Disentangeling the properties of the exotic double-gamma nuclear decay


Reporting their finding in Nature Communications [ https://www.nature.com/articles/s41467-020-16787-4 ], an collaboration lead by Dr P.-A. Söderström (ELI-NP) measured with the electromagnetic details of the rare two-photon decay with high-precision placing strict constraints on the possible second-order nuclear excitation underlying this decay mode.

The exotic decay studied is a very rarely occurring type of decay in which a radioactive quantum state of an atomic nucleus, instead of decaying directly to a lower energy state with the emission of a single gamma-ray, emits two gamma rays and changes to an energetically lower state via a quantum double-leap through a virtual coupling to a higher-energy state. Such second-order double-decays are a unique consequence of quantum mechanics, first predicted in the doctoral dissertation of Nobel laureate Maria Göppert-Meyer under the supervision of Max Born in Göttingen University in 1931. In this particular case, both single and double decays are allowed. Thus, this rare process is therefore called competitive double gamma decay by the scientists and is designated with the symbol "γγ/γ". On October 15, 2015, a team of nuclear physicists from Technische Universität Darmstadt announced the discovery of this process in the Ba-137 nuclide in the magazine Nature, showing that this process only occurs there with a probability of around two double-decays per one million single-decays.

A Romanian-led international research group has now studied this type of decay in more detail, during a performance test of the ELIGANT detectors within a 50 days measurement period, and made an unexpected observation. The measurements were carried out on the Extreme Light Infrastructure - Nuclear Physics (ELI-NP) facility, a part of the Romanian National Laboratory IFIN-HH near Bucharest by an international group of scientists working at ELI-NP in collaboration with Technische Universität Darmstadt. The more precise measurement data provide new insights into the specific properties of this type of radioactivity. By measuring the energy distribution to the two gamma quanta more precisely, new conclusions about their electromagnetic radiation character could be reached. The latest data show that γγ/γ decay in the nuclide Ba-137 is a combination of electrical octupole and magnetic dipole radiation from the atomic nucleus. This observation contradicts simple core structure models that were used to interpret the previous data.

The experimental results were explained by a joint effort of nuclear theorists at ELI-NP, Dr N. Tsoneva in collaboration with Universität Gießen who together have pioneered the application of energy-density functional plus quasiparticle-phonon theory, and a group of scientists from Japan led by Dr T. Otsuka at the RIKEN research centre in Tokyo, world leaders in the nuclear shell model approach using complex calculations on the Japanese supercomputer K. These two, very different, approaches to the atomic nucleus together have now been able to explain the dominance of electrical octupole and magnetic dipole radiation at γγ/γ decay in nuclide Ba-137. The computer models show that the γγ/γ decay processes are susceptible to changes in the protons and neutrons configurations in an atomic nucleus and are therefore suitable as precision tests of the structure created by the nuclear forces.

Schematic illustration of the single-γ and the double-γ decay processes, separated into E3M1 (octupole-dipole) and M2E2 (quadrupole-quadrupole) components. These components have been compared with the new experimental data and the sizes of the components have been narrowed down to a large octupole-dipole and a small quadrupole-quadrupole character.