PREX, the Lead Radius Experiment, will use parity violating electron scattering from a Pb-208 target to determine the radius of that nucleus’s neutron distribution to 1% accuracy. In heavy nuclei, the neutrons are expected to be distributed over a slightly larger radius than the protons, but experimental evidence for this neutron “skin” is not very good. We expect to nail it down decisively. While you might expect this to be of interest only to nuclear physicists, the neutron radius turns out to be an important parameter in the analysis of atomic parity violation experiments (which give us among the best tests of the Standard Model at low energy) and in the understanding of neutron stars. So atomic physicists, elementary particle physicists, and astrophysicists are enthusiastic about getting this measurement.
It’s not an easy experiment, though. The idea is to measure the relative difference in the scattering rate for electrons polarized parallel and antiparallel to their motion, and that quantity is expected to be about 500 parts per billion. To measure such a small asymmetry at the desired level of precision requires taking data for about 700 hours with the accelerator delivering 50 μA of electrons at an energy of 850 MeV. 50 μA isn’t a lot for Jefferson Lab, and 850 MeV is pretty low, but even so that’s about 40 watts of power being dumped into a piece of lead foil. And lead is kind of famous for having a low melting point. If the target isn’t designed correctly, JLab’s accelerator will become the world’s most expensive soldering iron, and a whole lot of people will be mad at us.
Meanwhile, to minimize our systematic errors, we need to measure the polarization of the electron beam to higher accuracy than ever; unfortunately, our main polarimeter’s performance gets rapidly worse at lower energies, and as I said, this experiment runs at a pretty low energy. It needs a new, higher energy laser, a new detector, and a new way of collecting the data.
We also need to monitor the beam position, angle, and energy, because changes in these can easily overwhelm the effect we’re looking for. For the small asymmetry we’re dealing with, we need to monitor them extremely well: position differences between the two electron beam states need to be measured to within 0.1 nanometers averaged over the run. That’s about the diameter of a hydrogen atom.
We think we can do all this, but it’s a challenge. At the end of this month we’ll be testing some of our equipment and procedures. Tomorrow I’ll be driving to Newport News to help get the test run ready, and I’ll return the first weekend of February.