Background reduction in FEL-generated γ-ray beam experiments using giant high-peak power pulses

A new method to eliminate beam-uncorrelated background in photonuclear experiments has been developed at the High Intensity Gamma-Ray Source (HIγS). Background suppression of over 3 orders-of-magnitude has been achieved by using a gain modulation technique to generate giant high-peak power (GHPP) pulses in the OK-4/ Duke storage ring FEL. The time structure of these laser pulses was tuned so that their duration was ∼100 μs, repeated every 50 ms. The associated FEL photons were backscattered from 780 MeV electrons to produce linearly polarized 40 MeV γ-rays having the same time structure as the FEL pulses. As an initial test of the technique, a nearly background-free measurement of the analyzing power in the Compton scattering reaction ¹⁶O(γ→,γ)¹⁶O was performed.