Direct radiochemical measurements

A strong specific adsorption of sulfate was later confirmed in direct radiochemical measurements. Specific chloride adsorption was also observed, but it was found to be weak and practically vanished in the presence of sulfate of about the same concentra-tion[361]. 

In summary, four experimental/calculational techniques, ICP-OES, ICP-MS, SIMS-SEM-EDX and NAA with radiochemical analysis, were used to determine the concentration of Li in reactor steels either by direct measurement or by calculation from measured values. Li measurements from the first two techniques, ICP-OES, and ICP-MS, were discarded due to excessive values/excessive limit of detection. SIMS/SEM-EDX measurement of Li and two approaches to neutron activation of steels samples with radiochemical measurement of values used to calculate Li concentrations yielded 10 valid data points spread over one order of magnitude, see Table 4. In particular, the two neutron activation approaches using differing flux, time and temperature conditions yielded good consistency of calculated values of Li. Consequently, the average Li concentration in Magnox RPV steel is considered to be < 1 ng g" and estimated as 0.4 0.2 ngg- x(lB). 

In addition, the experimental difficulties involved in the analytical determination of very low concentrations of a pure P emitter of low energy in the presence of a large excess of other P , y—active radionuclides have to be mentioned. Since a direct radiation measurement of the activity in such a mixture is not possible, a sophisticated radiochemical separation has to be performed the details of which will be summarized below. The same problems arise in the determination of the content in irradiated nuclear fuel. On the other hand, both radionuclides show similar chemical and radiation properties so that their determination can be carried out using similar analytical procedures. 

Three common quantitative applications of radiochemical methods of analysis are considered in this section the direct analysis of radioactive isotopes by measuring their rate of disintegration, neutron activation, and the use of radioactive isotopes as tracers in isotope dilution. 

Radiochemical methods of analysis take advantage of the decay of radioactive isotopes. A direct measurement of the rate at which a radioactive isotope decays may be used to determine its concentration in a sample. For analytes that are not naturally radioactive, neutron activation often can be used to induce radioactivity. Isotope dilution, in which a radioactively labeled form of an analyte is spiked into the sample, can be used as an internal standard for quantitative work. 

A radiochemical method for the determination of Rn-220 in fumarolic gas is studied. Both condensed water and non-condensing gas are collected together and Pb-212 is precipitated as PbS. After dissolving the precipitate in conc.HCI, it is mixed with an emulsion scintillator solution for activity measurements. As Pb-214 is simultaneously measured, the observed ratio of Pb-212 /Pb-214 gives Rn-220/Rn-222. This method is superior to the method of directly measuring Rn-220 for the samples in which Rn-220/Rn-222 ratios are less than unity. This method and the previously proposed direct method were applied in the field, and new data obtained. An attempt was also made to understand the formation and transport of radon underground. 

What are the advantages of the radiochemical method compared with other in situ techniques It offers a direct relationship between surface radiation (N ) and surface concentration, which allows a direct measurement of the amount of adsorbed molecules on the electrode, a condition difficult to determine with other in situ techniques. The main limitation of the technique is the availability of radioactive forms of the compound the experimenter wants to study. In this respect, the type of radiation preferred is of the P-type, mainly because of the ease of detection and minimal safety hazards. Typical P-emitters used are H, C, S, Cl, and P, which as constituents of molecules, open a great variability of compounds for study. Figure 6.21 shows some experimental results obtained for the measurement of adsorption on single crystals using this radiochemical method. 

Third, a curious and subtle concept was explained, the concept of surface excess, r. This is not to be confused with adsorption, although the surface excess may become nearly identical to the total amount adsorbed under certain limiting conditions. The surface excess of a particular species is the excess of that species present in the surface phase relative to the amount that would have been present had there been no double layer. The surface excess, therefore, represents the accumulation or depletion of the species in the entire interphase region. Further, electrocapillaiy measurements and radiochemical experiments permit a direct experimental description of the surface excess of a species. 

Common Features of NAA Procedures. In all of the procedures discussed in this article, irradiations are made in a high thermal neutron flux (1011 to 1013 neutrons cm"2 sec 1) simultaneously with the samples and standard(s) sealed in polyethylene containers for a short irradiation or in silica containers for a long irradiation. The standard is a known amount, or solution of known concentration, of a pure compound of the element to be determined. The concentration of the element in the sample is determined by comparing its radioactivity with that of the standard, which is either subjected to the same radiochemical separation as the sample with an inactive matrix or diluted. The radioactivity is counted directly if the sample is measured in solution. The radiochemical yield of precipitated samples is determined directly by weighing and that of solutions of samples by aliquot re-irradiation. 

Fr (Nozaki, 1984 Browne and Firestone, 1986). Be (Measures and Edmond, 1982), Mg (Carpenter and Mannella, 1973), Ca (Riley and Tongudai, 1967), Sr (Riley and Tongudai, 1967), Ba (Chan ef a/., 1976) and Ra (Chung and Craig, 1980). The list to the right of each profile indicates the dominant solution species and particulate forms in the water column. The dotted line shown for Fr indicates a profile based upon calculation from radioactive decay systematics rather than direct measurement. The disintegration rates of radiochemical species are given in Becquerels (Bq) with units of s 1. 

Direct measurement of dietary zinc availability in humans requires development of the stable isotope tracer methodology. Several aspects of this integrated methodology are considered and briefly discussed. These are analytical isotopic measurement methodology, consequences of the finite precision of isotopic measurements, validation of in vivo measurements, and several aspects of biological labeling of human foods. It is shown that Radiochemical Neutron Activation Analysis provides a suitable method for accurate measurement of the stable isotopes Zn,... 

Radiochemical group separation has a major advantage over individual element separation in that it is far less time consuming, yet it can permit suflBcient separation to allow precise analysis. For example, by simply separating the rare earth elements (REE) as a group after neutron activation, it is possible to measure most of the rare earth spectra by direct counting and thus determine their distribution. 

The essential apparatus for pressure measurement and analysis, and other important aspects such as furnaces and temperature control, are reviewed for thermal, photochemical and radiochemical systems. The latter two also involve sources of radiation, filters and actinometry or dosimetry. There are three main analytical techniques chemical, gas chromatographic and spectroscopic. Apart from the almost obsolete method of analysis by derivative formation, the first technique is also concerned with the use of traps to indicate the presence of free radicals and provide an effective measure of their concentration. Isotopes may be used for labelling and producing an isotope effect. Easily the most important analytical technique which has a wide application is gas chromatography (both GLC and Gsc). Intrinsic problems are those concerned with types of carrier gases, detectors, columns and temperature programming, whereas sampling methods have a direct role in gas-phase kinetic studies. Identification of reactants and products have to be confirmed usually by spectroscopic methods, mainly IR and mass spectroscopy. The latter two are also used for direct analysis as may trv, visible and ESR spectroscopy, nmr spectroscopy is confined to the study of solution reactions... 

The radiochemical yield of primary Si—CH3 fission could be determined by IR analysis from the number of Si—H bonds formed. A G value of 4, only slightly temperature-dependent between —40 and +100°C was measured. The G value of =Si—OH end groups resulting from direct breaking of the main chain according to... 

Quality is directly related to the labeling yield, which is measured by the amount of unbound Tc activity. Limits of radiochemical impurities are stated in the official monographs (Council of Europe 2005). 

After the cooling period, the sample is either counted directly or some chemical manipulation is performed before counting. The first procedure is known as instrumental neutron activation analysis (INAA), whereas the latter is referred to as radiochemical neutron activation analysis (RNAA). In RNAA a stable carrier for the element to be determined may be added to the sample after irradiation. The carrier is equilibrated with the element in the sample (often by fusing it with Na202, or treating it with strong acid). Then the element of interest is separated along with the carrier. The chemical yield of the separation is determined from the amount of carrier recovered, and this correction is applied to the measured activity. 

The second general category of radiochemical analysis involves adding a radioactive substance to the sample, manipulating the sample by chemical or physical means, measuring the radioactivity, and ultimately calculating the amount of the component of interest. This category includes direct and inverse isotope dilution analysis, radiochemical titrations, and radiorelease methods of analysis. 

Radiochemical analysis of "Tc is challenging because, unlike radionuclides such as 137Cs, it cannot be measured directly and nondestructively by gamma-ray spectral analysis. The usual processes of dissolution, purification, and preparation for counting can be complex for "Tc because it has multiple possible oxidation states, notably cationic Tc4+ and anionic pertechnetate TCO4 (see Section 6.4.1). 

Direct isotope dilution analysis is applied if an amount of an analyte cannot be separated quantitatively for analytical determination. A known amount of a radioactive isotope of the element of interest is added to the sample containing the analyte. Then a portion of the analyte is isolated in high purity from the sample. This separation step need not be quantitative. The mass and activity of the isolated portion are measured and used to calculate the amount of analyte in the original sample. There are several varieties known of this radiochemical method, e.g., reverse isotopic dilution. 

The Cs concentration in foods, plants, com samples, etc., can be determined by direct measurement of radioactivity with a y-ray spectrometer. This method requires a higher Cs concentration than in the radiochemical procedure, and radioactive impurities whose y-rays have energies similar to that of cesium y-rays (such as Zr) can be present only in negligible amounts. The spectrometric method can be used either directly on the original sample or after concentrating Cs, depending on the detection efficiency and on the Cs concentration in the sample. 

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