Extremely Rare Decay

International researcher team confirms TU findings on a new kind of radioactivity


Five years ago a team of the Institute for Nuclear Physcis of TU Darmstadt discovered the new, comparably rarely-occurring kind of radioactivity, the „competitive double-gamma decay“, which in 1937 had been predicted in theory by the Nobel Prize winner Maria Goeppert-Mayer. The findings which had been published in „Nature“ have now been confirmed and measured more precisely by an international team of researchers at the European Extreme Light Infrastructure-Nuclear Physics near Bucharest, Romania. The results are released in „Nature Communications“. One of the authors is the TU professor and at the time „joint-discoverer“, Norbert Pietralla.

“K Computer” of the Japanese research centre RIKEN near Tokyo, where the theoretical calculations took place.

In October 2015 a team of the Technische Universität Darmstadt published the discovery of a new variation of radioactivity in the „Nature“ magazine. In this very rarely occurring variation of gamma radiation a radioactive quantum state of an atomic nucleus simultaneously emits two gamma quanta, hence, highly-energetic light particles with wave lengths below the x-ray range. This way it makes a quantum leap into an energetically lower-lying state, although this leap would have been possible by emission of only one gamma quantum.

This rare process has the scientific name „competitive double-gamma decay“ and is denoted by the symbol „γγ/γ“. The Darmstadt nuclear physicists observed that in the nuclide Ba-137 this process ouccurs with a probabiliy of only two to one million.

Led by a former TU postdoctoral researcher

Measurement setup at the European Extreme Light Infrastructure – Nuclear Physics.

Now an international research team from Romania, Japan, Italy and Germany confirms the existence of the γγ/γ decay. The nuclear measurements were conducted at the EU-funded European Extreme Light Infrastructure – Nuclear Physics (ELI-NP) at the Romanian National Laboratory IFIN-Horia Hulubei near Bucharest, employing a dedicated experimental setup which has been constructed over two years.

The experiment was led by the ELI-NP employed Swedish scientist Pär-Anders Söderström, who previously gained scientific qualification as a postdoctoral researcher at the Institute for Nuclear Physics of TU Darmstadt within the research group of Professor Norbert Pietralla, one of the discoverers of the new decay. The new data confirm the probability of occurrence of this new variation of radioactivity, and yield a more precise value.

Nuclear structure models need revision

The more precise data give new insight into the properties and possibilities of scientific exploitation of this new kind of radioactivity. By a comparison of the – compared to Darmstadt data – more precise measurement of the distribution of energy over the two gamma quanta in the double-gamma degay, new conclusions on their electro-magnetic radiation character were drawn. The new data sho that the radiation emitted in the γγ/γ decay from the nuclide Ba-137 is a combination of electric octupole and magnetic dipole radiation. This observation is at contrast to simple nuclear structure models, that the Darmstadt discovery had first been compared to, and which found a combination of electric and magnetic quadrupole radiation to be underlying the γγ/γ decay in the nuclide Ba-137.

A world-leading group of scientists in nuclear theory from Japan was now able to give an explanation for the dominance of electric octupole and magnetic dipole radiation in the γγ/γ decay in the nuclide Ba-137 through demanding calculations on the Japanese super-computer K at the Tokio research center RIKEN. The computer models show that the γγ/γ decay processes are very sensitive on the interactions among protons and neutrons in an atomic nucleus, and therefore are especially suitable for precision tests of our scientific understanding of the structure of atomic nuclei and the nuclear forces.

The publication

The new data are published in “Nature Communications”:

P. -A. Söderström, L. Capponi, E. Açiksöz, T. Otsuka, N. Tsoneva, Y. Tsunoda, D. L. Balabanski, N. Pietralla, G. L. Guardo, D. Lattuada, H. Lenske, C. Matei, D. Nichita, A. Pappalardo & T. Petruse, “Electromagnetic character of the competitive γγ/γ-decay from 137mBa”, NATURE COMMUNICATIONS 11, 3242 (2020).

Publication from 2015

C. Walz, H. Scheit, N. Pietralla, T. Aumann, R. Lefol, V.Yu. Ponomarev, “Observation of the competitive double-gamma nuclear decay”, Nature(London) 526, 406 (2015).