Preparation of calibrated reference sources containing "fresh" fission products

Summary

NPL was recently asked to provide reference sources containing &ggr;-ray emitting fission product radionuclides, with certification completed within 50 days of the irradiation. Furthermore, the reference sources were to contain fission products, but no fissile material. The samples in the first reference source were to contain a mixture of fission products with only the ²³⁵U removed, leaving the fission products as 'undisturbed' as possible. A number of radionuclides were reported in this sample and included: ⁹¹Y, ⁹⁵Zr, ^95mNb, ^95gNb, ⁹⁹Mo, ^99mTc, ¹⁰³Ru, ¹⁰⁶Ru, ¹²⁷Sb, ^129mTe, ^129gTe, ¹³¹I,¹³²Te, ¹³²I, ¹³⁷Cs, ¹⁴⁰Ba, ¹⁴⁰La, ¹⁴¹Ce, ¹⁴⁴Ce, and¹⁴⁷Nd. Of these nuclides, NPL provided certified values for ⁹¹Y, ⁹⁵Zr, ^95mNb, ^95gNb, ⁹⁹Mo, ^99mTc, ¹⁰³Ru, ¹⁰⁶Ru, ¹³²Te, ¹³²I, ¹³⁷Cs, ¹⁴⁰Ba, ¹⁴⁰La, ¹⁴¹Ce, ¹⁴⁴Ce, and ¹⁴⁷Nd. The second reference source focused on a smaller subset of radionulides: ⁹⁵Zr, ⁹⁹Mo, ¹⁰³Ru, ¹⁰⁶Ru, ¹⁴⁰Ba, and ¹⁵⁵Eu, although ^95gNb, ^99mTc, ¹³²I and ¹⁴⁰La were present as daughter radionuclides and ⁹¹Y, ¹²⁷Sb, ^129mTe, ^129gTe, ¹³¹I, ¹³²Te, ¹³⁷Cs, ¹⁵²Eu and ¹⁵⁴Eu were present as impurities. This paper describes the preparation of these sources, based on a combination of cation- and anion-exchange chromatography with selective precipitation. The separation techniques were used to (1) produce the 'fresh' fission product mixture from irradiated ²³⁵U, and (2) isolate the specific set of radionuclides in the second exercise from mixtures containing irradiated ²³⁵U and fission products. To enable accurate assay of parent-daughter systems, integrated ingrowth and decay equations were derived from original Bateman equations and tested by observing the ¹⁴⁰Ba-¹⁴⁰La system over time.