• Number 347  |
• October 3, 2011

Result tickler: Lead nucleus may bury positive side under neutral facade

A novel experiment performed last year at DOE's Jefferson Lab suggests that the nucleus of a lead atom buries its positive "personality" beneath a neutral exterior. The preliminary result is consistent with the idea that neutrons form a kind of "neutron skin" around the protons in the nucleus in heavy nuclei, such as lead.

The Lead (Pb) Radius Experiment (PREx) collaboration announced the preliminary result at a seminar at Jefferson Lab and at the American Physical Society's April Meeting in Anaheim. The result is important for the understanding of the structure of heavy nuclei and for the theoretical equations that describe the life cycles of neutron stars.

"What this first measurement implies is that neutrons occupy a larger volume than the protons in heavy nuclei," said Krishna Kumar, a professor at the University of Massachusetts, Amherst, and a spokesperson for the experiment.

Prior to this experiment, the best estimates of the neutron radius came from nuclear theory, where models were constrained primarily by data other than measurements of neutron radii. In a heavy nucleus, such as lead (208Pb), the fractional difference between the neutron radius and the proton radius was previously expected to be several percent.

PREx measured the neutron radius in a clean and model-independent way, providing the first quantitative evidence of the presence of the neutron skin. The PREx result suggests that neutrons form a neutron skin around the protons inside heavy nuclei, but only at the 95 percent confidence level.

"The PREx measurement provides a first independent check of basic nuclear theory," said Kent Paschke, an assistant professor at the University of Virginia and a spokesperson for the experiment.

Bob McKeown, Jefferson Lab deputy director of science and technology, agreed. "Although higher precision is desirable to further constrain nuclear theory, the PREx result is a substantial experimental statement – one that we could not state before."

In the experiment, a beam of polarized (spinning) electrons was sent into a sandwich-style, carbon-lead-carbon target. The carbon sandwich allowed experimenters to draw heat from the target, keeping it from melting from the heavy bombardment of the high-intensity electron beam. Lead has a low melting point.

Electrons in the beam could interact with nuclei in the target through one of two fundamental forces: the electromagnetic force or the weak force. Throughout the experimental run, the polarized electron beam was flipped from one polarization (electrons spinning in one direction) to its opposite. This polarization flip allowed experimenters to exploit differences in how electrons interact with target particles via the electromagnetic versus the weak force.

The electromagnetic force is mirror-symmetric, whereas the weak force is not. Therefore, electrons that interacted with the target through the electromagnetic force did so regardless of the polarization. However, electrons that interacted with the target through the weak force preferentially interacted when the beam polarization was in one direction versus the other. Further, and even more importantly, the weak force preferentially interacts with neutrons.

By probing the difference in the number of electrons that interacted with the lead target in one polarization state versus the other, experimenters were able to tease out a measurement of the neutron radius in lead.

"The techniques that we used capitalized on two unique capabilities only available at Jefferson Lab: high-intensity, clean electron beam and the ability to flip the electron beam's polarization," said Robert Michaels, a spokesperson on the experiment and interim leader of Jefferson Lab's Hall A. "No one has done a parity-violation experiment on a heavy nucleus before using a clean probe, such as electron scattering. Jefferson Lab's unique capabilities made this experiment possible."

The ultimate goal of PREx was a measurement of the neutron radius to 1 percent accuracy. According to Kumar, this first run proved quite challenging.

"The experiment encountered several technical issues. Therefore, this result didn't achieve the full statistics due to the time required to solve these technical issues," Kumar said. "We now know how to make a more precise measurement than we have made so far."

During the first run, the experimenters ran into trouble stemming from damaged equipment due to the high-intensity electron beam and the thick target used, including a malfunctioning vacuum system, a cracked O-ring and other problems.

"For instance, due to the complexity of the downstream beamline, we originally used a soft O-ring near the target. The O-ring was eventually damaged by the radiation. So, to solve that technical issue, we plan to change out the O-ring for a metal seal," Kumar explained.

Michaels said the collaboration will present a new run plan to the Program Advisory Committee, a panel of scientists who prioritize proposed experiments for Jefferson Lab's CEBAF accelerator and experimental halls.

"We have now demonstrated the capability to perform this experiment. We now have all of the technical issues under control and can perform this experiment at a much higher precision. We plan to tweak the design of the experiment and apply for another run," Michaels said.

Kumar agreed. "Through our experience over the course of the first experiment run, we can demonstrate to the Program Advisory Committee that we have all of these problems under control and that we can run with the efficiency that was originally projected."

More than 100 researchers from more than 30 institutions worked on the experiment, which was installed, commissioned and conducted March-June 2010.

This work was supported in part by the Department of Energy's Office of Science, the National Science Foundation and the Istituto Nazionale di Fisica Nucleare.

[Kandice Carter, 757.269.7263,
kcarter@jlab.org]