New precision measurement program advances understanding of proton halos
In May 2022, the Facility for Rare Isotope Beams (FRIB) at Michigan State University (MSU), launched its precision measurement program. Staff from FRIB’s Low Energy Beam and Ion Trap (LEBIT) facility take high-energy, rare-isotope beams generated at FRIB and cool them to a lower energy state. Afterward, the researchers measure specific particles’ masses at high precision.
The LEBIT team, led by Ryan Ringle, adjunct professor of physics at FRIB and in the MSU Department of Physics and Astronomy and senior scientist at FRIB, and Georg Bollen, University Distinguished Professor of Physics and FRIB Experimental Systems Division director, published a research paper that used the facility to take a step in verifying the mass of aluminum-22.
Researchers think this exotic isotope demonstrates a rare but interesting property—specifically, that the nucleus is surrounded by a “halo” of protons that loosely orbit the nucleus. This halo structure reveals distinctive physical properties during its fleeting existence.
“This program requires a lot of extra beam preparation to perform experiments, and this is the first measurement in FRIB’s science program,” Ringle said.
“This measurement could not have been done in a reasonable time at FRIB’s predecessor, the National Superconducting Cyclotron Laboratory, and it highlights our facility’s potential moving forward. Considering this was done with one-eightieth of FRIB’s power specification, this was like a warm-up before exercising.”
The team published its results in Physical Review Letters in a paper titled “Precision Mass Measurement of the Proton Dripline Halo Candidate 22Al.”
Capturing elusive proton halos
While most atoms have electrons tightly orbiting the nucleus, protons and neutrons are part of the nucleus itself. However, when atoms encounter many of the same charged particles under certain conditions, they can create halos that orbit the nucleus beyond the pull of the strong nuclear force—the force that would normally keep these particles within the nucleus.
While all halo structures are rare fleeting phenomena, neutrons are usually observed as halo particles. A nucleus’s positive charge usually repels protons’ positive charges, meaning that halos made of protons are even rarer.
Measurements on nearby isotopes suggested that aluminum-22 might be an isotope that could form a proton halo, but researchers needed to verify this directly in other experiments.
To achieve this, the team creates a high-energy isotope beam of aluminum-22 using a process called “projectile fragmentation” at FRIB. The researchers create a beam from a heavy, stable atomic nucleus of a given element—in this case, an isotope of argon—then accelerate the beam to half the speed of light.
The beam then hits a target with these ultra-fast-moving particle projectiles. This violent collision creates rare, short-lived isotopes that the researchers can shepherd into an instrument to filter out the particle of interest.
They then lower the temperature to slow them down into a uniform beam and measure particle mass accurately.