Measurement of Thermal Neutron Cross Section and Resonance Integral of the Reaction 137Cs(n,.GAMMA.) 138Cs.

概要

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To obtain fundamental data for the research of the transmutation of nuclear waste, the thermal neutron cross section and the resonance integral of the reaction 137Cs(n, γ)138Cs have been measured by means of a modified Cd-ratio technique. Targets of about 0.4 MBq 137Cs were irradiated with neutrons in the JRR-4 swimming pool reactor by using a pneumatic tube equipped with a movable Cd shield, and neutron fluxes and their epithermal neutron fractions were monitored with activation detectors (Co/AI and Au/AI) whose sensitivity to epithermal neutrons differed from each other. After irradiation, the 137Cs samples were purified chemically and their γ-ray spectra were measured with a high purity Ge detector. The resulting yields of 138Cs for different neutron spectra and the neutron flux data have yielded that the thermal neutron cross section (for 2, 200 m/s neutrons) is 0.25±0.02 b and the resonance integral 0.36± 0.07 b. The results are consistent with the effective cross section (σ=0.250±0.013 b) of our previous measurement. An uncertainty due to neglecting the production of the short-lived isomer of 138Cs was calculated on the basis of the isomeric yield ratio given by the Huizenga and Vandenbosch model. A probable uncertainty was estimated to be +1.2% or +3.9%, depending on the spin of the compound nucleus produced in the s-wave neutron capture.
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