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

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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