QUASI-MONOCHROMATIC PHOTON-BEAM USED IN PHOTO-NEUTRON EXPERIMENTS FROM 20-120 MEV AT THE 600-MEV SACLAY LINAC

A beam of 20-130 MeV positrons, with average intensities between 10 nA and 50 nA, is used at the 600 MeV Saclay Linac to create a quasi-monochromatic photon beam with a continuously variable energy. This beam was used to measure photoneutron cross sections and the corresponding photonuclear facility is first described. The computer-controlled methods, implemented to measure the energy spectrum and the emittance of the positron beam are described. The quasi-monochromatic photon lines are produced by the annihilation in flight of monoenergetic positrons in two annihilation radiators with different Z successively. The photon beam emission angle θ is shown to be the most critical parameter in the search for an optimum overall signal to background ratio for a specific photoneutron experiment. The choice of an angle θ{reversed tilde equals}4° is explained for absolute measurements of σ(γ, xn) cross-sections, for which th used average intensities of monochromatic photons were thus purposely reduced to {reversed tilde equals}5 x 103 s-1., with an energy resolution {reversed tilde equals}12%. © 1979.