Extraordinary systems in the atomic nucleus

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Physicists from the Jagiellonian University are conducting crucial experimental research on the interactions between protons and neutrons, i.e., the elements of an atomic nucleus

The team composed of physicists from various academic centers in Poland and abroad, co-ordinated by Professor Stanisław Kistryn from the Institute of Physics of the Jagiellonian University (JU), provides the most accurate results in experiments analyzing the dynamics of interactions between the basic particles constituting an atomic nucleus – nucleons, i.e., protons and neutrons. This is a new generation of research, important and valued by groups of experts throughout the world.

Calculation models used by physicists to describe (reproduce the properties) of the world have to be simplified by their very nature. When physical phenomena from our direct surroundings are explained (e.g. the loads to which the armchair that we are sitting in is subjected), classical Newtonian mechanics is sufficient. In the case of more complicated problems it is necessary to take into account Albert Einstein's theory of relativity. A similar situation, when more detailed insight requires the introduction of a more complex description, is the case of analyzing the forces inside an atomic nucleus.

Are two enough?

As far as understanding the forces inside the atomic nucleus is concerned, the interactions between two nucleons have been understood and described quite well for a long time, regardless of whether we refer to two protons, two neutrons or a deuteron – a special instance of the proton-neutron system. Calculations employing such models are used, for example, for the purposes determining radiation doses in cancer therapy or irradiation of food products (which is used do disinfect food and prolong its shelf-life). They are also used for the configuration of shields of complex installations such as scientific accelerator systems (e.g. CERN) or equipment for the neutralization of nuclear power plant waste.

Theoretical models generally describe only certain aspects of reality. Their correctness and accuracy as well as the scope of application are verified by means of observing the given phenomena in nature. The point is to achieve the best possible compliance between theory and practice while constantly striving for perfection.

In 1996 the team from the Institute of Physics started studying specific nuclear reactions in which deuterons collide with protons. Results obtained through data analysis provided us with a large amount of valuable information about the interactions within a system composed not of two, but three nucleons. The physicists showed, for the first time in history, that research on a system consisting of three nucleons provides surprising information. They proved that it is not enough to take into account only the forces between pairs of nucleons in order to describe the results correctly but that it is also necessary to consider three-body forces.

Core part of the detection system – a set of nearly 150 detectors,
which simultaneously forms a vacuum reaction chamber.
The "mess" of cables serves to supply every detector with high
voltage (of about 2000 V) and to collect the electronic signals,
carrying the valuable physical information. Photo: E. Stephan

Three means more than just three pairs

After years of studying nuclear forces it turned out that in order to fully describe systems consisting of three nucleons simply adding up the forces in three possible pairs of partners proves insufficient. Such a method is very common – this is how, for example, orbits in the Sun-Earth-Moon system are calculated: gravity forces between each pair (Sun-Earth, Earth-Moon and Moon-Sun) are taken into account. In the case of nucleons such an approach is inadequate as it turns out that there is an additional contribution to the total energy of interaction. Theoreticians call this addition a three-body force. This force is much weaker than the standard interaction between two nucleons. This is why only very precise experiments, such as those carried out by the group supervised by Professor Kistryn, were able to prove its existence. Nowadays, theoreticians are able to create a physical/mathematical model that also includes this additional component of the dynamics of a three -nucleon system and to integrate it with previously used descriptions.

"Only such complete calculations provided us with outcomes that are perfectly compliant with our measurement results," says Professor Stanisław Kistryn. "This is a mutual success of experimental scientists and theoreticians which significantly improves our understanding of the mechanism of the processes occurring in systems composed of three nucleons," the physicist adds.

The method is the secret

Research conducted by physicists from the Jagiellonian University guarantees obtaining precise results thanks to the method which ensures high experimental accuracy. This is possible due to the development of adequate experimental procedures, which are themselves an achievement, adapted by other research teams for the purposes of projects in other fields of studies. What makes us happy is the fact athat there is a prospect of continuing the studies on systems consisting of several nucleons in Krakow. On Professor Kistryn's initiative, the experimental system used for the measurements described here was transferred to the Jagiellonian University and installed in the hall of the new cyclotron at the Institute of Nuclear Physics of the Polish Academy of Sciences in Bronowice. Since mid- 2013 an international research team has been conducting preparations for the continuation of the research project in Kraków.

Understanding the details of the interactions in systems consisting of several nucleons is crucial for the transition to an even more fundamental level, where the nucleon is treated as a complex structure, consisting of quarks that interact mutually throughout gluon exchanges. The attempt to describe the interactions between these elementary objects of matter and to transfer such a description to the level of more complex objects, such as nucleons or atomic nuclei, is a fundamental problem of contemporary physics.

To complete this image it is worth noting that on the level of quarks physicists do not expect the existence of three-body forces, which they currently use only as auxiliary tools enabling them to account for the incomplete knowledge about the structure of nucleons. It would be quite similar in the case of gravity forces in a planetary system. If we treated the Earth (as well as the Sun and the Moon) as a sphere without any structure, we would not be able to calculate the orbit of the Moon correctly, because we would ignore the influence of tides, i.e., the changes in the shape (internal structure) of the interacting bodies. However, if we were forced to apply such a simplified model of structure of celestial bodies, the introduction of a model of effective "three-planetary forces" would enable us to conduct correct astronomic calculations.

Research team from Institute of Physics, Jagiellonian University in Kraków:Professor Stanisław Kistryn; Professor Andrzej Magiera; Professor Kazimierz Bodek; Jacek Zejma, PhD; Izabela Ciepał, PhD; Aleksandra Wrońska, PhD; Rafał Sworst, PhD; Wiktor Parol, MSc; Ganshyambai Khatri, MSc

Cooperating institutions: Institute of Physics, University of Silesia, Katowice; Institute of Nuclear Physics, Polish Academy of Sciences, Kraków; Institute of Physics, University of Warsaw; Institute of Nuclear and Accelerator Physics (KVI), University of Groningen, The Netherlands; Tata Institute of Fundamental Research, Mumbai, India; University of Tokyo, Japan; Tohoku University, Japan