MEASUREMENT OF RADIATIVE CAPTURE RESONANCE INTEGRALS IN A THERMAL REACTOR SPECTRUM, AND THE THERMAL CROSS SECTION OF Pu-240

Abstract

An apparatus is described which detects γ-rays emitted by a thin target placed in a well-denned neutron beam. It has been used to determine the Cd ratios of Au and Pu-240, from which the ratio of the resonance integral to the 2200 m/s cross section for radiative neutron capture in Pu-240 has been deduced, using Au as a reference standard. Using this ratio and previously measured values of the resonance integral of Pu-240 and its effective cross section in two positions in the NRX reactor, three separate estimates of the 2200 m/s cross section of Pu-240 have been made. The mean value is 270 ± 17 barns.In an auxiliary experiment to indicate the shape of the epithermal spectrum of the neutron beam, the activation Cd ratios of Mn and In were compared with that of Au. These results, combined with the known 2200 m/s capture cross sections of these nuclides, yield new values of the radiative capture resonance integrals for both Mn and In.