Epithermal irradiation

With the neutron activation procedure, the measurement of P is reliable only when there is very little Al (<20 ppm), or when an epithermal irradiation (Cd-covered) is used to correct for the direct activation of 28Al from the Al. For badly contaminated samples, the X-ray fluorescence data are more suitable for the P determination. The sequential irradiation procedures (with and without cadmium) showed that many of the mummy samples were contaminated to some extent and contained more than 20 ppm of Al. 

INAA has been used for routine determination of about 30-48 elements in coal., soils, fly ash with excellent accuracy and precision proved by analysis of NIST and IAEA standard reference materials (Germani et al., 1980 Allan and Sansoni, 1990 Shtangeeva, 1994). Combination of pile and epithermal irradiations inereases the precision and accuracy and detection limits for a number of elements (Kostadinov and Djingova, 1980, 1981a). Detection limits are in the low mg/kg and xg/kg range and the precision is 1-8%. Analysis of soil EDTA and acetic acid extracts has been performed in (Bajo et al., 1989) and the content of 29 elements is determined with a precision of 4-10%. For some of the elements however (Tb, Ti and Zn) accuracy was not good (20-40%). 

The several polymeric metal carbonyls studied have led to some surprisingly high yields [e.g., Fe3(CO),2 and Ruj(CO)j2 in Table IV] but to no substantiated mechanisms. The 17% yield of Fe3(CO),2 in neutron-irradiated Fe(CO)j was interpreted as a reaction of Fe(CO)4 with the Fe(CO)5, but no further evidence is available. The study of Mn2(CO),o has been fruitful (44, 46). The insensitivity of the parent yield MnMn(CO),o to heat indicates that the molecule is formed by a reaction quite early in the sequence, perhaps epithermal. The discovery (46) of a species which reacts rapidly with I2 and exchanges with IMn(CO)5 led to the conclusion that the Mn(CO)5 radical is produced prominently (4.5%) by nuclear reactions in the solid decacarbonyl. The availability of this labeled Mn(CO)5 has made possible several interesting observations about the exchange properties of this radical in the solid (45) and in solution (42). 

In general, activation analysis relies on the use of standards that are irradiated under the same conditions and in the same position, and are also measured under the same conditions. Monoelement standards contain a known amount of one element. If they are applied to the evaluation of other elements the ratio of the cross sections o x/o s under the special conditions of irradiation and the ratio H /Hs of the relative abundances of the decay processes that are measured must be known (subscript x is for the sample and subscript s for the standard). Knowledge of the ratio o x/o s may cause problems, because the cross sections may vary drastically with the energy of the projectiles, for instance in the energy range of epithermal neutrons. These problems are not encountered with multielement standards that contain all the elements to be determined. However, the preparation of such multielement standards may be time-consuming. 

Another approach to the determination of Li in RPV steel was by neutron activation analysis (NAA) followed by radiochemical analysis for H. This was far more complex than the ICP-MS analysis as it involved the irradiation of the inactive steel specimens in the CONSORT reactor at Ascot and transport of the activated steel to the Springfields Laboratory for radiochemical analysis. Six inactive specimens of RPV steel from TRA were irradiated for 63 hours in a neutron flux comprising flux values of 960 x 10 n cm s 44 X 10 n cm s and 300 x 10 n cm s for thermal, epithermal and fast regions of the energy spectrum respectively. 

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. 

Samarium-153 has relatively high neutron capture cross-sections (with a thermal capture cross-section of 206 b and an epithermal capture cross-section of 3000 b), thus enabling production of high specific activity with minimal long lived radionuclidic impurities. The activities of various samarium targets obtained post-irradiation in PARR-I are given in Table 12.4. 

Instrumental Neutron Activation Analysis (INAA) was applied to the determination of the platinum metals as part of a study of their uptake, accumulation and toxicity in plants. Long irradiations are described for iridium, osmium and ruthenium determinations in specially grown plant materials. An interference in the determination of platinum in plant matrices by this method is reported also. Short irradiations, utilising thermal and epithermal fluxes are investigated for rhodium and palladium determinations, with further studies using cyclic activation analysis. 

INAA of rhodium and palladium by the epithermal cyclic activation method is very satisfactory. Osmium and iridium appear to be satisfactorily determined by the long irradiation method. However, the most interesting element, platinum, appears to suffer from an interference which is still under investigation. 

If a sample contains N nuclei of a particular stable isotope, the rate of formation of its (n,y) product nuclei is Nc in which is the neutron flux density of the thermal and epithermal neutron flux density in n-cm -s (O = Oth + epi) and a the (n.y) cross section (generally expressed in barns, one barn equalling 10 cm ) of the target nuclei in the neutron spectrum in which the sample is irradiated a = + lo epi/ ) ... 

An excellent review on the use of INAA in the analysis of aerosols can be found in (Allan and Sansoni, 1985, 1990). Over 25 elements are easily and reliably determined in different aerosol filters. The use of epithermal and thermal irradiations increases the analytical parameters of the analysis for some of the elements (Allan et al., 1984, 1985). Forty-one elements have been successfully determined in combustion effluents from power plants (James and Acevedo, 1993). 

However, irradiation of biological materials with thermal and epithermal neutrons also results in high background activities from Na, C1, and activation... 

Setup for a lateral BNCT brain Irradiation ushg the fission converter-based epithermal neutron beam at the Massachusetts Institute of Technology with a 12 cm diameter aperture. 

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. ... 

The assumption that the activation cross section varies with neutron energy as 1/v in the thermal neutron region is valid for most (n,y) reactions. The two reactions that deviate the most from the 1/v assumption are Lu(n,y) Lu (typically +0.4%/K) and Eu(n,y) Eu (typically —0.1%/K). The reaction rates for these two reactions, relative to a monitor reaction like Au(n,y) Au, will depend on the thermal neutron temperature in the irradiation channel used. A new set of equations, the Westcott formalism (Westcott 1955), was developed to account for these cases and used the Westcott g T ) factor, which is a measure of the variation of the effective thermal neutron activation cross-section relative to that of a 1/v reaction. In the modified Westcott formalism, the following differences are also included the Qo(a) value of the Hogdahl formalism is replaced by the So(< ) value, and the thermal to epithermal flux ratio, f, is replaced by the modified spectral index, r a) TJTo). To use this formalism with the kg method (De Corte et al. 1994), it is necessary to measure the neutron temperature, r , for each irradiation and a Lu temperature monitor should be irradiated. The Westcott formalism needs to be implemented only when analyzing for Lu and Eu. There are several other non-1/v nuclides Rh, In, Dy, Ir, and Ir, but for these the error... 

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