Radioactivity induced, measurement

Radioactivity. Methods based on the measurement of radioactivity belong to the realm of radiochemistry and may involve measurement of the intensity of the radiation from a naturally radioactive material measurement of induced radioactivity arising from exposure of the sample under investigation to a neutron source (activation analysis) or the application of what is known as the isotope dilution technique. 

Neutron activation (NA). The sample is bombarded with neutrons and the radioactivity induced in the sample is then measured. Both /3 and y radiation can be monitored, but y radiation is more frequently used because of the discrete wavelengths associated with emission that can be used to identify the emitter. 

The measurability criterion used by the FDA is a reasonable, conservative, and compromise criterion because although radioactivity is known in principle to be produced at energies below 10 m.e.v., such radioactivity has not only been nonmeasurable in food samples but also is probably not radio-logically significant. The criterion also simplifies control of the process considerably and eliminates the need for on-line monitoring of radioactivity induced by the process. 

The only apprehension of prospective industrial radiation processors with the nonmeasurability criterion comes from the analogies with the history of pesticide residues in foods U). Pesticides are cleared on the basis of either zero tolerance or finite tolerance, and use is rated as safe for a zero-tolerance pesticide when the residue on foods cannot be measured. However, improvements in measurement techniques with time have changed the definition of zero tolerance, sometimes abruptly. Radiation processors are apprehensive of similar experiences with radioactivity induced in food. 

Several investigators have used neutron activation analysis (NAA) to determine the aluminium content of biological specimens both with and without some chemical processing. Instrumental neutron activation analysis involves the bombardment of a sample with neutrons and the measurement of the radioactivity induced by nuclear reactions. No chemical processing is required. Upon activation Al (100% isotopic abundance) forms the radioactive AI nuclide by a (n,y) reaction. There are a number of attractive features in this technique which include excellent sensitivity with relative independence from matrix effects and interferences. Also, there is relative freedom from contamination since the sample is analyzed directly with minimal handling. One major problem is the need to... 

This method involves bombarding the sample with neutrons and measuring the radioactivity induced in the sample (commonly using gamma-ray spectrometry). In order to understand the principles of neutron activation analysis, some pertinent properties of neutrons and their interactions with matter will first be discussed. 

The scattered neutrons were detected by measuring the radioactivity induced in the C (w, 2n)... 

After the irradiation remove each sample portion from its bombardment container in preparation for the detection and measurement of the specific radioactivity induced in the element of interest. Two methods of preparation are usually followed (o) A simple transfer of the radioactive material to a counting vial and the use of a direct (or nondestructive) technique of radiation detection and measurement and (b) the use of chemical separations to isolate the radioelement free from all other induced radionuclides prior to the detection and measurement of its radiations. 

Figure 34. Rf-resonance signal of induced magnetic dipole transitions between hfs levels with F = 5/2 and F = 7/2 of the 4s4p state of radioactive Ca measured with the setup shown in Figure 33. The solid line represents the best fit to the experimental...

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. 

In terms of atomic spectrometry, NAA is a method combining excitation by nuclear reaction with delayed de-excitation of the radioactive atoms produced by emission of ionising radiation (fi, y, X-ray). Measurement of delayed particles or radiations from the decay of a radioactive product of a neutron-induced nuclear reaction is known as simple or delayed-gamma NAA, and may be purely instrumental (INAA). The y-ray energies are characteristic of specific indicator radionuclides, and their intensities are proportional to the amounts of the various target nuclides in the sample. NAA can thus... 

As a result of slow (thermal) neutron irradiation, a sample composed of stable atoms of a variety of elements will produce several radioactive isotopes of these activated elements. For a nuclear reaction to be useful analytically in the delayed NAA mode the element of interest must be capable of undergoing a nuclear reaction of some sort, the product of which must be radioactively unstable. The daughter nucleus must have a half-life of the order of days or months (so that it can be conveniently measured), and it should emit a particle which has a characteristic energy and is free from interference from other particles which may be produced by other elements within the sample. The induced radioactivity is complex as it comprises a summation of all the active species present. Individual species are identified by computer-aided de-convolution of the data. Parry (1991 42-9) and Glascock (1998) summarize the relevant decay schemes, and Alfassi (1990 3) and Glascock (1991 Table 3) list y ray energy spectra and percentage abundances for a number of isotopes useful in NAA. 

A method has been developed for the determination of technetium-99 in mixed fission products by neutron activation analysis Tc is separated from most fission products by a cyclohexanone extraction from carbonate solution, the stripping into water by addition of CCI4 to the cylohexanone phase, and the adsorption on an anion exchange column. Induced Tc radioactivity is determined using X-ray spectrometry to measure the 540 and 591 keV lines. The sensitivity of the analysis under these conditions is approximately 5 ng. The method has been successfully applied to reactor fuel solutions. 

This assay is normally carried out only if positive effects have been obtained in earlier in vitro tests. The UDS test measures the DNA repair synthesis which occurs after excision and removal of a stretch of DNA containing the region of damage, induced in hepatocytes of animals treated with the test chemicals. UDS is measured by the uptake of radioactively labelled nucleotide, usually tritium-labelled th)unidine, into the DNA of the damaged hepatocytes. Animals, usually male rats, are treated with the test chemical, and... 

Ionization detector. A chromatographic detector in which the sample measurement is derived from the current produced by the ionization of sample molecules. This ionization may be induced by thermal, radioactive, or other excitation sources. 

Further support for the hypothesis that Ca2+ plays a central role in regulating phytoalexin accumulation is provided by experiments in which the turnover of phosphatidylinositol was measured in the plasma membrane of elicitor-treated carrot cells [17]. The carrot cells were first labelled with [3H]myo-inositol and, after the addition of elicitors, acid extracts of the cells were analyzed chromatographically for the production of inositol trisphosphate (IP3). In cells treated with elicitor, the release of radioactive IP3 increased with time and attained a maximum at 3 - 5 min after treatment. Phospholipase activity responsible for the degradation of phosphorylated phosphatidylinositol increased correspondingly. Several reports have shown that IP3 induces rapid release of Ca2+ from intracellular stores in animal cells [18, 19]. Studies on plant cells have also demonstrated that exogenous IP3 releases Ca2+ from microsomal preparations at micromolar concentrations, although only limited... 

Radiocarbon Dating. This is a method of estimating Ihe age of carbon-containing materials by measuring the radioactivity of the carbon in them. The validity of this method rests upon certain observations and assumptions, of which the following statement is a brief summaiy. The cosmic rays entering the atmosphere undergo various transformations, one of which results in the formation of neutrons, which in turn, induce nuclear reactions in the nuclei of individual atoms of the adnosphere. The dominant reaction is... 

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