A Method for the Accurate Determination of Relative Initial Conversion Ratios

An experimental method of determining “initial conversion ratios,” the number of Pu²³⁹ atoms produced for each U²³⁵ atom destroyed, is described. The measurements are made relative to the conversion ratio that would be obtained for thin uranium in a thermal flux. The precision is about ± %. The relative neptunium and fission product activities induced in a representative cross section of the fuel material (a thin foil of natural uranium) and in a foil in a thermal flux are compared. The neptunium is counted by a coincidence method which suppresses the counting rate due to fission products and natural background 10 times relative to the neptunium counting rate.