenic Gas Stopper (ACGS). Next, the beam was sent to the Low Energy Beam and Ion Trap (LEBIT) facility, where the ions were injected into a device known as a Penning trap, which uses electric and magnetic fields to store the ions in space. The researchers then measured the mass of the ions with high precision by observing the ions’ motion in the trap. The team used a detection technique newly implemented at LEBIT called the Phase Imaging Ion Cyclotron Resonance (PI-ICR) technique. This enabled a measurement with a precision of better than 20 parts per billion, a challenge given the very short half-life of aluminum-22, at only 91 milliseconds.

This work demonstrates the potential of FRIB when combined with state-of-the art beam stopping, using ACGS and mass measurements with LEBIT. In the future, FRIB will ultimately provide two orders of magnitude more beam current, increasing the reach of LEBIT to even more exotic areas of the nuclear landscape.

Funding

This material is based upon work supported by the Department of Energy Office of Science, Office of Nuclear Physics, and the U.S. National Science Foundation.