Production of the isomeric state of 138Cs in the thermal neutron capture reaction 137Cs(n, γ)138Cs

In order to obtain precise data of the neutron capture cross section of the reaction 137Cs(n, γ)138Cs, the production probability of isomer state 138mCs was measured in this work. Targets of about 0.37 MBq 137Cs were irradiated for 3 min in the pneumatic tube facility (Pn-3) of Kyoto University Reactor (KUR). The 1,436 ke V γ-ray emitted from both of 138gCs and 138mCs was measured. A ratio of the production probability between 138gCs and 138mCs was deduced from time dependence of peak counts of 1,436 keV γ-ray by making use of difference of half-lives of 138gCs (33.41 min) and 138mCs (2.91 min). The production probability of 138mCs was obtained as 0.75±0.18 and this value revised the effective cross section upwards by 9±2%. The effective cross section σand the thermal neutron capture cross section σ0 were obtained as σ=0.29±0.02 b and σ0=0.27±0.03 b with taking into account the production of 138mCs. (author)