A Method for Absolute Determination of ²³⁸U Capture Rates in Fast Zero-Power Reactors

A method for the measurement of absolute ²³⁸U capture rates is described. The principle is based upon the fact that ²⁴³Am and ²³⁹U decay via ²³⁹Np into ²³⁹Pu. The α disintegration rate of ²⁴³Am is determined absolutely by low geometry α-particle counting. From this, the absolute ²³⁹Np disintegration rate follows directly, which is measured by the 106-keV γ-x-ray coincidence technique. Therefore, the explicit knowledge of detector efficiency and decay-scheme data is not necessary, and also radiochemical separation is avoided. The absolute ²³⁸U capture rate can then be obtained from the known ²³⁹Np activity. The γ-x-ray self-attenuation within thick uranium foils is treated quantitatively and an optimum foil thickness is derived. Special attention has been given to the reduction and correction of background due to fission-product γ activity. A detailed error analysis shows an overall accuracy of about 1% for the absolute ²³⁹Np disintegration rate including γ attenuation effects within the foils. To ascertain the reliability of low-geometry α calibration, comparison was made with the α-γ coincidence method. Agreement was found within the error limits of 1%.