International
Tables for
Crystallography
Volume H
Powder diffraction
Edited by C. J. Gilmore, J. A. Kaduk and H. Schenk
International Tables for Crystallography (2018). Vol. H, ch. 2.10, pp. 219-221

Section 2.10.2.4. Isotopes, absorption and activation

P. S. Whitfield,a* A. Huqb and J. A. Kadukc,d,e

aEnergy, Mining and Environment Portfolio, National Research Council Canada, 1200 Montreal Road, Ottawa ON K1A 0R6, Canada,bChemical and Engineering Materials Division, Spallation Neutron Source, P.O. Box 2008, MS 6475, Oak Ridge, TN 37831, USA,cDepartment of Chemistry, Illinois Institute of Technology, 3101 South Dearborn Street, Chicago, IL 60616, USA,dDepartment of Physics, North Central College, 131 South Loomis Street, Naperville, IL 60540, USA, and ePoly Crystallography Inc., 423 East Chicago Avenue, Naperville, IL 60540, USA
Correspondence e-mail:  pamela.whitfield@psi.ch

2.10.2.4. Isotopes, absorption and activation

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There are usually multiple sizes of vanadium cans available, the most common sizes being 6, 8 and 10 mm diameter. The choice of can depends on the quantity of sample available and the nature of the elements. Some elements such as cadmium, gadolinium and boron have extremely large absorption for neutrons and may not be feasible for measurements. Some elements have high absorption for the naturally occurring isotope mixes, but by choosing an alternative isotope one can often reduce or eliminate the absorption problem completely. For example, natural lithium is mostly 7Li and has an absorption cross section of 70.5 b. However, pure 7Li has an absorption cross section of 0.0454 b. The difference is due to the small amount of highly absorbing 6Li (absorption cross section = 940 b) present in naturally occurring lithium. With commercially sourced lithium salts it is not always safe to assume a natural abundance of lithium isotopes. 6Li is used in the production of tritium and, depending on their history, commercial lithium salts may be deficient in 6Li. Given the significantly different scattering lengths of 6Li and 7Li, this can have serious consequences for a structure refinement, so it may be necessary to perform an isotopic analysis to verify the 6Li:7Li ratio. The various neutron scattering cross sections are available on the NIST website http://www.ncnr.nist.gov/resources/n-lengths/ and in Table 4.4.4.1[link] in International Tables for Crystallography Volume C (2006)[link]. If the sample contains any element with a large absorption and is not isotopically substituted, it is prudent to calculate by how much the neutrons will penetrate the sample. Tools are available at the NIST website http://www.ncnr.nist.gov/resources/activation/ to carry out this calculation. The input information is the composition of the compound, the density (generally a 50% packing fraction and hence ½ of the calculated density is a good approximation for planning purposes) and the wavelength of the neutrons used. Fig. 2.10.53[link] shows a calculation for Li3N. Natural lithium has a quite large absorption, but in this case 1 Å neutrons will penetrate through 5 mm and so the use of a 6-mm diameter can is appropriate.

[Figure 2.10.53]

Figure 2.10.53 | top | pdf |

Calculation of the penetration of neutrons into Li3N using the online tool at https://www.ncnr.nist.gov/resources/activation/ .

However, if the penetration depth 1/e is 1–2 mm, one has to reconsider the choice of sample holder. For a high-intensity beamline capillaries can be considered. The other option is to use an annular holder made using co-axial, thin-walled vanadium or aluminium cylinders, or flat-plate mounts based on aluminium foils where the total depth of the sample is approximately the calculated 1/e. Perhaps the best way to maximize the transmission and improve the signal-to-noise ratio is to use silicon flat-wafer sample holders. When loading a can it is also important to record the weight and height of the sample in the can so that the absorption correction for the sample can be calculated. Alternatively, the neutron transmission through the sample can be measured using a pinhole mask and a detector downstream from the sample.

One of the other factors to keep in mind for neutron sample preparation is that hydrogen is a very special element in terms of its interaction with neutrons. Hydrogen has a very large incoherent scattering cross section (σinc) of 80.26 b, while its coherent cross section (σcoh) is 1.758 b. In comparison, for deuterium σinc = 2.05 b and σcoh = 5.59 b. If the scattering nuclei contain a mixture of isotopes or have a non-zero nuclear spin, the neutron scattering length consists of a coherent component, which is the average over all spins, and an incoherent part, which gives the deviation from this average value. In other words, coherent scattering describes interference between waves produced by the scattering of a single neutron from all the neutrons in the nuclei of the sample. On the other hand, incoherent scattering involves correlations between the position of an atom j at time zero and the position of the same atom at time t, and so the scattered waves from different nuclei no longer interfere. Thus incoherent scattering provides an excellent tool for studying processes involving atomic diffusion, but produces large backgrounds for diffraction experiments. Based on the available flux at the instrument of choice and the atom% hydrogen present in the sample, complete or partial deuteration of the sample may be necessary.

When illuminated by a neutron beam, some nuclei are converted into other radioactive nuclei (activated). Thus it may not be possible to return the specimen to the home laboratory, but it may have to be treated as radioactive waste. A sample-activation calculator is also available at http://www.ncnr.nist.gov/resources/activation/ .

References

International Tables for Crystallography (2006). Volume C, Mathematical, Physical and Chemical Tables, 1st online edition, edited by E. Prince. Chester: International Union of Crystalloagraphy. doi:10.1107/97809553602060000103 .Google Scholar








































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