Tracking down the size of the helium nucleus with highly specialized analysis technology

Researchers from the renowned Swiss Paul Scherrer Institute (PSI), in collaboration with colleagues from the Johannes Gutenberg University (JGU) in Mainz, have developed a measurement method that allows the most precise insights into the structure of atoms. The object of the investigation was the atomic nucleus of helium. With the unique measurement technology, the size of this nucleus could be determined five times more precisely than in previous experiments. Take part in fascinating insights into the world of particle physics.

Relevant building blocks for the fundamental theories of physics

Many fundamental theories and numerous constants of nature derive from investigations in basic physics. How atoms are made up of protons, neutrons and electrons, how these particles interact with each other and what their size is, is of fundamental importance here. Only through such findings can the chemical and physical processes in the world be understood and assumptions for natural constants be verified.

A good candidate for such fundamental investigations is helium. The lightest of all noble gases, it is the second most abundant element in the universe, surpassed only by hydrogen. More than a quarter of all atomic nuclei formed after the Big Bang were helium nuclei. Therefore, the study of the helium nucleus is of fundamental importance.

The trick with the muons

To determine the size of the helium nucleus more precisely, the international team of researchers developed a unique analysis technique. The basis is first the structure of the positively charged helium nucleus, which consists of two protons and two neutrons. The helium nucleus is surrounded by two electrons. However, since these two electrons are very high-energy, very small and very fast, they are difficult to study. Therefore, the scientists used a trick - they replaced the two electrons with a so-called muon.

This exotic particle is the so-called big brother of the electrons with comparable properties, but about 200 times heavier than the nimble electrons. In addition, muons can also be produced in low-energy states, so they are slower and easier to study.

Unique facility for the production of muons

The particle accelerator at PSI in Switzerland is the only one of its kind that can produce these low-energy muons in sufficiently large numbers. Only the low-energy muons can be stopped in the apparatus and thus interact with helium. If the particles were high-energy, they would simply fly through, this difference is what makes the facility in Switzerland so unique.

When the specifically generated muons hit the helium gas, the two electrons in the electron shell can be displaced and replaced by a muon. This creates the so-called muonic helium. This can now be studied with the second highly specialized technology - the laser system.

With laser technology the energy of the muons

The muons can be brought into an energetically higher state by targeted bombardment with laser light. If they then fall back from this to their original state, X-rays are emitted, which can be recorded by a detector.

However, the fact that the muons change their energy state, i.e., switch from low to high energy, only works if the irradiated energy corresponds exactly to the difference between the high- and low-energy states. The irradiated energy is directly related to the frequency of the laser light. In the experiment, therefore, the laser frequency is varied until the correct energy is found. This is then visible in the X-ray detector, since many signals from muons that have fallen down again can be detected in it.

With this measured so-called resonance frequency, the energy difference between the lower-energy and higher-energy states can now be calculated directly. And because this energy difference is directly related to the size of the helium nucleus via theoretical formulas, the size of the helium nucleus can now also be determined. The combination of outstanding technologies resulted in a value for the mean charge radius of the helium nucleus of 1.67824 femtometers - five times more accurate than ever before. A milestone for the study of fundamental theories of chemistry and physics.

Theories and constants put to the test

The new findings can now be used to test new theoretical models of nuclear physics. The precisely determined helium radius can serve as a good reference value for reconciling experiment and theory. Only through such examples can the validity of new theoretical approaches be verified.

Next, the researchers will again try their hand at normal helium atoms and helium ions. For this, investigations on electronic helium have already begun. From the comparison between the muonic and the electronic values, further insights can then be gained. Thus, this can also have a high relevance for the determination of fundamental constants of nature. An example of such a constant is the Rydberg constant, which plays a major role in quantum mechanics.

 

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