Using a method called atomic spectroscopy, an international team of physicists has produced a new value for the charge radius of the α particle, the nucleus of the helium-4 atom.
The trace of an α particle obtained by the nuclear physicist Wolfhart Willimczik with his spark chamber specially made for α particles.
The α particle, the nucleus of the helium-4 (4He) atom, consists of two protons and two neutrons, tightly bound by the strong nuclear force.
It is one of the most-studied atomic nuclei and its properties are of great importance for understanding the nuclear forces and the development of modern nuclear physics.
Its simplicity makes the α particle a favorable target for a variety of precision studies.
“The helium nucleus is a very fundamental nucleus, which could be described as magical,” said Dr. Aldo Antognini, a physicist in the Institute for Particle Physics and Astrophysics at ETH Zurich and the Paul Scherrer Institute.
“Our previous knowledge about the helium nucleus comes from experiments with electrons,” said Dr. Randolf Pohl, a physicist in the Max Planck Institute of Quantum Optics and QUANTUM at the Johannes Gutenberg-Universität Mainz.
“However, we have for the first time developed a new type of measurement method that allows much better accuracy.”
For their measurements, they used muons — particles that are very similar to electrons but 200 times more massive.
“The idea behind our experiments is simple,” Dr. Antognini said.
“Normally two negatively charged electrons orbit the positively charged helium nucleus.”
“We don’t work with normal atoms, but with exotic atoms in which both electrons have been replaced by a single muon.”
“So with muonic helium, we can draw conclusions about the structure of the atomic nucleus and measure its properties.”
The researchers measured the charge radius of the α particle to be 1.67824 femtometers.
“We present the measurement of two 2S-2P transitions in the muonic helium-4 ion that yields a precise determination of the root-mean-square charge radius of the α particle of 1.67824(83) femtometers,” they said.
“This determination from atomic spectroscopy is in excellent agreement with the value from electron scattering, but a factor of 4.8 more precise, providing a benchmark for few-nucleon theories, lattice quantum chromodynamics and electron scattering.”
The results are reported in the journal Nature.
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J.J. Krauth et al. 2021. Measuring the α-particle charge radius with muonic helium-4 ions. Nature 589, 527-531; doi: 10.1038/s41586-021-03183-1
