Measurement of k₀ values for europium, lutetium and iridium at FRM II with a very well thermalized neutron spectrum

The k₀ values of 6 non-1/v nuclides (¹⁵²Eu, ¹⁵²mEu, ¹⁵⁴Eu, ¹⁷⁷Lu, ¹⁹²Ir and ¹⁹⁴Ir) were determined using the extended Hogdahl formalism at the research reactor FRM II with very high f values. Standards were irradiated in 4 channels at different local temperatures between 40 °C and 55 °C measured using temperature sensitive irreversible labels. A good agreement with the recommended values was found for ¹⁵²Eu, ¹⁵⁴Eu and ¹⁷⁷Lu using the original g(T_n) factors by Gryntakis, however, the k₀ values for ^152mEu in this work were 7% higher. New k₀ values were also determined using the g(T_n) factors by Van Sluijs. Differences up to 6% were found for Eu isotopes compared with the recommended values. The recommended k₀ values for ¹⁹²Ir and ¹⁹⁴Ir could be confirmed using g = 1. The theoretical k₀ values for ¹⁷⁷Lu were calculated using new nuclear data. They are up to 6% less than the recommended values. The present k₀ values determined in this work showed a similar trend. The influence of different g(T_n) factors on the determination of the k₀ values was investigated. (author)