The effective resonance integrals of 186W and 184W as contained in natural tungsten were obtained for sample thicknesses from 0.25 to about 10−7 cm. Thin-sample activations yielded dilute resonance integrals of 441 ± 22 b for 186W and 14.9 ± 1.6 b for 184W.

Since the 18.8-eV resonance of 186W constitutes 92% of the dilute resonance integral for this isotope, and since the neutron scattering width has been measured accurately, the measured dilute integral provides a radiative capture width of 0.041 ± 0.003 eV for this resonance. The scattering widths for the important resonances of 184W are also known accurately; an average capture width of 0.062 ± 0.007 eV for 184W is obtained from the measured dilute integral.

In the thick-sample region, values of effective resonance integrals for 186W computed by the Nordheim integral method are found to overestimate experimental values for 186W as contained in natural tungsten. This overestimate may be caused by overlap of resonances in 182W and 186W and by inadequacy of the flat source assumption in the calculation. A determination of 76.4 ± 0.7 days was made for the half-life of 185W.