The atomic nucleus is a tough nut to crack. The strong interaction between
the protons and neutrons that make it up depends on many quantities, and
these particles, collectively known as nucleons, are subject to not only
two-body forces but also three-body ones. These and other features make the
theoretical modeling of atomic nuclei a challenging endeavor.

In the past few decades, however, ab initio theoretical calculations, which
attempt to describe nuclei from first principles, have started to change our
understanding of nuclei. These calculations require fewer assumptions than
traditional nuclear models, and they have a stronger predictive power. That
said, because so far they can only be used to predict the properties of
nuclei up to a certain atomic mass, they cannot always be compared with
so-called DFT calculations, which are also fundamental and powerful and have
been around for longer. Such a comparison is essential to build a nuclear
model that is applicable across the board.

In a paper just published in Physical Review Letters, an international team
at CERN's ISOLDE facility shows how a unique combination of high-quality
experimental data and several ab initio and DFT nuclear-physics calculations
has resulted in an excellent agreement between the different calculations,
as well as between the data and the calculations.

"Our study demonstrates that precision nuclear theory from first principles
is no longer a dream," says Stephan Malbrunot of CERN, the first author of
the paper. "In our work, the calculations agree with each other, as well as
with our ISOLDE data on nickel nuclei, to within a small theoretical
uncertainty."

Using a suite of experimental methods at ISOLDE, including a technique to
detect the light emitted by short-lived atoms when laser light is shone on
them, Malbrunot and colleagues determined the (charge) radii of a range of
short-lived nickel nuclei, which have the same number of protons, 28, but a
different number of neutrons. These 28 protons fill a complete shell within
the nucleus, resulting in nuclei that are more strongly bound and stable
than their nuclear neighbors. Such "magic" nuclei are excellent test cases
for nuclear theories, and in terms of their radius, nickel nuclei are the
last unexplored magic nuclei that have a mass within the mass region at
which both ab initio and DFT calculations can be made.

Comparing the ISOLDE radii data with three ab initio calculations and one
DFT calculation, the researchers found that the calculations agree with the
data, as well as with each other, to within a theoretical uncertainty of one
part in a hundred.

"An agreement at this level of precision demonstrates that it will
eventually become possible to build a model that is applicable across the
whole chart of nuclei," says Malbrunot.

## Reference:

S. Malbrunot-Ettenauer et al, Nuclear Charge Radii of the Nickel Isotopes
Ni58−68,70, Physical Review Letters (2022).
DOI: 10.1103/PhysRevLett.128.022502

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