A Measurement of the Photon Asymmetry in d(gpol,n)p at Low Energies

Brad Sawatzky

University of Virginia
(for the HI$\vec{\gamma}$S GDH Collaboration)

Potential models, effective field theory calculations, and phenomenological R-Matrix calculations all predict a rapid fall off of the M1 cross section for the reaction $d(\vec{\gamma},n)p$ as the energy increases from threshold to 5 to 10 MeV. A detailed knowledge of the M1 component of this cross section is important since it determines the cross section at the energies relevant to Big-Bang nucleosynthesis as well as the dominant threshold contribution to the GDH sum-rule for the deuteron. Linearly polarized gammas from the High Intensity Gamma Source (HI$\vec{\gamma}$S) at the Duke Free Electron Laser Laboratory (DFELL) were incident upon a deuterated water target and the neutrons detected in the newly commissioned "Blowfish" detector. This detector consists of 88 liquid scintillator cells located on the surface of a sphere centered on the target in 8 uniformly spaced arms of equal azimuthal angle $\phi$. The 11 cells in each arm are distributed uniformly between polar scattering angles $\theta$ of 22.5$^\circ$ and 157.5$^\circ$. The broad coverage (25% of 4$\pi$ sr) of the detector and its ability to be rotated about the beam axis permit high statistical precision as well as accurate determination of systematic effects. Results from recent measurements will be presented and compared to recent theoretical predictions.