Helium is a very important element, much more than just a way to make balloons float or increase the pitch of our voices. It is the second most abundant element in the universe and the lightest of the noble gases. Physicists have now announced a breakthrough in our understanding of this element: The most precise measurement of its nucleus yet.
An international team of researchers has established that the radius of the helium nucleus is 1.67824 femtometers – or 1.67824×10-15 meters (5.506×10-15 feet). To give you a comparison, if the nucleus was the size of your thumbnail, your actual thumbnail would then be the size of the Earth’s orbit. This new measurement is 4.8 times more precise than previous estimates. The findings are reported in Nature.
The helium atom, in its most abundant form, is made of a nucleus comprised of two protons and two neutrons surrounded by two electrons. The electrons have a negative electric charge, while the protons are positive. Neutrons have no electric charge but are key to keep the protons from repelling each other. Protons and neutrons are made of quarks, and quarks interact via the strong nuclear force.
This will tell you that the particles in the nucleus are not just staying still, and the nucleus hasn’t got a well-defined boundary. Its radius is estimated by working out the interaction between the nucleus and negatively charged particles.
In this latest experiment, researchers tweaked the helium by switching electrons with muons. Muons have the same electric charge as the electron but they are 200 times more massive. This difference allowed researchers to make more precise measurements.
"We don't work with normal atoms, but with exotic atoms in which both electrons have been replaced by a single muon.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," senior author Aldo Antognini, from the Paul Scherrer Institute, said in a statement.
This same approach with muon was used a few years back to obtain a more precise measurement of the proton. This led to a bit of contradiction in the size of the proton. Researchers thought that this was just an experimental error in older measurements, but they were open to the idea that maybe it was a hint of more complex physics.
There is no disagreement in the measurement of the helium – the latest number is a clear and simple improvement on previous estimates. This strengthens the proton results, making it unlikely to be a product of unknown physics.
"Our measurement can be used in different ways," says Julian Krauth, first author of the study. "The radius of the helium nucleus is an important touchstone for nuclear physics."
This work is a perfect testbed for several physical theories, from theoretical models of nuclear structure to refining our understanding of the strong nuclear force in fundamental physics.