The polarization of the gamma-ray beam plays a critical role in experimental
photonuclear research by probing angular momentum. For example, the 80Se(g,n)79Se differential reaction cross-section can be measured as a function of the azimuthal angle relative to the plane of polarization. This provides information about the electromagnetic multipolarities involved in the reaction. [1] Dynamic control over gamma beam polarization will open new opportunities in nuclear research, particularly by allowing relative asymmetries to be calculated without the uncertainty introduced by relative detector
efficiency. A gamma-ray beam with rotational linear poarization and high polarization purity (Plin ~ .99) has been demonstrated at the High Intensity Gamma-ray Source (HIGS) [2].
Without active tuning by an accelerator physicist, polarization quality is degraded due to decoupling of the free-electron laser (FEL) axis and the electron beam orbit. We are developing an active feedback system that is sensitive to the small centroid motions of the FEL optical axis. Preliminary measurements of the FEL response matrix have been conducted, showing good sensitivity to cavity mirror adjustments. Ongoing work will utilize
this feedback system to automatically sustain controllable gamma-ray polarization for nuclear physics experiments.
This research is supported in part by the U.S. Department of Energy under grant no. DE-FG02-97ER41033.
[1] Yates, S.A. et al., “Measurement of the 80Se(γ,n) reaction with linearly polarized γ rays” Physical Review C, 98, 054621 (2018).
[2] Yan, J. et al., “Precision control of gamma-ray polarization using a crossed helical undulator free-electron laser”, Nature Photonics, 13, 629-635 (2019).
L. Fortunato
