Measurement of k 0 values for caesium and iridium

Abstract

The OPAL research reactor in Australia has been used to determine k 0 values for 134mCs, 134Cs, 192Ir and 194Ir. Values for 24Na have also been measured for quality control. The neutron flux at the irradiation positions was very highly thermalised (f > 2,000), resulting in almost negligible activation by epithermal neutrons. As a consequence, the contribution to the total uncertainty of the k 0 values from epithermal-related factors such as Q 0 and \( \bar{E}_{\text{r}} \) was very small. The measured caesium k 0 values have been compared with the library values as well as with recent measurements by St Pierre et al. and Farina Arbocco et al. While there are k 0 values for 194Ir in the library, no 192Ir values have been measured previously. Despite 192Ir having a higher sensitivity than 194Ir, k 0 values were not measured during the establishment of the k 0-method because the nuclear data available at the time indicated that the activation cross-section of 191Ir deviated significantly from 1/v behaviour (g(T n ) ≠ 1), which would result in unacceptable errors if k 0 analysis were to be carried out using the Hogdahl convention. However later nuclear data compilations showed that 191Ir has better 1/v behaviour than previously reported, making it suitable for k 0 analysis using the Hogdahl convention. For completeness, k 0 values have been determined using both the Hogdahl and modified-Westcott conventions and these have been compared with library (194Ir) and calculated values.