The quasi-monochromatic photon beam used in photoneutron experiments from 20–120 MeV at the 600 MeV Saclay Linac

Abstract

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⋍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 ⋍5 x 10 3 s −1 ., with an energy resolution ⋍12%.