Silicon INAA analysis

Atomic absorption spectroscopy of VPD solutions (VPD-AAS) and instrumental neutron activation analysis (INAA) offer similar detection limits for metallic impurities with silicon substrates. The main advantage of TXRF, compared to VPD-AAS, is its multielement capability AAS is a sequential technique that requires a specific lamp to detect each element. Furthermore, the problem of blank values is of little importance with TXRF because no handling of the analytical solution is involved. On the other hand, adequately sensitive detection of sodium is possible only by using VPD-AAS. INAA is basically a bulk analysis technique, while TXRF is sensitive only to the surface. In addition, TXRF is fast, with an typical analysis time of 1000 s turn-around times for INAA are on the order of weeks. Gallium arsenide surfaces can be analyzed neither by AAS nor by INAA. 

During the late 1960s and early 1970s, neutron activation analysis provided a new way to measure bulk chemical composition. Neutron activation analysis utilizes (n,y) reactions to identify elements. A sample is placed in a nuclear reactor where thermal neutrons are captured by atoms in the sample and become radioactive. When they decay, the radioactive isotopes emit characteristic y-rays that are measured to determine abundances. Approximately 35 elements are routinely measured by neutron activation analysis. A number of others produce radioactive isotopes that emit y-rays, but their half-lives are too short to be useful. Unfortunately, silicon is one of these elements. Other elements do not produce y-ray-emitting isotopes when irradiated with neutrons. There are two methods of using neutron activation to determine bulk compositions, instrumental neutron activation analysis (INAA) and radiochemical neutron activation analysis (RNAA). 

Chemical Data. The oxide concentrations given in Appendix C were obtained by DCP-OES. This method of analysis allows easy determination of the oxides of silicon, aluminum, magnesium, calcium, and phosphorus in addition to the 15 oxides originally analyzed by INAA. In the study presented here, this additional concentration data on major constituents was very useful in conjunction with the study of the mineralogy by microscopic examination and by x-ray diffraction analysis. 

In instrument neutron activation analysis (INAA) a small fraction of the stable atomic nuclei present in the sample are made radioactive by irradiation with neutrons or other particles. By measuring the resulting radioactivity, the original elements present can be determined. Reactor or thermal neutrons are usually used. The method does not work well for certain key elements of environmental interest including silicon, sulfur, and lead. Table 6.2 shows INAA detection limits for various elements and typical concentrations of these elemenis in urban air. In atmospheric samples, the fiiiiii of detectability for a particular clement depends on the quantities of the other elements in the filter matrix. The table is based on a total air sample of 17 actually used in the measurements. The tabulated results show that all elements listed could be detected in this air volume with the exception... 

The availability of high flux thermal neutron irradiation facilities and high resolution intrinsic Ge and lithium drifted germanium (Ge(Li)) or silicon (Si(Li)) detectors has made neutron activation a very attractive tool for determining trace elemental composition of petroleum and petroleum products. This analytical technique is generally referred to as instrumental neutron activation analysis (INAA) to distinguish it from neutron activation followed by radiochemical separations. INAA can be used as a multi-elemental method with high sensitivity for many trace elements (Table 3.IV), and it has been applied to various petroleum materials in recent years (45-55). In some instances as many as 30 trace elements have been identified and measured in crude oils by this technique (56, 57). 

INAA is especially suited for the multielement analysis of matrices that, on neutron irradiation, do not produce inten.se gamma radiation or produce only short-lived ones. Examples of these matrices are graphite, silicon, and aluminum, or their oxides. A typical irradiation counting scheme allows the determination of 55 elements at submicro-gram-per-gram concentrations in wafers of silicon [37]. Also, other semiconductor or alloying materials such as Se, Ge, Sb, Te, and ultrapure metals such as Cu, Fe, Ti, Mo, Nb, Ni, Ga Zn, Sn, Zr, Bi, Pb, and noble metals have been analyzed. Often a postirradiation separation of the matrix activity is required, or a radiochemical separation scheme must be applied based on ion ex-... 

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