They have been hunted for almost 50 years and for 20 years they seem to be pointing more and more. Tetraneutrons, nuclei composed only of neutrons, much as if they were the eponymous stars, again appear to have been made in the laboratory.
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[EN VIDÉO] Nucleosynthesis, or how stars make atoms Stefano Panebianco, research engineer at CEA, talks about the nuclear astrophysics of stars. These gigantic natural thermonuclear fusion reactors produce future atomic nuclei from hydrogen and helium. The more often the manufacturing stops with the iron, the more stable. But beyond that, for heavier nuclei, certain mysteries remain.
The physical nuclear has not yet revealed all its secrets. As Futura explained in the previous articles below, the modern heirs of Rutherford, Chadwick, Fermi or Segrè continue to explore the nuclear reactions and the products of these reactions during collisions between nuclei, in search of what are called tetraneutrons.
As the name suggests, these are cores that would be made up of just four neutrons. It is well known that there are nuclear forces which bind the protons and neutrons in nuclei. These forces are necessary to explain why the protons do not separate while they repel each other prodigiously since they are positively charged and therefore undergo forces electrostatic mutual repellants. They also explain why neutrons, which are not charged, are stuck to other neutrons and to protons in nuclei.
We note the existence of deuterium nuclei with one proton and one neutron, and tritium nuclei with one proton and two neutrons. We could therefore naively expect the existence of nuclei with two protons or two neutrons only. In his quantum physics course, the Nobel Prize in Physics Richard Feynman explains why the Pauli exclusion principle and the nature of the nuclear forces between two protons does not allow a nucleus 2He, therefore without neutrons, to exist. There should also be no heavier nuclei composed only of protons.
The question arose of the existence of nuclei composed only of neutrons and although the physicists generally agree that this was probably not the case, this statement was not as solid that in the case of 2He, so the hunt for these cores has been on for decades.
Two teams of physicists, French in 2002, and Japanese in 2016, (see the previous article below) had precisely found indications of the existence of tetraneutrons but without providing definitive proof. Today, it is a team of physicists from the Technical University of Munich (TUM) who are bringing new elements to the debate on this subject as evidenced by a publication in Physics Letters B .
Unstable nuclei produced by nuclear collisions
Physicists used the Van de Graaff Tandem Particle Accelerator, i.e. a type of accelerator using electrostatic fields only, and therefore of the type that preceded the cyclotrons and synchrotrons arrived in the 1930s and 1940s. Electrostatic accelerators are useful, but they do not allow for much rise in temperature. energy.
The one available on the Garching research campus has nevertheless made it possible to accelerate the nuclei of lithium 7 to make them reach about 12% of the speed of light. During collisions between pairs of these nuclei, it does seem that a production of a carbon 10 nucleus in its first excited state and a tetraneutron with a binding energy of 0.42 megaelectronvolt (MeV) has occurred for these collisions.
Theorists suspected it and now the measurements seem to indicate it, giving more weight to previous tetraneutron detections advanced for about 20 years. Tetraneutrons that may have been observed again also appear to be quite fragile and unstable, disintegrating rapidly. Calculations by the team responsible for the likely discovery suggest that the tetraneutron is about as stable as a free neutron, which has a half-life of 450 s.
Let us remain cautious, the detection still has a statistical significance of 3σ, it normally takes 5 for a solid discovery and confirmations at this level by several independent teams.
But, if this is the case, then tetraneutrons are sort of mini neutron stars on Earth. Certainly, the giant and massive atomic nuclei that are these neutron stars giants to use their name in the founding works of Robert Oppenheimer in 1939 (in collaboration with his students at the time: On Massive Neutron Cores, with Georges Volkoff, and On Continued Gravitational Contraction, with Hartland Snyder) owe their cohesion to the strength of gravitation and in addition in the field of intensity requiring the general relativity, which is not the case for tetraneutrons. But their study should reveal secrets about thestate of matter nuclear power in neutron stars, assuming of course that tetraneutrons do exist, which seems increasingly likely.
What you must remember
- Nuclei formed from only two neutrons or two protons are too fragile to exist, according to the theory of nuclear forces.
- Theoretical and experimental arguments however converge towards the proof of the existence of nuclei formed of only four neutrons: tetraneutrons.
An exotic nucleus with 4 neutrons, it’s possible!
Article by Laurent Sacco published on 07/26/2017
Theoretical calculations support the idea that the Japanese laboratory Riken would have observed the creation of a nucleus exotic researched for decades: the tetraneutron. There is still work to be done to talk about a real discovery.
The nuclear forces that bind neutrons and protons in atomic nuclei are more complex than one might think Hideki Yukawa. He began to unravel the mystery of their nature by postulating the existence of a new force conveyed by a boson loaded scalar, the ” meson pi ”, also known as a“ pion ”.
These forces make more or less fragile, or on the contrary solid, combinations in different proportions of protons and neutrons. They do not allow, for example, nuclei formed of only two protons or of only two neutrons, which are far too weakly linked, but make possible the existence of the deuteron (a proton and a neutron) or of the ‘helium 4 (two neutrons and two protons).
The situation is less clear as regards the theoretical possibility of the existence of certain exotic cores, like the one that would be formed of four neutrons: the tetraneutron. As is often the case in this type of situation, one of the most effective ways of deciding the question is to experiment. In 2002, an international team, led by physicists from the corpuscular physics laboratory in Caen, announced that it had obtained indications of the existence of this exotic nucleus (see article below) in experiments carried out with the particle beams of the Great national accelerator ofions heavy, in Caen (Ganil, CEA-CNRS).
A very unstable tetraneutron?
In 2016, it was their colleagues from Radioactive Isotope Beam factory, from the famous Japanese Scientific Research Institute, the Riken, who announced that they had even more convincing indications of the existence of tetraneutron, to the point that we are bordering on a discovery “at 5 sigmas”, as physicists say in their jargon.
A team of American and French theorists has just made it known, with an article on arXiv, that it was in fact possible to also theoretically justify the existence of the tetraneutron, which consolidates the interpretation of the experimental results of the past year. However, the combination of the two results suggests that the tetraneutron is very fragile, so much so that one might well end up questioning its real existence as a true nucleus. To be continued …
Is there a nucleus made up only of neutrons?
CNRS article published on 04/24/2002
An international team, led by physicists from the Corpuscular Physics Laboratory in Caen (CNRS / IN2P3-ISMRA), presents experimental results suggesting that there could be a bound atomic nucleus made up of four neutrons (a “tetraneutron”).
These results, soon to be published in Physical Review C, have been obtained thanks to the use of exotic beams from the Large National Heavy Ion Accelerator in Caen (Ganil, CEA-CNRS). If confirmed, this discovery, which would challenge current theoretical models, will have important implications in nuclear physics.
One of the major challenges of nuclear physics is to understand how atomic nuclei are constructed from their constituents, the nucleons (protons and neutrons). A few simple facts are now established: on the one hand, all nuclei heavier than the nucleus ofhydrogen (consisting of a single proton) include both protons and neutrons. On the other hand, a system comprising only two neutrons is not linked … but not much: a slight increase in attraction between the two particles would lead to the formation of a linked structure, the “dineutron” . Finally, the study of nuclei with more than two neutrons shows that, very often, the addition of an additional neutron increases the stability of the structure.
The question then arises whether a neutron system composed of more than two neutrons could exist. Based on current knowledge of the interaction between nucleons, the theoretical answer is probably no. In fact, for forty years, all attempts at experimental evidence have failed. However, the advent of high-energy exotic nucleus beams over the past decade has made it possible to design new experiments, as it is believed that very neutron-rich nuclei may contain aggregates composed only of neutrons within them.
These aggregates could be released when breaking very exotic nuclei in collisions with other nuclei. One of the problems is the detection and identification of such neutral objects because they can easily be mistaken for simple neutrons, also released during the collision. Physicists have developed a method similar in many ways to that used by James Chadwick when he discovered the neutron in the 1930s. It is based on the fact that a frontal collision of a proton with a system of four neutrons communicates to this proton an energy much greater than that which would be communicated to it by the shock with a simple neutron.
A careful analysis of the data collected at Ganil, with the British multidetector Charissa and the Franco-Belgian neutron detector Demon, revealed six events compatible with the characteristics of a tetraneutron which would be produced during the breaking of beryllium 14 nuclei. number of events is greater than the estimated background noise level based on the possibility of other processes occurring. Given the small number of events observed, it is essential to continue specific research experiments for this tetraneutron. If these experiments confirmed the current result, they would challenge current models of nucleon-nucleon interaction.
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