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There is an old question in physics. The question is whether there is an atomic nucleus that contains only neutrons. In nature, there are atomic nuclei that contain only protons, but no atomic nuclei that contain only neutrons. However, recently, an international joint research team was able to observe a nucleus (tetra neutron) that contained only four neutrons. The discovered neutron tetra is expected to bring a major turning point in nuclear physics.
Atoms are the basic particles that make up matter. Atoms consist of a nucleus and electrons. And the nucleus is again made of protons and neutrons. A positively charged proton and an electrically neutral neutron. The harmony created by two nucleons in a small space that the human eye can never see or feel is colorful and mysterious.
If you think of the size of an atom as a playground, the size of an atom’s nucleus is only about the size of a football. How can protons and neutrons collect in such a small space?
An electromagnetic force (repulsive force) that repels each other like the same magnetic poles acting between protons. Magnets that repel each other need a force greater than the force of the magnets to hold them together. The same is true when an electric charge causes the same nucleons to stick together. The force that overcomes the electromagnetic force between protons and makes it possible to form an atomic nucleus is called the ‘nucleon-nucleon interaction’ or ‘residual strong nuclear force’ (abbreviated as ‘nuclear force’ in this text ).
The reason why neutrons have not been detected so far
An international joint research team including the Korea Institute of Basic Science (IBS) succeeded in observing ‘tetra neutrons’ from the heavy ion accelerator at the RIKEN in Japan. A tetra neutron is a neutron that contains only four neutrons and no protons. Dong-A Science DB/Data provided by Japan Institute of Physics and Chemistry
The first start of the periodic table, which lists all the elements discovered so far, is ‘hydrogen (H)’ with atomic number 1. In hydrogen, one proton forms the nucleus of an atom. Since protons have an infinite lifetime, they can exist stably in nature.
If an atomic nucleus contained only neutrons, the atomic number would have started from 0. However, atomic nuclei containing only neutrons cannot be found in nature. This is because neutrons are more stable to convert into protons. After the half-life of neutrons (about 614 seconds), half of the neutrons are converted into protons.
For a neutron to remain stable in the nucleus, it must coexist with a proton. Except for hydrogen, all atoms have protons and neutrons in their nucleus. For example, the nucleus of deuterium (²H) has one proton and one neutron. Like deuterium, a state in which protons and neutrons are not collapsed by the nuclear force and maintain a stable state is called the ‘bonded state’.
Thanks to its intrinsic angular momentum a neutron can remain bound when it is with a proton. Nucleons have an inherent property of intrinsic angular momentum. A stable nucleus can be created if the directions of intrinsic angular momentum of the nucleons that make up the nucleus are opposite. On the other hand, if only nucleons with the same direction of intrinsic angular momentum, such as proton-proton or neutron-neutron, are formed, they cannot exist in a bound state.
Of course, this does not mean that there are no structures that contain only neutrons in nature. On a scale larger than the atomic unit, a system consisting mainly of neutrons can be found. A typical example is a ‘neutron star’ created by a supernova explosion in the final stage of a star’s evolution. A neutron star has a very high density of neutrons. The strong gravitational force of a neutron star keeps the neutrons together without turning them into protons inside.
Neutrons present for ’10 to the power of 22′ seconds in a heavy ion accelerator
Whether a nucleus containing only neutrons could exist has been one of the long-standing questions in the field of nuclear physics. Evidence has recently been found to answer this question. An international joint research team that included 25 domestic and foreign institutions including the Darmstadt Institute of Technology in Germany, RIKEN in Japan, and the Rare Nuclear Research Center in Korea Institute of Basic Science (IBS) succeeded in observing a nucleus (tetra neutron) made of only four neutrons.
In the study of tetra neutrons, the heavy ion accelerator RIBF (Rare Isotope Beam Factory) of the Japanese Institute of Physics and Chemistry was used. The first aim was to create a ‘neutron-rich nucleus’ with more neutrons than a normal atomic nucleus. Helium-8 (He) was made by colliding oxygen-18 (O) ions with beryllium metal in a heavy ion accelerator. The nucleus of helium-8 contains 2 protons and 6 neutrons. There are 4 more neutrons than helium (helium-4), which is common in nature.
The researchers found that when helium-8 collided with a proton target, it split into helium-4 and tetra neutrons, and found that a nucleus containing only neutrons could exist for a very short time of approx. 3.8 × 10 to -22 square seconds.
Dong-A DB/Data Science IBS provided
Tetra neutrons could be a major turning point in nuclear physics
New discoveries always fascinate researchers. It also shows that the scientific knowledge we know is not an absolute and immutable truth. The discovery could revolutionize many areas of modern physics, from nuclear forces to models of neutron stars.
In particular, the field of nuclear physics is expected to reach its greatest turning point. Until now, when calculating the nuclear force, we did not only calculate the case of neutrons, but the discovery of tetra neutrons meant that it was necessary to study more extensively on different interactions between nucleons.
The question of the existence of tetra neutrons is not over yet. First of all, in order to explain the tetraneutron, the current theory of the nuclear force must be greatly modified. Also, the resonance state of tetraneutrons
It is also possible to consider the possibility that it is not due to neutrons, but due to other quantum physics actions.
It is not easy to say that new discoveries are different from existing scientific theories. In order to revise a scientific theory, it is essential that it is supported by a range of evidence. Therefore, in order to solve the questions asked by scientists so far, we need a theoretical model that takes into account the forces between several nucleons and more varied and precise experimental results. We are excited to see what kind of answers nuclear physicists will offer based on the new experimental results and how the way we understand the world we live in will change in the future.
*Resonance state: A state where nucleons such as protons and neutrons interact with each other. When a resonance state appears, the nucleon can be said to be bound.
※ About the author
Dahi Kim, Institute for Basic Science (IBS) Rare Nuclear Research Center Research Fellow, Postdoctoral Research Fellow, Deuk-soon Ahn, Director