A Method of Nondestructive Measurement of the Absolute Cumulative Barium-140 Yield of Uranium Irradiated in Thermal and Fast Neutron Spectra

The ¹⁴⁰Ba cumulative yield of irradiated uranium has been determined absolutely by a nondestructive method. Thin uranium foils are irradiated in the fast thermal zero-power reactor STARK. To avoid radiochemical separation, the 1596.5-keV gamma activity of ¹⁴⁰La, the daughter nucleus of ¹⁴⁰Ba, is measured with a high resolution (2.6 keV fwhm) Ge (Li) detector. From this ¹⁴⁰La activity the absolute ¹⁴⁰Ba yield is evaluated from the corresponding absolute fission rate and. the detector efficiency known for the 1596.5-keV photopeak. The uranium fission rate in the foil is normalized to the absolute fission rate measured with a calibrated parallel-plate chamber. Calibration of the gamma spectrometer is performed with activated thin La₂O₃ foils whose absolute activities are determined by 2π beta counting. The ¹⁴⁰Ba cumulative yield data obtained are 6.29 ± 0.14% for ²³⁵ U thermal neutron fission and 6.10 ± 0.15(%) and 6.34 ± 0.41(%) for fast neutron fission of ²³⁵U and ²³⁸U, respectively. They are in good agreement (within ±1%) with the values reported to date.