MEASUREMENT OF THERMAL-NEUTRON CROSS-SECTION AND RESONANCE INTEGRAL OF THE REACTION CS-137(N,GAMMA) CS-138

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 (Cs(n, gamma)Cs)Cs-137-Cs-138 have been measured by means of a modified Cd-ratio technique. Targets of about 0.4 MBq Cs-137 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/Al and Au/Al) whose sensitivity to epithermal neutrons differed from each other. After irradiation, the Cs-137 samples were purified chemically and their gamma-ray spectra were measured with a high purity Ge detector. The resulting yields of Cs-138 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 (sigma = 0.250 +/- 0.013 b) of our previous measurement. An uncertainty due to neglecting the production of the short-lived isomer of Cs-138 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.