Noyau atomique peau neutrons

The thickness of the lead core is measured قلب

Some measurements with precise scales, on the order of a femtometer (10-15th meters) have reflections on astronomical scales. The Prex collaboration, led by Kent Buchke of the University of Virginia, USA, evaluated the thickness of the lead nucleus’ “shell”, the neutron-rich outer layer. This measurement provides information about conditions in the atomic nucleus, but it also affects other levels in the structure of a particular type of very dense star, neutron stars.

In the simplified view of the nucleus of an atom, its components (protons and neutrons) are randomly distributed. However, in heavy nuclei, neutrons, which are often more protons, are subjected to a pressure that pushes them slightly outside the nucleus. The latter therefore has a surface layer rich in neutrons, referred to as the “skin”. The Prex collaboration used facilities at the Jefferson Laboratory to measure the thickness of this outer layer in the core of lead-208, a stable isotope that has 82 protons and 126 neutrons.

The distribution of protons in nuclei has been well studied for several decades. In fact, in so-called “elastic” scattering experiments, an electron beam bombards a nucleus whose electrically charged protons knock electrons out of their path. Analysis of this diffusion makes it possible to determine the distribution of the protons and, in particular, the radius of the sphere they occupy.

This approach does not apply to the distribution of neutrons, which as their name suggests do not carry an electric charge. The idea is to perform the same kind of experiments with a probe sensitive to the strong interaction, because neutrons are made of quarks that interact through this force. The problem is that the theory describing the strong interaction, quantum chromodynamics, suffers from significant uncertainties that make accurate measurements difficult.

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So the Prex collaboration relied on another fundamental strength, weak interaction. Neutrons are likely to be deflected from electrons by exchanging Z bosons, which are the carrier particles of the weak interaction. An electron can also interact with protons Across This strength, but with less potential.

Physicists have measured the violation of symmetry, parity, and specificity of the weak interaction. Electrons with ‘right’ polarity (spin points in the particle in the direction of their motion) are often scattered more by a neutron than electrons with ‘left’ polarity (spin alignment in the opposite direction of motion). particle). The researchers measured this asymmetry using an electron beam with an energy of 953 megaelectronvolts and polarization sometimes left, sometimes right. The benefit of this measurement is that the amplitude of the asymmetry is directly related to the distribution of neutrons in the lead nucleus. The effect is very small and requires complete control of all sources of uncertainty in the experiment. The challenge the team successfully tackled.

Thus, the researchers obtained a radius of 5.8 femtometers for the distribution of neutrons in the nucleus. Subtracting the known radius from elsewhere for the proton distribution, they conclude that the skin of the lead 208 core is 0.283 femtometers thick. This finding comes on the heels of a first test on lead 208 in 2012, but increases its accuracy significantly.

How interesting is this result in the study of neutron stars? The skin of the nucleus is formed by the pressure of the internal “symmetry” of the nucleus and due to the Pauli exclusion principle. This prevents identical particles, in this case neutrons, from occupying the same quantum state within a system as the atomic nucleus. This pressure increases with increasing nuclear density. However, in a neutron star, the density is higher than in the nucleus of an atom. Precisely determining how symmetry pressure evolves with nuclear density will make it possible to better estimate the radius of a neutron star for a given mass. Taking into account the effect of the skin, such a star could be even greater than its proposed single mass.

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Good knowledge of the structure of neutron stars is critical to modeling the merger of these stars, from events to the next level.The origin of gravitational waves recorded by giant interferometers end to end And the Virgo on the earth. For example, a larger neutron star may deform differently during a merger and the gravitational signal emitted varies accordingly.

The study of skin cores can also be useful in the search for physical phenomena beyond the Standard Model, by reducing uncertainty in measurements of the interactions that occur in different experiments. In direct detection of dark matter, one tracks virtual particles propagating over atomic nuclei. Similarly, the experience coherentIn the United States, the scattering of neutrinos from nuclei by exchange of Z bosons is examined. In such an interaction, the nucleus is subjected to a slight recoil. Any anomalies in the recoil energy could be a sign of “new physics”.

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