Determination of Eta by Comparison of for U²³³ and Pu²³⁹ with for U²³⁵ in a Flux Trap Critical Assembly

The values of eta for U²³³ and eta for Pu²³⁹ have been determined by a reactivity coefficient measurement. An aqueous solution of each isotope was introduced axially into a critical cylindrical annular flux trap reactor, and the resulting reactivity change was measured by period determinations. From these data the ratios and were obtained. Using recently measured values of eta for U²³⁵ and the absorption cross sections in this ratio, the thermal values of 2.317 ± 0.040 for eta of U²³³ and 2.032 ± 0.053 for eta of Pu²³⁹ are obtained. Correction to a neutron velocity of 2200 meters/sec by using the appropriate g-factor gives a value of 2.317 ± 0.040 for eta of U²³³ and 2.082 ± 0.054 for eta of Pu²³⁹.