Effective Resonance Integrals of Separated Tungsten Isotopes from Reactivity Measurements

Measurements of effective resonance integrals for separated tungsten isotopes enriched in ¹⁸²W, ¹⁸³W, ¹⁸⁴W, and ¹⁸⁶W, and for natural tungsten and gold were made over a range of sample size. The effective integrals were determined by using a small homogeneous reactor to measure epicadmium reactivities for samples located at the center of the core. For calibration of the reactivity data, and to extend the measurement for gold over a wide range of sample size, relative γ-ray activities of cadmium-covered thin samples of gold were also measured.

The experimental results were compared with effective resonance integrals calculated by the Nordheim integral method and using recently measured isotopic resonance parameters. Agreement between the calculations and experimental results for ¹⁸²W, ¹⁸³W, and ¹⁸⁴W were generally good. However, for the ¹⁸⁶W samples and for thick samples of natural tungsten, the calculations overestimated experimental effective integrals by 10 to 20% because of failure of the flat source approximation employed. For ¹⁸⁶W, the use of a radiation width of 0.044 ± 0.004 eV for the highly scattering 18.8-eV resonance brought calculations and experiment into better agreement. For natural tungsten, the experimental results indicated significant overlap effects for resonances of constituent isotopes in thick samples.