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What’s the True Nature of Atomic Nuclei?
August 29, 2014   
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Protons and neutrons are the basic constituents of atomic nuclei. Are they distributed homogeneously or perhaps in quartets consisting of two protons and two neutrons? Physicists from Poland and Spain have recently put forward an idea of how this can be determined through experiments.

According to textbooks, protons and neutrons in atomic nuclei are placed uniformly and move independently from one another. However, there are many indications that nucleons in many nuclei bind into small clusters, for example into helium nuclei (alpha particles) formed of two protons and two neutrons. Direct measurements of this phenomenon are difficult and the results have been ambiguous so far. Researchers are still baffled by the true nature of the atomic nucleus.

In an article published in Physical Review Letters, physicists from the Polish Academy of Sciences’ Institute of Nuclear Physics in Cracow and the University of Granada in Spain described a novel method that may help reveal experimentally whether protons and neutrons in atomic nuclei indeed do cluster, or whether they tend to “live on their own.”

Suggestions that nucleons may group into clusters in atomic nuclei first appeared more than 80 years ago. In 1931, Russian-born American nuclear physicist George Gamow came up with a hypothesis that atomic nuclei are made up of alpha particles. After many decades there is still no unambiguous experimental evidence of this fact. However, advanced computer simulations suggest that the nucleus of beryllium 9Be, for example, is formed of two alpha clusters and an extra neutron, so it is shaped more like a dumbbell than a sphere. Some experiments carried out with the use of accelerators point to the presence of clusters in heavier nuclei, for example three in carbon 12C, four in oxygen 16O, ten in calcium 40Ca, or 14 in nickel 56Ni.

“We believe that if the structure of atomic nuclei is formed of alpha clusters, we will be able to see its traces in the spectra of particles formed in ultra-relativistic collisions of properly chosen nuclei,” says the Institute of Nuclear Physics’ Prof. Wojciech Broniowski, who co-authored the paper.

In these ultra-relativistic collisions, atomic nuclei move with velocities very close to the velocity of light. For that reason their spatial configuration is “frozen” during the extremely short reaction time. As a result of the collision, quark-gluon plasma is formed, which behaves like a fluid that pours out in all directions. In turns out, however, that the velocity of this flow is not the same in all directions: in some it is faster, and in some slower. These differences reflect the original shape of the colliding nuclei.

“After a few femtoseconds [i.e. several millionths of one billionth of a second] we arrive at an interesting moment,” says Broniowski. “The flowing plasma cools down and freezes into hadrons which are then observed by detectors. Their velocities are somewhat higher in those directions where the flow is higher. We have shown that by measuring very accurately the particle velocities it is possible to recover the information on the initial shape of the colliding nuclei.”

The researchers modeled the collisions of the carbon 12C nuclei on lead 208Pb. The choice of carbon 12C is not accidental: if this nucleus is composed of three alpha clusters, it should have a triangular shape, the researchers say. In that situation the velocities of the produced hadrons should clearly depend on the direction, moving faster in the direction perpendicular to the edges of the triangle, and slower in the directions indicated by its corners. On the other hand, the very heavy 208Pb nucleus was necessary to guarantee the formation of the quark gluon plasma, necessary for the flow.

Prof. Enrique Ruiz Arriola of the University of Granada said, “Our method should also be applicable to heavier nuclei, such as oxygen 16O, which probably has a pyramidal shape. However, the more clusters, the more spherical the nuclei tend to be, and the differences in the hadron velocities are more difficult to detect.”

The joining of objects into groups is a universal mechanism that is common in nature at all distance scales. Upper and lower quarks group into triplets to form nucleons, nucleons join into atomic nuclei, atoms connect into molecules, droplets of water freeze into snow flakes. At cosmic scales, the stars form galaxies and galaxies form clusters.

The Institute of Nuclear Physics’ Broniowski said, “We still don’t know if protons and neutrons form alpha clusters in nuclei. However, we now know a method that will allow us to find out. The next step on the road to understanding the structure of the atomic nucleus now belongs to researchers ready to conduct experiments.”
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