Medium Energy (MEP) Group - Polarized 3He Target Experiement
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Experimental Setup:
This experiment is being planned as part of the High-Intensity Gamma Source (HIGS) program at the Duke Free Electron Laser Laboratory (DFELL) . The DFELL is a high-energy, high-intensity polarized gamma source. It uses accelerated electrons in a storage ring to generate photon intensities of 107 photons/sec at 15 MeV. These mostly monochromatic photons are generated by injecting the accelerated electrons in to a cavity where the photons are extracted from the electrons using a magnetic undulator and the the mirrors in the cavity help build up the energy of a selected wavelength of light.
The west storage ring of the DFELL
Because the Compton scattering cross-section for nucleon scattering grows as photon energy increases, an energy of >100 MeV is necessary to get rates high enough to complete the experiment in a reasonable amount of time. Fortunately, upgrades to the DFELL in the near future will make this possible.
The experiment uses a polarized 3He target as an effective neutron target. The target is designed to be 40 cm long with a number density of 2.7x1020 / cm3 and capable of a polarization of greater than 40%. This target is currently under construction and more can be read about it here .
The experiment will measure the asymmetry of circularly polarized photons on a polarized target. These photons are measured by two Neutral Meson Spectrometers (NMS) . The detectors consist of 60 CsI shower counters, with a set of tracking chambers and a bismuth germinate detector for triggering on photon events. To seperate photons scattered from a quasi-free neutrons from photons scattered from an intact 3He nucleus, the neutron is detected by a set of liquid scintillator neutron detectors. However, the asymmetry from elastic scattering from the 3He nucleus is also an important quantity of increasing theoretical interest.