This report describes the construction, operation, and typical uses of Topsy, the versatile, remotely controlled, critical assembly machine at Pajarito. Section 1 covers the mechanical design of the machine, and the hydraulic and electrical operation of the various components. Section 2 describes how Topsy is used for investigating reacting metal assemblies. Procedures for establishing a delayed critical configuration and operation at delayed critical are illustrated for the oralloy-tuballoy system. Also included are brief descriptions of oralloy-nickel, plutonium-tuballoy, and low density and concentration assemblies that have been made on the machine.

Neutron detector traverses of the untamped Oy (Godiva) and the Tu- or Ni-tamped Oy (Topsy) metal critical assemblies have been obtained by counting the following types of radiation : 1) y-activity of U 235 and U238 fission products; 2) fission fragments of Np 237 in a spiral chamber; 3) and Beta -activity of Au and S. At a few positions within the assemblies, and for the U235 fission spectrum, cross section ratios of a number of pairs of fissionable isotopes were determined by means of a comparison fission chamber. At the center of Godiva, af(25)\af(28) = 6.2, and this ratio for the Topsy Oy-Tu assembly ranges from 6.8 at the center to 76 at a radius of 8-1/4”. Results of radiochemical analyses for fission and other reaction products, by Group J-11, are listed in an appendix.