How to measure the atomic nucleus with extreme accuracy? The Swiss team exchanged two electrons in the helium atom for a single muon. Muons are negatively charged leptons, like electrons, only about two hundred times heavier. This made it possible to measure the size of the helium core extremely accurately. Measures 1.67824 femtometer.
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Helium lamp. Credit: Pslawinski / Wikimedia Commons.
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Helium is ubiquitous in the universe, and after hydrogen it is the second most abundant element of all. It is very important in many respects, including, of course, that it can be used to make floating balloons. Physicists strive to know all possible parameters of helium as well as possible, as they can have far-reaching consequences.
Aldo Antognini from the Swiss Paul Scherrer Institute together with an international team of experts measured the size of the helium core with the greatest accuracy in history. They concluded that the helium nucleus measured 1.67824 femtometers, i.e. ten to minus fifteen meters. This figure is about 4.8 more accurate than the known value. The results of the extreme measurement were published in the journal Nature.
Aldo Antognini. Credit: Paul Scherrer Institute.
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The helium atom, in the form of the most common isotope, consists of two positively charged protons with two neutrons, around which two negatively charged electrons orbit. Neutrons do not have an electric charge, but they contribute to the stability of the nucleus. The nuclei of atoms, with the exception of helium, are not solid, well-defined entities. The particles that make up the core are not icy. Therefore, the nucleus does not have a clearly defined boundary, and its size is estimated by scientists from the interactions between the nucleus as a whole and negatively charged particles, usually electrons. Which is not entirely simple.
When measuring the core size again, Antognini et al. they exchanged both electrons for a single muon in the helium atom. Muons are leptons, just like electrons, and are also negatively charged. The difference is that muons are about 200 times heavier than electrons.
Paul Scherrer Institute, logo.
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And this has made it easier for physicists to make their ultra-accurate measurements. According to Antognini, muon helium allowed them to study the structure of the nucleus of the helium atom and its properties. A similar approach was used a few years ago to measure proton size, although it ended in controversy.
The new indication of the size of the nucleus of the helium atom is no surprise. It doesn’t even bring new physics, unfortunately. It is a refinement of previous measurements. But that does not mean that such research is useless. As the authors emphasize, their results can be used in a number of other studies. The size of the helium nucleus is crucial in many respects, in a number of models and hypotheses of nuclear physics.
Literature
Paul Scherrer Institute January 27, 2021.
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