The Lead Radius Experiment II

208Pb Radius Experiment II
(PREX-II)

The Motivation

In heavy nuclei, such as 208Pb, it is thought that the extra neutrons are found on the surface where they form a 'neutron skin'. The thickness of this skin provides important information about nuclear structure. The goal of this experiment is to procure an accurate measurement of the radius of neutrons, RN, in a 208Pb nucleus. By subtracting RP, the radius of protons, from RN, the thickness of the neutron skin can be deduced. RP is well known from electron scattering experiments, but because neutrons carry no electric charge, it is difficult to collect information about RN. PREX is designed to collect an accurate measurement of RN using parity violating electron scattering. Measurement of RN could provide insight into QCD, dynamics of neutron rich nuclei, and the structure of neutron stars.

Parity Violating Weak Interactions

In quantum electrodynamics, QED, when electrons are scattered off of nucleons the interaction is mediated by the photon. This process is used to determine the distribution of protons in the nucleus, and neutrons are mostly ignored because they have no electric charge. In this interaction parity is conserved. While neutrons have no electric charge, they do have a weak charge that is much greater than that of a proton. In order to observe the distribution of neutrons in a nucleus, the electron scattering interaction can be approached in terms of electroweak theory. In this case, the protons are mostly ignored. This electroweak interaction is mediated by the Z0 boson instead of the photon. While the photon is symmetric by construction, the Z0 boson prefers left-handed particles, and thus the cross sections for left-handed electrons differs from that of right handed electrons. This leads to parity violating asymmetry, APV, which can be measured by firing a polarized beam of electrons through a polarized electron target. RN can then be deduced from the measured APV.

RPI's Contribution

Our responsibility for PREX-II is to upgrade the Møller Polarimeter that is used to measure the polarization of the electron beam. This includes the redesign, simulation and implementation of the device. The Møler Polarimeter measures the beam polarization using electron-electron scattering (Møller Scattering), by measuring the asymmetry in the scattering of the polarized beam off of a polarized target. The Møller Polarimeter is being upgraded so it can be operated at higher energies.