State-of-the-art laser spectroscopy reveals intricate patterns in the sizes of radioactive isotopes of copper

As outlined in the paper, measuring the size of short-lived copper isotopes is not an easy task. Postdoctoral researcher Ruben de Groote sheds light to the intriguing stages of the research:

“In this research we measure the nuclear size, technically the nuclear charge radius, through a measurement of the properties of the electrons bound to that nucleus. Specifically, we use lasers to measure the energy it takes to excite one of these electrons to a higher-lying atomic state. As the number of neutrons in the underlying nucleus changes, so does this energy”, de Groote says.

Over the years, many laser spectroscopy methods have been developed at radioactive ion beam facilities, with gradual improvements in the sensitivity of the techniques bringing key isotopes to within reach.

“Certain atomic nuclei have what we call a ‘magic’ nature: the number of protons and neutrons in the core is such that it provides an enhanced stability. One of these magic nuclei is 78Ni, which has 28 protons and 50 neutrons. As this nucleus is currently out of reach for optical methods, we opted to study this nucleus indirectly, by measuring the sizes of the copper (29 protons) nuclei close to it.”

Not a single theory that can explain all properties of all the isotopes discovered

The study of the exotic copper isotopes was performed at the ISOLDE laboratory in CERN, using the Collinear Resonance Ionization Spectroscopy (CRIS) apparatus.

“It’s a complicated machine, which I started working on during my PhD. We combine the principle of collinear laser spectroscopy, which yields excellent resolution, with pulsed laser ionization, which provides us with excellent selectivity and experimental efficiency,” Ruben de Groote informs.

“That’s really the key – otherwise it’s impossible to study these short-lived isotopes. Even at ISOLDE, which boasts some of the highest production rates of radioactive isotopes in the world, we can only get about 20 copper-78 isotopes per second. Only with the development of CRIS, done in my PhD work, can we study this isotope.”

From the experiments, the team could deduce that the charge radii of the copper isotopes increases as neutrons are added to the nucleus, but that close to neutron number 50, this increasing trend stops. Also, the typical ‘zig-zag’ pattern that is observed throughout the nuclear landscape seems to reduce in size.

“This was perhaps the most unexpected feature in the data, which is why we went to great lengths to explore this using state-of-the-art nuclear theory as well. I think that’s the biggest strength of the paper: we combined cutting edge experimental and theoretical tools”, de Groote explains.

The complexity of the atomic nucleus means there is not a single theory that can explain all properties of all the isotopes discovered so far. Two important branches in nuclear theory, called Density Functional Theory (DFT) and the Valence-Space In-Medium Similarity Renormalization Group (VS-IMSRG) framework, were used to investigate the nuclear sizes of copper in more detail.

The relation between the global behaviour of charge radii and the saturation density of nuclear matter could be investigated in detail, and the local charge radii variations were found to naturally emerge from many-body calculations fitted to properties of mass A ≤ 4 nuclei, where A is the sum of the number of protons and neutrons. These observations provide an important step in the understanding of the nuclear binding energy and charge radii, and thus a major milestone on the path towards a predictive nuclear theory.

Pioneering laser spectroscopy tools come from Jyväskylä

Currently Ruben de Groote is working to develop new spectroscopic techniques at the Accelerator Laboratory in the University of Jyväskylä. He tells what is following next in the research:

“We are planning to extend these measurements to other elements as well. As always, reaching the next goals will require further experimental developments. That’s what’s great about this job – the job is never done, there is always more to learn."

The Accelerator Laboratory in JYU is sure to play an important role there.

“In the laboratory here in Finland, there is a real spirit of innovation. I don’t think it’s a coincidence many of the pioneering laser spectroscopy tools were originally developed here. I hope to continue that trend!”

Link to the article in Nature Physics, April 2020:
https://www.nature.com/articles/s41567-020-0868-y

More information on the Accelerator Laboratory at the University of Jyväskylä:
https://www.jyu.fi/science/en/physics/research/infrastructures/accelerator-laboratory

For further information:
Ruben de Groote, The University of Jyväskylä, ruben.p.degroote@jyu.fi, 358408054079