The exterior of the nucleus is covered in a thin layer of neutrons

Mar 5, 2012 14:56 GMT  ·  By
The vessel containing the lead sample in the PREX experiment (left) and the massive spectrometers used to detect the electrons scattered from the lead nuclei and measure the nuclei's skin
   The vessel containing the lead sample in the PREX experiment (left) and the massive spectrometers used to detect the electrons scattered from the lead nuclei and measure the nuclei's skin

The nucleus of each large atomic nucleus is covered by a neutron skin, a very thin, nearly-pure layer of neutrons that act as a hard shell protecting the gooey interior. The latter contains a mixture of protons and neutrons. For the first time ever, experts measured the thickness of atomic nuclei's neutron skins.

The approach was relatively direct. We currently do not possess the technology to conduct direct studies at such small scales with any relevant degree of accuracy. In the latest investigation, scientists were able to approximate this measurement with more precision than ever before.

Scientists with the research team say that the technique they used could theoretically be refined even further, allowing an even higher degree of precision in future measurements. The new study could be of critical importance to understanding neutron stars.

But the most useful application for finding out this value would be to use it as an extremely important constraint on theoretical models, which are currently used to explain a wide array of physical phenomena, Science Now reports.

The announcement was made by University of Tennessee in Knoxville nuclear theorist Witold Nazarewicz, who was not a part of the study team. The study was conducted by experts with the Thomas Jefferson National Accelerator Facility (JLab) Lead Radius Experiment (PREX).

Thus far, experts relied on uncertain theoretical model and limited experiments to learn more about the neutron skin. They used to bombard atomic nuclei with pions, antiprotons and protons, all of which are particles that interact with nuclei via the strong nuclear force.

However, each model yielded a different result. Unlike its predecessors, the PREX experiment fires electron beams at very thin sheets of an isotope called lead-208. The nucleus of each lead-208 atoms contains 126 neutrons and only 82 protons, which made researchers classify it as neutron-rich.

As such, it provides an amazing target for such studies. By studying the patterns in which beam electrons are deflected via the weak nuclear force, researchers can establish the distribution of protons within the nucleus (the proton radius).

But the PREX team was also able to measure the thickness of the neutron skin, by subtracting the proton radius from the neutron radius. This has never been done before, but the team is still not satisfied, saying that its results are not yet as precise as they could be.

The neutron radius of lead-208 was established to be around 5.78 millionth of a nanometer, while the neutron skin thickness was established to be around 0.33 millionths of a nanometer (with a degree of error of 50 percent). The study is detailed in the latest issue of the journal Physical Review Letter.