Uranium determination radionuclide activity measurement

The errors inherent in measuring the activity of polonium, uranium, and plutonium can be assessed by determining radionuclides in CRMs in international laboratory exercises and using the CRM produced by the International Atomic Energy Agency... 

In natural U, the radionuclides of the uranium family and the actinium family are present, and sometimes also radionuclides of the thorium family. Therefore, direct determination of U in ores without chemical separation is difficult, especially since the absorption of the radiation depends on the nature of the minerals. Generally, the samples are dissolved and Th is separated, e.g. by coprecipitation or by extraction with thenoyltrifluoroacetone (TTA). Radioactive equilibrium between " Th and the daughter nuclide 234mp jg rather quickly attained, and the high-energy yS" radiation of the latter can easily be measured. A prerequisite of the determination of U by measuring the activity of either " Th or 234mp jg establishment of radioactive equilibrium. This means that the uranium compound must not have been treated chemically for about 8 months. 

The methods range from simple, inexpensive absorption spectroscopy to sophisticated tunable-laser-excited fluorescence and ionization spectroscopies. AAS has been used routinely for uranium and thorium determinations (see for example Pollard et al., 1986). The technique is based on the measurement of absorption of light by the sample. The incident light is normally the emission spectrum of the element of interest, generated in a hollow-cathode lamp. For isotopes with a shorter half life than and Th, this requires construction of a hollow-cathode lamp with significant quantities of radioactive material. Measurement of technetium has been demonstrated in this way by Pollard et al. (1986). Lawrenz and Niemax (1989) have demonstrated that tunable lasers can be used to replace hollow-cathode lamps. This avoids the safety problems involved in the construction and use of active hollow-cathode lamps. Tunable semiconductor lasers were used as these are low-cost devices. They do not, however, provide complete coverage of the spectral range useful for AAS and the method has, so far, only been demonstrated for a few elements, none of which were radionuclides. 

When an element has more than one radioisotope, determinations and data analysis are generally more complex because the isotopes may differ in half-life, especially when a series is involved, e.g., radium, thorium, polonium, radon, actinium, protactinium, and uranium. One possibility is to make measurements after the decay of the short-lived radionuclides, but this may require long waiting times. In favorable cases, it is more convenient to measure the activity of decay products (e.g., radon, thoron ( Rn), actinon ( Rn)), or correct the measurements of the short-lived radioisotopes after determination of the isotopic composition. 

The uranium concentration in hair samples of Brazilian residents (18 young females and 4 males) was determined by ENAA (Akamine et al. 2007). Hair samples were collected from the occipital part of the head, cut into 2 mm sections, rinsed to remove external contamination, and placed on filter paper for drying. The hair samples, and samples spiked with uranium standards, were placed in thin cadmium capsules and irradiated for 16 h. After 4 days, to allow for decay of interfering radionuclides, the gamma activity of the samples was measured for 50,000 s, and the uranium content was determined from the intensity of the 106 and 278 keV peaks of Np. Method validation was based on measur ent of CRM (NIST 1575—pine needles). The uranium concentration was 2.1-49.8 ngU g hair, with mean and median values of 15.4 and 10.7 ng g , respectively. 

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