The LEGS Facility

The Laser Electron Gamma Source (LEGS) has been producing tagged polarized gamma rays since 1990. Up until the fall of 1994, energies have been limited to 330 MeV. Recent upgrades, in both the electron ring and in the laser will increase the maximum gamma ray energy to 470 MeV. To date, 13 experiments have been completed. These have involved collaborations between US groups from Duke, Ohio University, North Carolina State, Rensselaer Polytech, South Carolina, Virginia, and Virginia Polytech. There has also been significant European involvement by groups from Catania, Frascati, Genova, Gießen, Grenoble, ISS-Rome, Pisa, And Rome-II/Tor-Vergata. Capitalizing on the high degree of beam polarization, experiments have produced the first definitive observations of electric quadrupole strength in the N transition (a signature of nucleon deformation), asymmetries have provided precise constraints on the short range part of the N-N tensor interaction, and exclusive measurements have made critical tests of 3-body currents in a calculable nuclear many-body system.

The future experimental program at LEGS will focus on beam-target double- polarization measurements on the nucleon. These will remove ambiguities in the N helicity structure, and directly measure the dominant contributions to the Spin-Polarizability () and Drell- Hearn-Gerasimov (DHG) sum rules. At present, there is a significant incompatibility between PT calculations of , the evaluation of the conventional DHG sum rule in terms of magnetic moments, and the predictions for these quantities using recent photo- production multipole analyses. To carry out this program, a novel frozen-spin polarized target, SPHICE (Strongly Polarized Hydrogen ICE), consisting of molecular HD in the solid phase, is being developed. With 80%H- polarization and 50%D-polarization, the target concentration (the fraction of target nucleons that are polarized) is dramatically enhanced over conventional ammonia or butanol targets. Since quality factors for double-polarization measurements scale with the squares of polarizations and target concentration, the LEGS+SPHICE combination will result in an order of magnitude improvement over bremsstrahlung beams and conventional polarized proton targets, and more than two orders of magnitude enhancement for polarized neutron measurements. Moreover, since the key physics issues with the least model dependence are in the proton-neutron difference, the ability to make simultaneous measurements on both species will lead to significant reductions in systematic uncertainties. The development of the SPHICE target will result in a new technology that will be applicable to other laboratories. Large solid angle coverage for reaction products is crucial in double-polarization experiments, and an 80%4 detector for neutrals and charged hadrons, SASY – the Spin ASYmmetry array, is being constructed for these measurements. The collaboration working on these projects consists of 45 physicists from 14 institutions and 4 countries. Costs are expected to be shared between DOE(50%), NSF (23%), and European agencies (27%). Construction began in FY 1994, and a staged development is planned through FY 1998, consistent with a modest but steady schedule of capital funding from DOE. A proposal to NSF is currently being reviewed, and funding commitments from European collaborators are already in place.