Measurements of thermal-neutron capture cross-section of Cesium-135 by applying mass spectrometry

The thermal-neutron capture cross-section (sigma(0)) and resonance integral (I-0) were measured for the Cs-135(n,gamma)Cs-136 reaction by an activation method and mass spectrometry. Because of difficulty in the preparation of pure Cs-135 samples, we used Cs-135 contained as an impurity in a normally available Cs-137 standard solution. An isotope ratio of Cs-135 and Cs-137 in a standard Cs-137 solution was measured by mass spectrometry to quantify Cs-135. Cesium-135 impurity along with the Cs-137 standard solution was irradiated at the hydraulic conveyer of the research reactor in the Institute for Integral Radiation and Nuclear Science, Kyoto University. Wires of Co/Al and Au/Al alloys were used as neutron monitors to measure thermal-neutron fluxes and epi-thermal Westcott's indices at an irradiation position. A gadolinium filter was used to measure the sigma(0), and a value of 0.133 eV was taken as the cut-off energy. Gamma-ray spectroscopy was used to measure induced activities of Cs-137, Cs-136 and monitor wires. On the basis of Westcott's convention, the sigma(0) and I-0 values were derived as 8.57 +/- 0.25 barn, and 45.3 +/- 3.2 barn, respectively. The value of sigma(0) obtained in the present study agreed within the limits of uncertainties with the past-reported value of 8.3 +/- 0.3 barn.