Threshold Photoneutrons and Photon Doorways

Abstract

It has long been recognized that the measurement of photonuclear cross sections with an energy resolution as fine as that obtained heretofore only for neutron cross sections would be a powerful probe for investigating the properties of nuclear states. This would bring to bear the electromagnetic selection rules as a spectroscopic tool, and open for investigation all the stable nuclei not accessible to neutron-induced reactions. This goal now has been accomplished, for the first MeV or so above the (γ,n) threshold, by the threshold photoneutron technique.¹ In this experiment, bremsstrahlung from a pulsed, nearly monoenergetic electron beam is directed at the sample under study. The neutrons ejected in the (γ,n) reaction are detected and their energy is measured by the neutron time-of-flight technique. The energy of the electron beam is adjusted so that the tip of the bremsstrahlung spectrum barely exceeds the (γ,n) threshold of the sample. Then neutron transitions from levels of the compound nucleus to excited states of the residual nucleus cannot take place — only ground state transitions are energetically possible. Thus a measure of the neutron energy uniquely determines the energy of the photon which induced the reaction.