NAA

Factors other tlian tire Si/Al ratio are also important. The alkali-fonn of zeolites, for instance, is per se not susceptible to hydrolysis of tire Al-0 bond by steam or acid attack. The concurrent ion exchange for protons, however, creates Bronsted acid sites whose AlO tetraliedron can be hydrolysed (e.g. leading to complete dissolution of NaA zeolite in acidic aqueous solutions). 

The presence of the half-odd quantum number j in Eq. (69) is potentially a physically measurable consequence of geomehic phase, which was first claimed to have been detected in the spectrum of Naa [16]. The situation is, however, quite complicated and the first unambiguous evidence for geometric phase in Nas was reported only in 1999 [17],... 

Equation 6.44 is written in terms of the concentrations of CH3COOH and CH3COO- at equilibrium. A more useful relationship relates the buffer s pH to the initial concentrations of weak acid and weak base. A general buffer equation can be derived by considering the following reactions for a weak acid, HA, and the salt of its conjugate weak base, NaA. 

Neutron Activation Analysis Few samples of interest are naturally radioactive. For many elements, however, radioactivity may be induced by irradiating the sample with neutrons in a process called neutron activation analysis (NAA). The radioactive element formed by neutron activation decays to a stable isotope by emitting gamma rays and, if necessary, other nuclear particles. The rate of gamma-ray emission is proportional to the analyte s initial concentration in the sample. For example, when a sample containing nonradioactive 13AI is placed in a nuclear reactor and irradiated with neutrons, the following nuclear reaction results. 

One of the important advantages of NAA is its applicability to almost all elements in the periodic table. Another advantage of neutron activation is that it is nondestructive. Consequently, NAA is an important technique for analyzing archaeological and forensic samples, as well as works of art. 

H. Mandel and N. Ewbank, Critical Constants of Diphenyl and Eerphenyl, Report No. NAA-SR-5129, Atomics International, Canoga Parks, Calif., Dec. 1960, p. 18. 

Instrumental Quantitative Analysis. Methods such as x-ray spectroscopy, oaes, and naa do not necessarily require pretreatment of samples to soluble forms. Only reUable and verified standards are needed. Other instmmental methods that can be used to determine a wide range of chromium concentrations are atomic absorption spectroscopy (aas), flame photometry, icap-aes, and direct current plasma—atomic emission spectroscopy (dcp-aes). These methods caimot distinguish the oxidation states of chromium, and speciation at trace levels usually requires a previous wet-chemical separation. However, the instmmental methods are preferred over (3)-diphenylcarbazide for trace chromium concentrations, because of the difficulty of oxidizing very small quantities of Cr(III). 

Neutron Activation Analysis (NAA) is one of the analytical methods recommended for low level Mo determination in biological materials. 

In this work some important factors affecting the uncertainty of results of Mo determination by radiochemical NAA in the materials of biological origin have been discussed. 

It has been proved lately that the NAA method of Mo determination meets the requirements of primary ratio methods of measurements. 

The REE values obtained for the U.S.G.S. standards by this method compare well with the literature values obtained by NAA, MS and other methods. Einear calibration curves in the range of about 0 to 200 956 g/g are obtained and detection limits in the range of 0.06 to 0.04 956 /g are achieved. 

The chemical composition of particulate pollutants is determined in two forms specific elements, or specific compounds or ions. Knowledge of their chemical composition is useful in determining the sources of airborne particles and in understanding the fate of particles in the atmosphere. Elemental analysis yields results in terms of the individual elements present in a sample such as a given quantity of sulfur, S. From elemental analysis techniques we do not obtain direct information about the chemical form of S in a sample such as sulfate (SO/ ) or sulfide. Two nondestructive techniques used for direct elemental analysis of particulate samples are X-ray fluorescence spectroscopy (XRF) and neutron activation analysis (NAA). 

NAA involves the bombardment of the sample with neutrons, which interact with the sample to form different isotopes of the elements in the sample (14). Many of these isotopes are radioactive and may be identified by comparing their radioactivity with standards. This technique is not quite as versatile as XRF and requires a neutron source. 

All the techniques discussed here involve the atomic nucleus. Three use neutrons, generated either in nuclear reactors or very high energy proton ajccelerators (spallation sources), as the probe beam. They are Neutron Diffraction, Neutron Reflectivity, NR, and Neutron Activation Analysis, NAA. The fourth. Nuclear Reaction Analysis, NRA, uses charged particles from an ion accelerator to produce nuclear reactions. The nature and energy of the resulting products identify the atoms present. Since NRA is performed in RBS apparatus, it could have been included in Chapter 9. We include it here instead because nuclear reactions are involved. 

In NAA the sample is made radioactive by subjecting it to a high dose (days) of thermal neutrons in a reactor. The process is effective for about two-thirds of the elements in the periodic table. The sample is then removed in a lead-shielded container. The radioisotopes formed decay by B emission, y-ray emission, or X-ray emission. The y-ray or X-ray energies are measured by EDS (see Chapter 3) in spe-... 

Most of the transition elements that are of primary interest in the semiconductor industry such as Fe, Cr, Mn, Co, and Ni, can be analyzed with very low detection limits. Second to its sensitivity, the most important advantage of NAA is the minimal sample preparation that is required, eliminating the likelihood of contamination due to handling. Quantitative values can be obtained and a precision of 1-5% relative is regularly achieved. Since the technique measures many elements simultaneously, NAA is used to scan for impurities conveniently. 

All NAA experiments are conducted in two steps irradiation and counting as indicated in Figure 1. Samples are made radioactive by placing them in a neutron field. Typically a research nuclear reactor provides the necessary neutron flux. Elements present in the sample capture neutrons, and often become radioactive isotopes. This part of the experiment is known as irradiation. A typical irradiation in a reac-... 

Elements with multiple stable isotopes may produce several radioisotopes that can be measured to assure the accuracy of the analysis. For example, Zn has five stable isotopes. The isotope Zn will produce the radioisotope Zn, and Zn will produce the radioisotope Zn. Both of these radioisotopes can provide an independent measurement of the Zn concentration and therefore can be used to check the consistency and quality of the analysis. On the other hand, Zn will produce Zn, which is nonradioactive and therefore cannot be used in NAA. 

All radioactive isotopes decay with a characteristic half-life. For example, Fe decays with a half-life of 45 days, while Cu decays with a half-life of 12.6 hours. As a result of the decay, signature high-energy photons or y rays are emitted from a given radioisotope. Thus, Fe emits two prominent y rays at 1099 and 1292 keV, " Na emits at 1368 and 2754 keV, and Zn emits at 1115 keV. Compilations of y rays used in NAA can be found in y-ray tables. 

NAA is a quantitative method. Quantification can be performed by comparison to standards or by computation from basic principles (parametric analysis). A certified reference material specifically for trace impurities in silicon is not currently available. Since neutron and y rays are penetrating radiations (free from absorption problems, such as those found in X-ray fluorescence), matrix matching between the sample and the comparator standard is not critical. Biological trace impurities standards (e.g., the National Institute of Standards and Technology Standard Rference Material, SRM 1572 Citrus Leaves) can be used as reference materials. For the parametric analysis many instrumental fiictors, such as the neutron flux density and the efficiency of the detector, must be well known. The activation equation can be used to determine concentrations ... 

NAA has been used to determine trace impurities in polysilicon, single-crystal boules, silicon wafers, and processed silicon, as well as plastics used for packaging. ... 

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