Neutron resonance integrals

A maximum specific activity of approximately 850 GBq/mg was achieved when irradiation was carried out at a thermal neutron flux of 1 x 10 n-cni -s for 21 d, which corresponds to around 21% of the maximum achievable specific activity. Tlie specific activity of the Lu obtained was significantly higher than the theoretically calculated value under the irradiation conditions employed (7.9 at.%), accounting for only thermal neutron capture. This could perhaps be attributed to the contribution from epithermal neutrons (resonance integral 1087 b), which is not accounted for in theoretical calculations. 

Neutron Resonance Integrals Resonance integrals for neutron capture reactions in barns (10 " cm ) or millibarns (mb). Proton, alpha production and fission reactions are designated by R.I., R L, R.I.j, respectively. Separate values are listed for isomeric production. 

There are several approaches to the use of NAA for iodine determination in foodstuffs and other biological materials. Of the two nuclear reactions of stable the reaction of T(n, ) I with thermal and epithermal neutrons is almost exclusively used, because favorable nuclear parameters (high activation cross-section and resonance integral for thermal and epithermal neutrons, respectively, cf. Table 2.3) provide much lower detection limits compared... 

The thermal neutron cross section, (Jq, and the resonance integral, lo, are conveniently measured in barn (b) units ... 

Tabulated values of thermal cross sections, resonance integrals, fission-averaged cross sections, and 14 MeV neutron cross sections are available in the literature (Mughabghab et al. 1981 Mughabghab 1984 IAEA 1987 OECD 1994). Recommended data for the most important nuclides can be found in the Appendix of this volume. 

This equation, first written by Simonits et al. (1980), has the same structure as the equation used with the classical relative method, but contains additional correction factors because the analyte nuclide is different from the monitor nuclide. The subscript m refers to the monitor, /is the ratio of thermal flux to epithermal flux, and Qo is the ratio of the resonance integral to the thermal neutron cross section (see Eq. (30.9)). 

In Eq. (30.38), the term Qo(a)//in the factor (1 + Qo( )//) corrects for activation by epithermal neutrons. This factor is the heart of the ko method and was the key innovation that made the method possible. It was necessary to predict, with high accuracy, the relative reaction rates for two different (n,y) reactions in any reactor neutron spectrum. For each reaction, the Qo value, the ratio of the resonance integral to the thermal neutron activation cross section, and / the ratio of thermal flux to epithermal flux for the irradiation channel used, is needed. ... 

Thermal Neutron Radiative Capture Cross Sections, g-Factors, and Resonance Integrals... 

O Table 36.2 gives thermal neutron capture cross sections om Mughabghab etal. (1981, 1984), and (2003). Atomic abundances are taken from tbe latest lUPAC compilation (Rosman and Taylor 1998). Tbe Westcott g-factors, wbicb are a measure of tbe deviation of tbe cross section from tbe regular 1/w-bebavior, were calculated by Trkov (2003) in tbe framework of a coordinated research project (IAEA 2003). Resonance integrals are from Mugbabgbab (2003), except for those in square brackets that are from the earlier compilation (Mughabghab et al. 1981,1984) but have been omitted in the later reference. The notations in Table 36.2 are the following ... 

It is obvious that the exact values of r and depend on the position of the sample within a given reactor. If the sample is present in substantial amounts, then the resonance integral may need to be corrected for self-absorption of neutrons at the resonance energies. 

In contrast, In/Al alloy wire can be manufactured relatively simply employing enriched In and the wire can be activated to contain the desired level of radioactivity in a moderate-size nuclear reactor within a rather short irradiation time. The thermal neutron cross section and epithermal resonance integral for In(n,y) In are 8.1 b and 220 b, respectively. The expected yield of "In for 1 week of irradiation at a neutron flux of 1 x 10 n cm s is 6.7 GBq mg ( 180 mCi mg ) of In. A typical seed wire, 2 cm in length, 0.03 cm in diameter, containing --- 0.12 mg of In (2% In), would contain 0.74 GBq ( 20 mCi) of In. Experimental results are summarized in O Table 38.5. As seen, theoretical calculations are within a factor of 2 of experimental results. 

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