Nuclide standards

The counting time of the activated sample, like the irradiation time, is usually comparable to the half-life of the radionuclide measured, or as much as a day for small quantities of long-lived nuclides. Standards are counted under identical conditions of counting efficiency. 

The recoilless nuclear resonance absorption of y-radiation (Mossbauer effect) has been verified for more than 40 elements, but only some 15 of them are suitable for practical applications [33, 34]. The limiting factors are the lifetime and the energy of the nuclear excited state involved in the Mossbauer transition. The lifetime determines the spectral line width, which should not exceed the hyperfine interaction energies to be observed. The transition energy of the y-quanta determines the recoil energy and thus the resonance effect [34]. 57Fe is by far the most suited and thus the most widely studied Mossbauer-active nuclide, and 57Fe Mossbauer spectroscopy has become a standard technique for the characterisation of SCO compounds of iron. 

In absorption Mossbauer spectroscopy, a source nuclide in a standard form (usually in a metallic matrix) is coupled with a sample to be investigated. This method requires at least 100 pg of Fe or Sn in the usual experimental setup even if a Mossbauer sensitive enriched stable isotope Fe-57 or Sn-119 is employed. In emission Mossbauer spectroscopy, however, 1 mCi of Co-57 or Sb-119, which corresponds nominally to 120 ng of Co-57 or 1.4 ng of Sb-119, is sufficient to permit measurement. This technique enables study of very dilute systems, especially those with ions directly bound to the substrate. 

Early studies of Mg isotope ratios in geological materials used the notation A Mg to mean per mil deviations from a standard as expressed in Equation (1) above, a convention that persists today (e.g., Elsu et al. 2000). The values assigned to A "Mg in those studies represent the level of mass-dependent isotopic fractionation relative to the standard. The same convention defined fi Mg as the per mil deviation from the standard after correction for the mass fractionation evidenced by A "Mg. In this system of nomenclature, A values refer to mass dependentfractionations while 5 values refer to deviations from mass-dependent fractionation (i.e., the S Mg defines excesses in Mg relative to mass fractionation attributable to decay of the extinct nuclide Al). In some cases A "Mg has been replaced by the symbol Fn,g (Kennedy et al. 1997) where the F refers to fractionation. ... 

The probability of inducing a nuclear transformation depends on the nuclide and the energy of the neutrons. These two factors are combined in a parameter called the effective cross-section. One of the simplest ways to bombard a sample with neutrons is to use an intense flux of 1014 to 1018 neutrons m 2 s-1, available in nuclear reactors of at least 100 kW. Sufficient activation levels can be obtained in a few minutes for low-concentration samples, even for elements with a long half-life. The process is, however, expensive and imposes constraints on the sample to be analysed. The latter must be thermally stable and preferentially in the solid form. The sample is placed in a tube with an external reference standard and introduced pneumatically into the nuclear reactor for irradiation. 

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