A Measurement of the Neutron Asymmetry in d(g,n)p Near Threshold

Sawatzky, Bradley David, Department of Physics, University of Virginia
Norum, Blaine, Department of Physics, University of Virginia
Liyanage, Nilanga, Department of Physics, University of Virginia

This dissertation describes a measurement of the neutron asymmetries and normalized differential cross sections for the d ¢¡γ£ n¤ p reaction near breakup threshold. This measurement served as the commissioning experiment for a new detector package called the BLOWFISH which was developed to facilitate large solid angle, low-energy neutron measurements at the High Intensity Gamma Source (HI¡γS) located on the Duke University campus. The BLOWFISH is a segmented neutron detector consisting of 88 BC-505 liquid scintillator cells distributed on the surface of an imaginary 40.6 cm (16") radius sphere centered on the target. The present configuration covers roughly 1/4 of 4π with the cells split among 8 "arms" equally distributed in φ, 11 cells to an arm. The choice of scintillator allows pulse-shape discrimination to enhance neutral particle ID, and the detector frame is free to rotate about the beam-axis to aid in the reduction of systematic errors. The solid angle coverage of the BLOWFISH array combined with the excellent γ-ray energy resolution and flux available at HI¡γS to make the systematic and statistical quality of the present d ¥¡γ£ n¤ p data unparalleled in the near-threshold energy region. Analysis of the these data revealed a striking discrepancy with prevailing low energy models of the nucleon-nucleon (NN) interaction. The most prominent feature of the disagreement lies in an observed front-back (polar angle) asymmetry in the center of mass photo-neutron yield that is not represented in the theory (e.g., [Are91, Rup00]). The magnitude of this discrepancy systematically rises as the γ-ray energy falls toward threshold. An underlying mechanism that might explain this difference remains elusive, but an analogous discrepancy between a recent (2002) deuteron electro-disintegration measurement at low momentum transfer [von02] and the same NN model ([Are91]) suggests that existing theories may be missing some details in the near-threshold region.

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PHD (Doctor of Philosophy)
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