A. Caracappa

Coherent π0 photo-production on 4He at intermediate energies with polarized photons

Abstract The reaction 4 He ( γ ,π 0 ) 4 He has been studied with the LEGS tagged, polarized gamma-ray beam. Data are presented at laboratory angles of 31°, 45°, 72.5°, 90°, 110° and 130° for incoming photon energies of 206, 219, 232, 245, 259, 274, 289, 306 and 322 MeV. The unpolarized cross sections are compared with the Δ-hole theoretical model results available in literature. The measured photon beam asymmetry is −1 as expected from conservation of angular momentum and parity.

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.

Target Polarization Measurements with a Crossed‐Coil NMR Polarimeter

We have performed a complete electronic circuit analysis of the crossed coil NMR polarimeter (CC‐meter), and from this analysis we have determined the optimum conditions for its operation. From this analysis, which is confirmed by NMR measurements on hydrogen, we conclude that the CC‐meter can be operated to give a very high signal‐to‐noise ratio (SNR) for thermal equilibrium polarization while also producing a highly linear response for fully polarized targets. In general, a well‐designed and properly constructed CC‐meter can provide a larger SNR at similar non‐linearity, compared to the more commonly used Q‐meter.

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.

Polarized Photon Scattering from 4He

We have measured, for the first time, the differential cross section of and the parameter of asymmetry for the reactions 4He ( \(\vec{\gamma }\), γ)4He with linearly polarized photon. The LEGS polarized and tagged gamma-ray beam of Brookhaven National Laboratory has been used in the energy region 180–310 MeV. Data are colleted for five laboratory angles 30°, 45°, 72.5°, 110° and 130°.

Precision HD Polarimetry with the LEGS Crossed‐Coil NMR Polarimeter

We have developed a precision NMR polarimeter to observe the proton and deuteron polarization of a frozen spin HD target used in photoproduction experiments at the LEGS facility. This polarimeter uses a pair of magnetically orthogonal (“crossed”) RF coils to reduce the signal to very small levels in the absence of polarization of the sample. This reduces the thermal noise in the total signal with polarization, which improves the statistical precision of the polarization determination. Care has been taken in the design of the instrument to reduce nonthermal sources of noise, with the result that a fractional polarization uncertainty of 4% can be maintained over the lifetime of the targets, which has been as long as 19 months. The non‐ideal response of the NMR circuit makes a negligible contribution to this uncertainty.

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.