Nuclear Elastic Scattering of Monoenergetic Neutron-Capture Gamma Rays

Abstract

A source of monoenergetic gamma rays produced by neutron capture in various elements is described. A search was made for elastic scattering of these neutron-capture gamma rays from Ta, Hg, Pb, and Bi. A large scattered signal was observed when 7.285-MeV gamma rays produced by neutron capture in iron were incident upon a lead target. Evidence is presented which indicates that this signal results from resonance scattering of these gamma rays by a level in ${\mathrm{Pb}}^{208}$ with an angular momentum quantum number of unity, and with a ground-state radiative width which is less than 4 eV. An argument is presented which indicates that it is probable that the spacing near 7 MeV in ${\mathrm{Pb}}^{208}$ of levels of the type observed is of the order of magnitude of 1400 eV.