L. Miceli

FIRST MEASUREMENT OF THE REACTION 3HE(GAMMA ,P)X WITH POLARIZED PHOTONS

The first measurement of the reaction He-3(gamma over arrow pointing right, p)X using linearly polarized photons is reported. Cross sections and beam-polarization asymmetries for theta(p)lab = 60-degrees - 100-degrees and E(gamma) - 195-304 MeV are compared with a microscopic calculation which includes one-, two-, and three-nucleon absorption mechanisms. One- and two-nucleon absorption alone fails to describe the data at low proton momenta. The inclusion of three-nucleon absorption significantly improves the comparison with the measured cross sections. However, some features of the asymmetry distributions are not explained.

Brute Force with a Gentle Touch: Vibration Isolation Techniques Used to Increase HD Target Polarization

The performance of statically polarized high‐field/low‐temperature targets is a strong function of the base temperature during polarization. At the Laser‐Electron Gamma Source (LEGS) facility, highly polarized Hydrogen Deuteride targets are created in a dilution refrigerator/15 tesla superconducting magnet system, and converted to a frozen spin state. This allows them to retain polarization when placed in a beam at a lower field (0.7 T) and higher temperature (1.3 K). An increase in temperature from the 0 T state to the 15 T state of the refrigerator suggested eddy currents were primarily responsible for heating of the cold finger. Vibration‐isolation techniques have been developed to reduce the level of eddy currents due to vibration inside the polarizing field. These techniques reduced the amplitude of vibration due to the pumping system by two orders of magnitude and lowered the cold finger temperature with field energized from ∼ 17 mK to ∼ 12 mK. The potential gain in polarization is substantial.

First beam-target double-polarization measurements using polarized HD at legs

A new polarized target using HD in the solid phase has been developed for studies of the nucleon spin structure at Q(2) = 0 using pion photo-production. In combination with the high quality LEGS photon beam and a large solid angle spectrometer this target allows practically background-free measurements on the proton and on the neutron. The first beam-target double-polarization data taken with this target are reported here.

Polarized Compton scattering from He-4 in the Delta region

Differential cross sections and beam asymmetries of Compton scattering from He-4 have been measured with linearly polarized photons in the energy range from 206 to 310 MeV . The quality of the results has the potential to provide strong constraints on the understanding of the reaction mechanism in the Delta resonance region. A phenomenological analysis of the experimental results has been performed fitting the data to a multipole expansion including dipole and quadrupole scatterings in the impulse approximation. Results indicate that quadrupole contributions should not be neglected to reproduce the general trend of the experimental results. Comparison with predictions from recent theoretical models shows that important discrepancies exist particularly at backward angles. The additional information carried by the incident photon spin increases the difficulty in achieving a comprehensive description of experimental data.

Recent Polarization Experiments and the GDH Sum Rule

Measurements of the spin‐difference cross sections entering the Gerasimov‐Drell‐Hearn (GDH) sum rule are reviewed. Results on the proton from Mainz and Bonn exceeded the GDH prediction by 22 μb, requiring as yet unmeasured canceling high‐energy components. Recent experiments with frozen‐spin HD at BNL reveal a different angular dependence for π0 production than what was assumed in Mainz analyses in lieu of direct measurements and integrate to a value that is 18 μb lower, suggesting a rapid convergence. Results for deuterium over limited energy ranges are consistent with large canceling contributions but differ from existing state of the art calculations.

Exclusive pion production from 16O using polarized photons

The (γ↘,π−p) reaction was measured for 16O at photon energies of 200 to 320 MeV, and angles ranging from about 20° to 160° for the proton and 32° to 135° for the pion. The photon asymmetry data are compared with calculations in a DWIA framework. The agreement between the present data and calculations is reasonable at quasifree kinematics, but disagree as the momentum transfer to the residual nucleus becomes large.

Status of the legs polarized HD target

A solid, polarized HD target has been developed for the measurement of double-polarization observables in the Delta resonance region. Since the use of the first target in-beam in November 2001, dramatic improvements have been made to the in-beam and transfer cryostats and the NMR circuit has been redesigned and is well understood.

Double-Polarization Experiments using Polarized HD at LEGS

A solid, polarized HD target has been developed for the measurement of double-polarization observables in the A resonance region. The focus of the experimental program is a comparison of pion photo-production amplitudes for the neutron and proton and the associated spin-dependent sum rules.

Status of Frozen‐spin Polarized HD Targets for Spin Experiments

The first experiments have been carried out with polarized HD targets at the Laser‐Electron‐Gamma‐Source (LEGS) facility. By holding targets at low temperature and high field (17 mK and 15 Tesla) in a Dilution Refrigerator (DF) for six weeks a frozen‐spin state was reached, with equilibrium polarizations for protons and deuterons of 70% and 17%, respectively. Multiple measurements of the relaxation times and multiple transfers of the targets reduced these values so that experimental runs were carried out with polarizations of 30% and 6%, respectively. The relaxation times for protons and deuterons were observed to be 13 days and 36 days, respectively, in the beam line cryostat at 1.3 K and 0.7 Tesla magnetic field. For the future runs significantly higher D polarizations are possible by transfer of spin from the proton to the deuteron using an rf forbidden adiabatic fast passage. Higher polarizations and longer relaxation times are expected from ongoing development.

First pion photoproduction double-polarization results with polarized (H)over-right-arrow(D)over-right-arrow at LEGS

A new polarized target using HD in the solid phase has been developed for studies of the nucleon spin structure at Q(2) = 0 using pion photo-production. In combination with the high quality LEGS photon beam and a large solid angle spectrometer this target allows practically background-free measurements on the proton and on the neutron. The first beam-target double-polarization data taken with this target are reported here.