Neutron cross sections of the elements

Sears (1992). Neutron News, 3, Neutron scattering lengths and 

Dianoux G. Lander, (Ed.) (2001). Neutron Data Booklet, Institut Laue-Langevin, Grenoble. 

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Hughes, D. J., and J. A. Harvey Neutron Cross Sections. Brookhaven National Laboratory Report BNL 325- This is a collection of total and reaction cross sections presented graphically as a function of energy. It extends and brings up to date the earlier report by Adair Neutron Cross Sections of the Elements. Rev. Mod. Phys. 22, 249 (1950). This latter report has a very clear introduction in which neutron scattering theory is summarised. 

Sensitivity to trace elements. The sensitivity obtained by activation analysis is a function of the neutron cross-section of the element in question, available neutron flux, length of irradiation, resolution of the detector, matrix composition, and the total sample size. Hence, increasing neutron fluxes, increased irradiation times, and the major advances in nuclear detector technology in the areas of increased efficiency and resolution have pushed the detection limits of most elements of interest to the very low levels. 

Laser stimulation of a silver surface results in a reflected signal over a million times stronger than that of other metals. Called laser-enhanced Raman spectroscopy, this procedure is useful in catalysis. The large neutron cross section of silver (see Fig. 2), makes this element useful as a thermal neutron flux monitor for reactor surveillance programs (see Nuclearreactors). 

With regurd to nuclear engineering, the sepamtion of zirconium and hafnium h.is been of considerable interest because of (he low neutron crass section of zirconium and the high neutron cross section of hafnium. Unfortunately, the separation of these two elements is perhaps the most difficult of any pair of elements. Explain why. 

The uranium and thorium ore concentrates received by fuel fabrication plants still contain a variety of impurities, some of which may be quite effective neutron absorbers. Such impurities must be almost completely removed if they are not seriously to impair reactor performance. The thermal neutron capture cross sections of the more important contaminants, along with some typical maximum concentrations acceptable for fuel fabrication, are given in Table 9. The removal of these unwanted elements may be effected either by precipitation and fractional crystallization methods, or by solvent extraction. The former methods have been historically important but have now been superseded by solvent extraction with TBP. The thorium or uranium salts so produced are then of sufficient purity to be accepted for fuel preparation or uranium enrichment. Solvent extraction by TBP also forms the basis of the Purex process for separating uranium and plutonium, and the Thorex process for separating uranium and thorium, in irradiated fuels. These processes and the principles of solvent extraction are described in more detail in Section 65.2.4, but the chemistry of U022+ and Th4+ extraction by TBP is considered here. 

Before samples can be analysed by INAA, they must be submitted to neutron irradiation. The interaction between neutrons and different elements in the sample, makes some of them become y-active and so measurable by a y-spectrometer. It depends on what kind of heavy metals are present in the sample, on the neutron flux and the neutron flux composition and on the cross-section of the relative n-y nuclear reactions. The neutron irradiation of the quartz microfibre filters is performed at the TRIGA MARK II nuclear research reactor at the ENEA... 

Radiation scattering by an assembly of centres is also characterized by another effect related to time fluctuations concerning either the sample or the incident radiation. In fact, in the course of time, the total spin state of the neutron-nuclei system may change, and the same remark applies to the orientations of the anisotropic polarizable elements. Thus, the cross-section of the assembly of scattering centres is averaged over a period of time. Two contributions appear. The first one, which is called coherent, reveals interference effects between scattered rays. The second one, which is called incoherent, is the sum of the cross-section of the various centres (considered as isolated). 

Tc is available through the /l -decay of Mo (Fig. 2.1.B), which can be obtained by irradiation of natural molybdenum or enriched Mo with thermal neutrons in a nuclear reactor. The cross section of the reaction Mo(nih,v) Mo is 0.13 barn [1.5], Molybdenum trioxide, ammonium molybdate or molybdenum metal are used as targets. This so-called (n,7)-molybdenum-99 is obtained in high nuclidic purity. However, its specific activity amounts to only a few Ci per gram. In contrast, Mo with a specific activity of more than in Ci (3.7 10 MBq) per gram is obtainable by fission of with thermal neutrons in a fission yield of 6.1 atom % [16]. Natural or -enriched uranium, in the form of metal, uranium-aluminum alloys or uranium dioxide, is used for the fission. The isolation of Mo requires many separation steps, particularly for the separation of other fission products and transuranium elements that arc also produced. 

Because of its neuronic, mechanical, and physical properties, hafnium is an excellent control material for water-cooled, water-moderated reactors. It is found together with zirconium, and the process that produces pure zirconium produces hafnium as a by-product. Hafnium is resistant to corrosion by high-temperature water, has adequate mechanical strength, and can be readily fabricated. Hafnium consists of four isotopes, each of which has appreciable neutron absorption cross sections. The capture of neutrons by the isotope hafnium-177 leads to the formation of hafnium-178 the latter forms hafnium-179, which leads to hafnium-180. The first three have large resonance-capture cross sections, and hafnium-180 has a moderately large cross section. Thus, the element hafnium in its natural form has a long, useful lifetime as a neutron absorber. Because of the limited availability and high cost of hafnium, its use as a control material in civilian power reactors has been restricted. 

The effect of the absorption by the 24 can be considered under the assumption that the 24 itself is not fissionable. In this case every atom 24 will eventually be transformed into 25 so that in equilibrium there appears to be a loss of 12% in efficiency due to the absorption by the 24 because in equilibrium,. 12 as many neutrons are absorbed by 24 than by the 23. Of course if the cross section of the 24 is very small this equilibrium will be reached only after a very long time and until this happens the loss in efficiency is correspondingly smaller. However, even when equilibrium has been reached the loss in efficiency will be very much smaller than the 12% quoted because the absorption leads to a fissionable element 25 and the loss in efficiency is caused only by the fact that the 25 has a smaller rj than the 23. When the absorption of 25 has come to an equilibrium one can consider the total effect in the following way. A 25 is formed by the absorption of two neutrons from the 24 and gives r/25 new neutrons. If we denote, as in Section A, the fraction of neutrons which survive all losses by 1 — p, the loss due to the absorption of 24 will be given at equilibrium by 2 X. 12 —. 12r/25(l — p) =. 25p —. 01. One sees that this absorption will not cause a considerable decrease in efficiency. Strictly speaking, one should further consider the formation of 26 from the 25, but this evidently will not cause any major effect. 

NAA is the most common form of activation analysis. The activation reaction is induced by the interaction of a neutron with the nucleus of an analyte element. Depending on the energy of the incident neutron and the reaction cross sections of the target elements, different types of reactions can take place, leading to activation products as described in O Sect. 30.2. This reaction is commonly followed by the measurement of a nuclide-characteristic de-excitation step (radioactive decay). It is this characteristic gamma-ray decay that is commonly used in the detection and determination of the element of interest. 

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