Exotics among the atoms

A new experiment at the Paul Scherrer Institute (PSI) has set out to better understand the atomic nucleus using muons. ‘QUARTET’, a collaboration of institutes from Switzerland, Belgium, Germany, France, Israel and Portugal, has just taken data for the second time and everyone involved is eagerly awaiting the results. Their ally is a new detector technology that can be used to explore much lower energy ranges.

The group around Andreas Knecht (PSI) and Katharina von Schoeler (ETH) has picked up a research thread that used to have a golden era: research with muonic atoms. A ‘normal’ atom consists of a number of protons and neutrons in the nucleus surrounded by a cloud of electrons. Instead of electrons, a muonic atom has a muon, a negatively charged relative of electrons. They are 200 times heavier than electrons, and if you introduce them into regular atoms, you can find out a lot about the atomic nucleus - for example how big it is. After all, this is still only a partially solved mystery.

Nuclear charge radius is the key word in this context. How far exactly does the positive charge of the atomic nucleus extend, i.e. where does the nucleus end? This can provide researchers with information on whether the standard model of particle physics is correct, for example, or provide accurate input for precision calculations of atoms. Measurements with muonic atoms have revealed significant discrepancies with the predictions of the standard model: the measured proton radius deviates from theoretical expectations, so could new particles or forces be at play? The nuclear charge radius cannot be measured directly, but is derived from experimental data. And muons are very helpful here because they orbit closer to the nucleus than the electrons due to their greater mass. One method to track down the radius is to excite the muons with lasers to a higher energy level. Detectors then measure the X-ray light that they emit when they leave this higher level - similar to spectroscopy with normal atoms. Another method is to use X-ray spectroscopy to detect the radiation emitted by the muon.

‘’Ever since people started experimenting with muon beams in the 1960s, they have measured everything they could,‘’ says Andreas Knecht. ‘By the mid-1980s, they were done - there was nothing more that could have been discovered with the existing technology. The research field of muonic atoms went quiet for a while, but with improved muon beams, such as those available at PSI, this research is experiencing a renaissance. For example, the atoms are no longer produced exclusively with the help of thick targets, but also with wafer-thin layers in the microgram range, and can therefore also be measured with radioactive material.

‘This gives us access to a whole new field of isotopes. We are looking for cases where there are two stable isotopes and one unstable, radioactive one. How do the nuclear charge radii change from isotope to isotope?' explains Katharina von Schoeler. With three reference radii, experiment can give theory a helping hand, improving the general precision and perhaps seeing things that had previously been lost in the noise.

One limiting factor has thus been eliminated, but the ‘QUARTET’ (QUAntum inteRacTions for Exotic aToms) experiment has made another improvement - namely in the detector. Instead of the traditional germanium detectors, the team uses a metallic magnetic micro-calorimeter. Like the germanium detectors, it specialises in measuring X-rays. Unlike the traditional detectors, however, this calorimeter is particularly good at measuring low energies. The threshold for conventional detectors is around 50 keV; the metallic magnetic micro-calorimeter can do this down to a few keV. Its resolution is therefore 20 times better than that of other detectors.

This is due to its technology: the entire detector has a size of no more than 4 x 4 millimetres. It consists of 8 x 8 pixels, each half a millimetre in size. Built at and constantly monitored by employees of the Kirchhoff Institute for Physics in Heidelberg, it measures tiny temperature changes caused by the absorption of energy. It has to be operated at very low temperatures - 10 to 15 milli-Kelvin, reached thanks to a commercially available dilution refrigerator - in order to get the best possible results from the data, and has an extremely high resolution. The key lies in the tiny temperature differences that the calorimeter can detect. The researchers not only want to find out which energies it can measure, but also how good the energy resolution is there. This is the key to differentiating between muonic lithium isotopes, for example. Based on the transition energies of the muonic atoms, the researchers can then draw conclusions about the nuclear charge radii.

The team has already measured lithium, beryllium and boron. ‘This year, we have set ourselves the goal of measuring lithium even more precisely for our two-week beam time. It's quite stressful because we naturally want to collect as much data as possible in such a short time, but everyone involved is also very excited,’ says Katharina von Schoeler. During the first run last year, the team first tested the principle of data collection (it works!) and identified areas for improvement. The first results of these measurements are expected in 2025 - and then, of course, there will be many other nuclei that need to be measured precisely!

Barbara Warmbein