Accurate measurements of the neutron skin of an atomic nucleus change knowledge of neutron stars

Fizycy z Thomas Jefferson National Accelerator (TJNAF – Jefferson Lab) They measured the thickness of the neutron “skin” that forms the sheath of the lead core with extraordinary accuracy. in pages physical review messages I reported that the thickness was 0.28 millionths of a nanometer. Their measurements are of great importance in determining the structure and size of neutron stars.

The nucleus of each element is made up of protons and neutrons. This includes identifying the properties of the elements and allowing us to distinguish them from one another. Physicists have long studied atomic nuclei to learn how protons and neutrons interact. The Jefferson Laboratory runs the Lead Radial Experiment (PREx), which aims to precisely study the distribution of protons and neutrons in the lead nucleus.

The question is where are the neutrons in the nucleus. Lead is a heavy element. It has extra neutrons. However, if we consider only the interaction of the nuclear forces that bind protons and neutrons in the nucleus, then the model in which the lead nucleus contains an equal number of protons and neutrons works best., says Professor Kent Paschke of the University of Virginia, a spokesperson for PREx.

In light nuclei, which have few protons, the number of protons and neutrons is usually equal in reality. However, the heavier the nucleus, the more neutrons it needs to remain stable than the protons. All nuclei of stable elements with more than 20 protons contain more neutrons than protons. Lead, on the other hand, is the heaviest element with stable isotopes. Its nucleus contains 82 protons and 126 neutrons. To understand how they all hold together, we need to know how the extra neutrons are distributed throughout the nucleus.

The protons in the lead nucleus are arranged in the shape of a ball. The neutrons form a larger sphere surrounding a smaller one. We called this larger sphere the neutron skinPaschke explains. This skin was first observed in Jefferson’s lab in 2012. Since then, scientists have been trying to measure its thickness with increasing accuracy.

Neutrons are difficult to study because many of the tools available to physicists record electromagnetic interactions, one of the four fundamental forces of nature. The PREx measurement experiment uses another fundamental force – the weak interactions. Protons have an electric charge that we can study using electromagnetic interactions. Neutrons do not have an electric charge, but they generate strong weak interactions compared to protons. So if you are able to use it, you can tell where the neutrons areBashk adds.

The authors of the new study used a precisely controlled beam of electrons that was fired toward a thin layer of lead cooled to extremely cold temperatures. The electrons rotate in the direction of the beam movement and interact with the protons and neutrons in the lead atoms. Electromagnetic interactions maintain the symmetry of the reflection, and the weak interactions do not. That is, an electron interacting with electromagnetic forces does so regardless of the direction of its rotation. However, when it comes to interactions through weak interactions, there is a clear preference for one direction of rotation. So we can use this asymmetry to study the strength of the interaction, and this allows us to determine the region occupied by the neutrons. So it tells us where the neutrons are in relation to the protonsProfessor Krishna Kumar of the University of Massachusetts Amherst says.

Experimentation requires high accuracy. Suffice it to say that the direction of rotation of the electrons in the current has changed 240 times per second, and the electrons, before reaching the tested lead sample, travel 1 km through the accelerator. The researchers knew the relative position of the electrons with different spins to each other with an accuracy of 10 atoms wide.

Thanks to this great precision, the scientists found that the diameter of the ball made up of protons is equal to About 5.5 femtometres. And the neutron sphere is slightly larger, about 5.8 femtometers. Therefore, the thickness of the neutron skin is 0.28 femtometers. This is about 0.28 millionths of a nanometer, says Bashki.

But how do these measurements translate into our knowledge of neutron stars? The results in Jefferson’s lab indicate that the neutron skin is much thicker than some theories have suggested. This, in turn, means that more pressure is required to compress the nucleus than previously thought, so that the nucleus itself is somewhat less dense. Since we cannot directly study the interior of neutron stars, we have to rely on calculations in which we use the known properties of the components of these stars.

The new discovery also has implications for data from gravitational wave detectors. Circular neutron stars emit gravitational waves discovered by LIGO. Once they are very close, in the last millisecond, the interaction of one star causes the other to become elliptical. If the neutron skin was larger, the star would take a different shape than if the skin was smaller. And LIGO can measure this shape. LIGO and PREx study completely different things, but they share a basic equation – the equation of state of nuclear matter.

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