Measurements of Decay Radioactivity of Long-Lived Isotopes

Measurements of long-lived radioactivity are required to generate reliable data-base for qualifying fusion materials for reactor applications. However, long half lives necessitate intense 14 MeV neutron source, long irradiation time, long cooling time, and long counting time under low background. A 32h12m long irradiation, at mean source neutron intensity of 1.13 × 10¹² n/s, was carried out on a number of foil packages kept near rotating neutron taiget source at FNS under USDOE/JAERI collaborative program in June 1989. Four identical foil packages were kept at 0, 45, 90, and ∼115 degrees to the d⁺ beam. Each package contained foils of Ag, Al, Dy, Hf, ¹⁵¹Eu, ¹⁵³Eu, Hf, Ho, Ir, Mo, Re, Tb, and W. The objective was to measure decay γ radioactivity from ^108mAg, ²⁶Al, ¹⁵⁸Tb, ¹⁵²Eu, ¹⁵⁰Eu, ⁹⁴Nb, ¹⁸⁶mRe, ^178m2Hf, ^192mIr, and ^166mHo, among others. The half lives of these products range from 13.3y (¹⁵²Eu) to 0.72My (²⁶Al). These foils were interspersed with dosimetric foils of Nb and Zr. An estimated average fluence of ∼0.83 × 10¹⁵ n/cm² (range: 0.47–1.65.10¹⁵ n/cm²) was obtained for the foil-package at zero degree. After cooling times ranging from 1.3 to 2 years, γ-spectroscopy of some of these foils has been completed. Analysis of measurements, done on foil package at zero degree, has been carried out using four radioactivity codes, REAC-2, DKRICF, ACT4 (THIDA-2), and RACC. REAC-2 is the only code that has data for most of the observed products; RACC has data for Al, Mo and W products only. Ratio of computed to experimentally measured activities varies from 4.10⁻⁵ to 377. Major update of all four cross-section libraries is recommended as waste classification of many fusion specific materials is likely to change dramatically.