Radioactivity from nuclides

Citric acid and nitriloacetic acid (NTA) lanthanide complexes were used in the earliest ion exchange separations of lanthanides from fission product mixtures (Kf = 3.2 for Ce(H3 Cit.)3 and Kf = 10.8 for CeNTA2) (Sillen and Martell, 1964). More recently such polyamino-polycarboxylic acids as ethylenediaminetetraacetic acid (EDTA), 1,2-diaminocyclohexaneacetic acid (DCTA), and diethylenetriaminepentaacetic acid (DTPA) have been prepared. Their lanthanide complexes are very stable (Table 3) and have been widely used in analysis and separation of lanthanide mixtures. They have also been used experimentally to remove internally-deposited 144Ce and other radioactive lanthanide nuclides from animals and man (Foreman and Finnegan, 1957 Catsch, 1962 Balabukha et al., 1966 Palmer et al., 1968 among others). 

Cosmic-ray exposure ages are determined from spallation-produced radioactive nuclides. Cosmic-ray irradiation normally occurs while a meteoroid is in space, but surface rocks unshielded by an atmosphere may also have cosmogenic nuclides. These measurements provide information on orbital lifetimes of meteorites and constrain orbital calculations. Terrestrial ages can be estimated from the relative abundances of radioactive cosmogenic nuclides with different half-lives as they decay from the equilibrium values established in space. These ages provide information on meteorite survival relative to weathering. 

The main contributors to the radioactivity of the effluent were Cs-137, Cs-134, and Ru-106. Most of the radioactivity from the low-level radioactive effluents could be removed by ED. Greater DF was achieved for cesium than for rathenium due to the nonionic nature of the latter [12]. The degree of decontamination increased with the number of electrodialysis stages performed. Salt content and radionuclide concentration did not have any marked influence on the decontamination factors of these nuclides [7]. The concentrate streams generated during electrodialysis contained 0.005-0.05 mCi/L of Cs-137, and the VRF achieved in the electrodialysis operation was ca. 10. 

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... 

Of the atomic species taking part in this cascade, only uranium-238 has a long lifetime. Apart from the stable, nonradioactive end-product lead-206, all other species decay rapidly. These include two other radioactive lead nuclides, Pb-210 and Pb-214, which have a transitory existence. Some nuclides have two alternative ways of decay and so the path occasionally branches. For example, Bi-214 can form Po-214 by a p-decay, or Tl-210 by an a-decay. [Not all of the possible branching reactions... 

Whereas ETAAS lacks the capability to perform isotope measurements, NAA has been used in the past and precision and accuracy ranging from 1% to 10% have been reported. In many cases, NAA can not provide data on all the isotope ratios due to the lack of a suitable radioactive daughter nuclide. For example, Mg, Ca, Ca, Fe, and are very useful tracers but they cannot be measured by NAA. Mass spectrometry occupies a unique position for determining not only the concentration of trace elements by isotope dilution but also for measuring the isotope abundances of the elements. 

Figure 1.31 Relative radioactivity from fission products as a function of decay time. Data are for thermal neutron fission of flux of 10 n cm s and irradiation time of 2 years. Several different nuclides may contribute to the curve for each element. Adapted with permission from Choppin and Rydberg (1980)...

Already the synthesis of heavy and superheavy elements is, from the technological point of view, very demanding. In order to gain access to the longer-lived isotopes of transactinide elements, exotic, highly radioactive target nuclides such as " Pu, " Am, or " Es are bombarded with intense heavy... 

Nuclear-physical methods ai e the basic ones in controlling environmental pollution which results from nucleai -power complexes and power plants work. Oil and gas production leads to the extraction of radio nuclides of natural origin in considerable amounts, which later spread from oil-slimes and water wastes in the neighborhoods of oil and gas producing entei prises. Similaidy, toxic and radioactive elements can pollute environment in case of mineral deposits extraction. 

The discoveries of Becquerel, Curie, and Rutherford and Rutherford s later development of the nuclear model of the atom (Section B) showed that radioactivity is produced by nuclear decay, the partial breakup of a nucleus. The change in the composition of a nucleus is called a nuclear reaction. Recall from Section B that nuclei are composed of protons and neutrons that are collectively called nucleons a specific nucleus with a given atomic number and mass number is called a nuclide. Thus, H, 2H, and lhO are three different nuclides the first two being isotopes of the same element. Nuclei that change their structure spontaneously and emit radiation are called radioactive. Often the result is a different nuclide. 

Half-lives span a very wide range (Table 17.5). Consider strontium-90, for which the half-life is 28 a. This nuclide is present in nuclear fallout, the fine dust that settles from clouds of airborne particles after the explosion of a nuclear bomb, and may also be present in the accidental release of radioactive materials into the air. Because it is chemically very similar to calcium, strontium may accompany that element through the environment and become incorporated into bones once there, it continues to emit radiation for many years. About 10 half-lives (for strontium-90, 280 a) must pass before the activity of a sample has fallen to 1/1000 of its initial value. Iodine-131, which was released in the accidental fire at the Chernobyl nuclear power plant, has a half-life of only 8.05 d, but it accumulates in the thyroid gland. Several cases of thyroid cancer have been linked to iodine-131 exposure from the accident. Plutonium-239 has a half-life of 24 ka (24000 years). Consequently, very long term storage facilities are required for plutonium waste, and land contaminated with plutonium cannot be inhabited again for thousands of years without expensive remediation efforts. 

The earliest studies in this field were conducted largely to benefit from the Szilard-Chalmers effect—namely, the separation of radioactive atoms from the bulk material—in order either to make nuclear chemical study of radioactive nuclides or to effect an enrichment of radioisotopes. In Table II are listed some selected works of this type. 

The only respect in which the hot atom chemistry of organometallic compounds has so far been applied to other fields of study is in the area of isotope enrichment. Much of this has been done for isolation of radioactive nuclides from other radioactive species for the purpose of nuclear chemical study, or for the preparation of high specific activity radioactive tracers. Some examples of these applications have been given in Table II. The most serious difficulty with preparation of carrier-free tracers by this method is that of radiolysis of the target compound, which can be severe under conditions suited to commercial isotope production, so that the radiolysis products dilute the enriched isotopes. A balance can be struck in some cases, however, between high yield and high specific activity (19, 7J),... 

As shown in Example, Equation is used to find a nuclear half-life from measurements of nuclear decays. Equation is used to find how much of a radioactive substance will remain after a certain time, or how long it will take for the amount of substance to fall by a given amount. Example provides an illustration of this t q)e of calculation. In Section 22-1. we show that Equation also provides a way to determine the age of a material that contains radioactive nuclides. 

Radioactive dating is most valuable in estimating the age of materials that predate human records, such as the Earth itself Calculating the age of a sample from the amounts of its radioactive nuclides and their decay products... 

If the amount of a radioactive nuclide in a rock sample is N, the sum of this amount plus the amount of its product nuclide is A/q. For argon dating, Nq is the sum of potassium-40 and argon-40 present in a sample of rock. Assuming that Ar gas escapes from molten rock but is trapped when the rock cools and solidifies, the lifetime obtained by substituting these values into Equation is the time since the rock solidified. Such analyses show that the oldest rock samples on Earth are 3.8 X 10 years old. 

Analyses of this type are correct only if all of the product nuclide comes from radioactive decay. This is not known with certainty, but when age estimates using different pairs of nuclides give the same age and samples from different locations also agree, the age estimate is likely to be accurate. Note also that 3.8 X 10 years agrees with the qualitative limits derived from naturally occurring radioactive nuclides. 

The uranium and thorium decay-series contain radioactive isotopes of many elements (in particular, U, Th, Pa, Ra and Rn). The varied geochemical properties of these elements cause nuclides within the chain to be fractionated in different geological environments, while the varied half-lives of the nuclides allows investigation of processes occurring on time scales from days to 10 years. U-series measurements have therefore revolutionized the Earth Sciences by offering some of the only quantitative constraints on time scales applicable to the physical processes that take place on the Earth. 

Calorimetry. Radioactive decay produces heat and the rate of heat production can be used to calculate half-life. If the heat production from a known quantity of a pure parent, P, is measured by calorimetry, and the energy released by each decay is also known, the half-life can be calculated in a manner similar to that of the specific activity approach. Calorimetry has been widely used to assess half-lives and works particularly well for pure a-emitters (Attree et al. 1962). As with the specific activity approach, calibration of the measurement technique and purity of the nuclide are the two biggest problems to overcome. Calorimetry provides the best estimates of the half lives of several U-series nuclides including Pa, Ra, Ac, and °Po (Holden 1990). 

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