The Yield of Short-Lived Gamma-Ray Emitting Nuclides from Fast- and Thermal-Neutron Fission

Yields have been determined for the short-lived gamma-ray emitting nuclides ⁹⁵Zr, ⁹⁷Zr, ¹⁰³Ru, ¹³¹I, and ¹⁴⁰Ba, produced in fission of ²³⁵U and ²³⁹Pu by both thermal and fast neutrons and in fission of ²³⁸U by fast neutrons. The dependence of the yields on neutron energy has been evaluated. The irradiations were carried out in an Argonne National Laboratory fast critical assembly and a thermal-neutron source reactor. The number of fission-product atoms produced in thin metal foils was established by direct gamma-ray spectrometric assays on the foils. In the fast-neutron studies, the number of fissions in the foils was derived from solid-state fission-track recorders. These recorders contained nanogram amounts of the fissionable materials and were irradiated in close proximity to the foils. In the thermal-neutron studies, the number of fissions was obtained from the ¹³⁷Cs content and the ¹³⁷Cs fractional fission yield. For certain fission products, e.g., ⁹⁵Zr, the yields are in close agreement with those of the stable daughters, whereas for others, e.g., ¹⁴⁰Ba, the yields are significantly lower. In the neutron energy range from thermal to 450 keV, relative changes of <1% are observed in the yields of all ²³⁹Pu fission products and of the major ones for ²³⁵U. The yields of ¹⁰³Ru and ¹³¹I from ²³⁵u increase significantly in this energy range and appear to have a logarithmic neutron-energy dependence.