How to uncover the true face of atomic nuclei?

Jul 10, 2014
How to uncover the true face of atomic nuclei?
Two models of the carbon 12C nucleus

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 presented an idea how this issue may be investigated in future experiments.

Singles or proponents of family life? According the textbooks, protons and neutrons in are placed uniformly and move independently from one another. However, there are many facts hinting that the nucleons in many nuclei bind into small clusters, e.g. into helium nuclei (alpha particles) formed of two protons and two neutrons. Direct measurements of this effect are, however, quite difficult and the results, up to now, ambiguous. How can we see the true face of the atomic nucleus?

In an article published in the prestigious Physical Review Letters and recommended by the editors, physicists from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow and University of Granada (UG), Spain, described a novel method which in future experiments may reveal if the nucleons in nuclei indeed do cluster, or if they "live on their own". The investigation was co-financed by the Polish National Science Center.

Suggestions that the nucleons may group into clusters in atomic nuclei appeared already more that 80 years ago. In 1931 George Gamow, a famous physicist, posed a hypothesis that the atomic nuclei are formed of the . After many decades there is still no unambiguous experimental evidence of this fact. However, from advanced computer simulations we know, for instance, that the nucleus of beryllium 9Be is formed of two alpha clusters and an extra neutron, so it has more a shape of a dumbbell than a sphere. The so-called fragmentation experiments carried out with accelerators hint on the presence of clusters in heavier nuclei, e.g. three in carbon 12C, four in oxygen 16O, ten in calcium 40Ca, or fourteen in nickel 56Ni.

"We claim that if the structure of atomic nuclei is formed of the 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 Prof. Wojciech Broniowski (IFJ PAN), a co-author of the paper.

In these ultra-relativistic collisions the 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, the quark-gluon plasma is formed, which behaves 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 we arrive at an interesting moment", notices Prof. Broniowski. "The flowing plasma cools down and freezes into hadrons which are then observed in 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 one may in a clever way recover the information on the initial shape of the colliding nuclei".

The authors of the publication modeled the collisions of the carbon 12C nuclei on lead 208Pb. The choice of 12C is not accidental: if this nucleus is composed of three alpha clusters, it should have a triangular shape. 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.

"Our method should also apply to heavier nuclei, such as oxygen 16O, which probably has a pyramidal shape. However, the more clusters, effectively the more spherical the nuclei are, and the differences in the hadron velocities will be more difficult to detect", says Prof. Enrique Ruiz Arriola (UG).

Joining of objects into groups is a universal mechanism of lowering the energy of physical systems subject to attraction. It is common in Nature at all distance scales. The up and down 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. In our life, we, the people, may form groups in winter to feel warmer.

"We still do not know if the protons and neutrons form the alpha clusters in nuclei. However, we now know a method how to learn it in certain cases. The next step in on the experimentalists' side", concludes Prof. Broniowski.

Explore further: Observation of unexpectedly deformed neutron-rich magnesium nuclei prompts rethink of nuclear shell structure

More information: "Signatures of Alfa Clustering in Light Nuclei from Relativistic Nuclear Collisions"; W. Broniowski, Enrique Arriola; Physical Review Letters, 112, 112501 (2014).

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axemaster
5 / 5 (5) Jul 10, 2014
The choice of 12C is not accidental: if this nucleus is composed of three alpha clusters, it should have a triangular shape. 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.


Clever, very clever. I love it when physicists can clearly explain how the experiment works. I will be looking forward to hearing the results of this experiment, when they get a chance to do it.
arom
1 / 5 (6) Jul 11, 2014
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?....

Singles or proponents of family life? According the textbooks, protons and neutrons in atomic nuclei are placed uniformly and move independently from one another. However, there are many facts hinting that the nucleons in many nuclei bind into small clusters, e.g. into helium nuclei (alpha particles) formed of two protons and two neutrons….
"We still do not know if the protons and neutrons form the alpha clusters in nuclei. However, we now know a method how to learn it in certain cases. The next step in on the experimentalists' side", concludes Prof. Broniowski.


According the textbooks, two protons and two neutrons forming to be helium nuclei, but there is no explanation how such forming process work; maybe understand this proposed mechanism could help the research ….
http://www.vacuum...=4〈=en
JohnGee
5 / 5 (5) Jul 11, 2014
It's called the strong force, idiot.
swordsman
1 / 5 (4) Jul 11, 2014
A name is just a name. The question of how a proton could attract a neutron has never been answered. A neutron is supposed to have neutral charge, while the proton has positive charge. This would seem to be a comparatively small force.
JohnGee
5 / 5 (3) Jul 11, 2014
No, actually it is the STRONG force.
TechnoCreed
3.7 / 5 (3) Jul 11, 2014
A name is just a name. The question of how a proton could attract a neutron has never been answered. A neutron is supposed to have neutral charge, while the proton has positive charge. This would seem to be a comparatively small force.
No, actually it is the STRONG force.

It is trough mesonic exchange (see quantum chromodynamics), a residual effect of the strong force, that the nucleus is held together; the strong force being effective only within the nucleons. The neutrons act as dampers between the positively charged protons who would otherwise stay apart.
Steve 200mph Cruiz
4.7 / 5 (3) Jul 12, 2014
Arom, swordsman,

They are held together by the strong force. The strong force is sort of like electricity, but instead of positive and negative, there are three "charges", red, blue, and green.
There are 3 quarks in protons and neutrons, and those quarks exist in one of each of those color charges. Gluons are the force carers of that force, as opposed to photons for electromagnetism. The quarks are constantly exchanging their color charges through these gluon, which is what holds protons and neutrons together. This attractive force is also exchanged between other nearby protons and neutrons within the nucleus of the atoms, which is what binds it together.
That's my basic understanding of it anyways.
The strong force has its name for a reason. There are no naked quarks (other than possibly the cores of neutron stars) in the universe, and it kicks electromagnetism around like it's nothing.
DoieaS
3.1 / 5 (8) Jul 12, 2014
The question of how a proton could attract a neutron has never been answered.
The bare fact remains, no force has been explained so far with mainstream physics. We only have formal regressions of it (the distance dependence of forces, which are called the Newton or Coulomb laws), which is analogy of botanics in understanding of plants. If you really want to understand the forces, then the binding force of proton and neutron probably is not the simplest example.
TechnoCreed
5 / 5 (2) Jul 13, 2014
@Steve 200mph Cruiz
When I said that the strong force exist only in the nucleons, I meant that there is no gluon outside of the nucleons. Nucleons interaction happens by mesons exchange; it is called the strong interaction and it is just a residual effect of the strong force.
Here is a video that explain how the nucleons interact (Note that instead of 'mesons' the guy says 'pions'; pions are π-mesons) https://www.youtu...OSMqGLlg

For quark gluon plasma in the cores of neutron stars, I cannot say no, but it is highly hypothetical.
Mimath224
5 / 5 (2) Jul 13, 2014
@TechnoCreed the geometric nucleonic arrangement seems to indicate that the researches are describing Symmetry, U(1) through SO(3). But I lack the knowledge of which when it comes to the qgp and the 'freezing' to hadrons afterwards. Any ideas on this?
TechnoCreed
5 / 5 (2) Jul 14, 2014
You are funny Mimath :-!
Why would anybody ask a question involving Lie group algebra on a GP comment board? Especially on qgp since there is no valid flavor hadronisation dynamics model yet. When you have a serious question to ask, go to the Physics Stack Exchange http://physics.st...nge.com/ But, for this one I would suggest that you ask Garreth Lisi, if you know what I mean.
Dr_toad
Jul 14, 2014
This comment has been removed by a moderator.