NEWPORT NEWS – Nuclear physicists have made a new, highly accurate measurement of the thickness of the neutron “skin” that encompasses the lead nucleus in experiments conducted at the U.S.
The protons and neutrons that form the nucleus at the heart of every atom in the universe help determine each atom’s identity and properties.
“The question is about where the neutrons are in lead.
Measuring how these extra neutrons are distributed inside the nucleus is key input for understanding how heavy nuclei are put together.
The PREx experiment result, published in Physical Review Letters in 2012, provided the first experimental observation of this neutron skin using electron scattering techniques.
Neutrons are difficult to measure, because many of the sensitive probes that physicists use to measure subatomic particles rely on measuring the particles’ electric charge through the electromagnetic interaction, one of the four interactions in nature.
“Protons have an electric charge and can be mapped using the electromagnetic force.
In the experiment, a precisely controlled beam of electrons was sent crashing into a thin sheet of cryogenically cooled lead.
That means that the electrons that interacted via electromagnetism did so regardless of the electrons’ spin direction, while the electrons that interacted via the weak interaction preferentially did so more often when the spin was in one direction versus the other.
“Using this asymmetry in the scattering, we can identify the strength of the interaction, and that tells us the size of the volume occupied by neutrons.
The measurement required a high degree of precision to carry out successfully.
The electrons that had scattered off lead nuclei while leaving them intact were collected and analyzed.
We are finding what we call a stiff equation of state – higher than expected pressure so that it’s difficult to squeeze these neutrons into the nucleus.
The unexpectedly stiff equation of state implied by the PREx result has deep connections to recent observations of colliding neutron stars made by the Nobel Prize-winning Laser Interferometer Gravitational-Wave Observatory, or LIGO, experiment.
And as they get close in the last fraction of a second, the gravitational pull of one neutron star makes the other neutron star into a teardrop – it actually becomes oblong like an American football.
and PAE, manages and operates the Thomas Jefferson National Accelerator Facility, or Jefferson Lab, for the U.S.