Measurement of activation resonance integrals at infinite dilution with a lithium-drifted germanium gamma-ray spectrometer and comparison with calculated values

Activation resonance integrals are cross-sections which can be used to confirm the accuracy of microscopic neutron cross-sections and neutron resonance parameters in the evaluation of reactor-neutron cross-sections. In the past the accuracy of neutron cross-sections has been frequently limited by the poor resolution of the measuring instruments used and the consequent inability to separate completely the desired activity from other activities in the sample. The high resolution attainable with a lithium-drifted germanium gamma-ray spectrometer makes it possible by activation analysis of the samples to select an energy region which is completely free from contaminants and furthermore to include the standard in the sample so as to avoid, by simultaneous measurement of photo-peaks relating to the standard and the unknown, the need for weight calibrations and corrections due to slight differences in irradiation position and counter geometry. The flux in the RA-1 reactor where the irradiations were made was sufficiently large (2 x 1012 n/cm2 s) to permit the use of samples with negligible self-shielding correction. The reactor spectrum at the irradiation site was calculated by means of multi-group diffusion theory and a correction factor allowing for the slight deviation from the 1/E spectrum was applied to the resonance integral. In consequence the systematic errors which have been common in the majority of previous resonance integral determinations, such as insufficient discrimination of spurious activities, self-shielding of epithermal neutrons and uncertain or incomplete description of the thermal spectrum at the irradiation site have been carefully avoided in the present method. This technique has been used to obtain an accurate determination of the ratio of the activation resonance integral to the thermal cross-section either by measurement of the cadmium ratio or by comparing the ratios in a reactor spectrum and in a quasi-thermal spectrum in respect of 28 isotopes (of mass number less than 220). Where serious doubts exist on the accuracy of the published values for thermal activation cross-sections, an absolute determination of the activation resonance integral was performed. Up-to-date neutron resonance parameters have also been used to calculate the resonance integrals for 23 isotopes. From the final results presented a better agreement is observed with the calculated values than in previous experimental determinations. Serious deviations from previous determinations are discussed in the light of present data on neutron resonance parameters. (author)