Role of isomeric state formation on the measurement of thermal neutron cross section and resonance integral

Abstract

The isomeric thermal neutron cross section and isomeric resonance integral of 109Ag(n, γ)110m Ag, 133Cs(n, γ)134m Cs and 134g Cs, and 136Ba(n, γ)137m Ba reactions were investigated together with 115In(n, γ)116m In monitor reaction. These residual nuclei have broad half-life time scale suitable for our investigation. Moderated neutrons from steady AmBe source were used for activation of samples with natural abundances. Field was monitored and mapped using gold and indium activation. The formulae used for neutron activation analysis were derived with emphasizing the different interpenetrations of cadmium transmission factor and self-shielding factors. The k 0-factor for γ-rays in the residual nuclei were measured. The isomeric thermal neutron cross section and resonance integral for 115In(n, γ)116m In were evaluated to be 162.6 b and 2585 b. These data were used to measure the k 0-factors; and compared to reported values to confirm the procedure. The thermal neutron cross section and resonance integral for 133Cs(n, γ)134m Cs were found to be 2.64 ± 0.11 b and 42 ± 1.7 b, respectively; while those of the 109Ag(n, γ)110m Ag reaction were 4.09 ± 0.35 b and 68 ± 6 b. Thermal neutron cross section for 136Ba(n, γ)137m Ba was identified as 0.032 ± 0.003 b, while the resonance integral could not be evaluated as a result of interfering reactions. Model calculations were done using EMPIRE code to simulate isomeric ratio and compared with the experimental results. The feeding of isomeric state from neutron reaction is more sensitive to the variation of neutron flux distribution than the nucleus formation. Steady neutron field could be retained with isotopic neutron source with moderation setup and geometry of suitable homogeneity and isotropy.