Isotopes for

To support them, therefore, immense activities are prompted both in pubh c private sectors with increasing importance on NDT. The particular application of radiography using Ir-192 isotopes for industrial production, construction maintenance of industries, power plants, oil and gas pipelines plants, railway, aviation systems, naval structures and vessels, etc is currently in the fore front for its reliabih ly, ease of application record keeping advantages. 

The uses of nuclear fuels to generate electrical power, to make isotopes for peaceful purposes, and to make explosives are well known. The estimated world-wide capacity of the 429 nuclear power reactors in operation in January 1990 amounted to about 311,000 megawatts. 

Equations 13.31 and 13.32 are only valid if the radioactive element in the tracer has a half-life that is considerably longer than the time needed to conduct the analysis. If this is not the case, then the decrease in activity is due both to the effect of dilution and the natural decrease in the isotope s activity. Some common radioactive isotopes for use in isotope dilution are listed in Table 13.1. 

Isotopes of an element are formed by the protons in its nucleus combining with various numbers of neutrons. Most natural isotopes are not radioactive, and the approximate pattern of peaks they give in a mass spectrum can be used to identify the presence of many elements. The ratio of abundances of isotopes for any one element, when measured accurately, can be used for a variety of analytical purposes, such as dating geological samples or gaining insights into chemical reaction mechanisms. 

A common mistake for beginners in mass spectrometry is to confuse average atomic mass and isotopic mass. For example, the average atomic mass for chlorine is close to 35.45, but this average is of the numbers and masses of Cl and Cl isotopes. This average must be used for instruments that cannot differentiate isotopes (for example, gravimetric balances). Mass spectrometers do differentiate isotopes by mass, so it is important in mass spectrometry that isotopic masses be used... 

The masses of the naturally occurring isotopes for lanthanum and cerium are shown. For lanthanum, the isotope at 138 is only present in 0.09% natural abundance and is isobaric with Ce. For this reason the isotope La is used to measure the amount of lanthanum. Similarly, Ce and Ce are present in low abundance "Ce is present in greatest abundance and is used to measure the amount of cerium. Another isotope of cerium, C, although quite abundant, is isobaric with Nd and is therefore not used for measurement. 

Almost any type of analyzer could be used to separate isotopes, so their ratios of abundances can be measured. In practice, the type of analyzer employed will depend on the resolution needed to differentiate among a range of isotopes. When the isotopes are locked into multielement ions, it becomes difficult to separate all of the possible isotopes. For example, an ion of composition CgHijOj will actually consist of many compositions if all of the isotopes ( C, C, H, H, 0, O, and 0) are considered. To resolve all of these isotopic compositions before measurement of their abundances is difficult. For low-molecular-mass ions (HjO, COj) or for atomic ions (Ca, Cl), the problems are not so severe. Therefore, most accurate isotope ratio measurements are made on low-molecular-mass species, and resolution of these even with simple analyzers is not difficult. The most widely used analyzers are based on magnets, quadrupoles, ion traps, and time-of-flight instruments. 

Other elements have atoms that can have different ratios of protons to neutrons. Indeed, hydrogen actually consists of three types of atoms. All hydrogen atoms have the same number of protons (one for hydrogen), giving each a mass of 1 Dalton, but some atoms of hydrogen also contain one neutron in the nucleus as well as the proton (mass of 2 Da), while yet others have two neutrons with each proton (mass of 3 Da). Thus hydrogen has three naturally occurring isotopes of mass 1, 2, and 3 Da. Chemically, there are only small differences between the reactivities of the different isotopes for any one element. Thus isotopes of palladium aU react in the same way but react differently from all isotopes of platinum. 

Atoms of many other elements contain nuclei that have different numbers of neutrons. For example, carbon (Z = 6) can have six neutrons (M = 6 + 6 = 12), seven neutrons (M = 13), or eight neutrons (M = 14). Atoms of the same atomic number but having different numbers of neutrons (and different atomic masses) are called isotopes. Thus, naturally occurring carbon has three isotopes, for which Z = P = 6 and N = 6 or 7 or 8. These are written. ... 

The phenomenon of multiphoton dissociation finds a possible application in the separation of isotopes. For this purpose it is not only the high power of the laser that is important but the fact that it is highly monochromatic. This latter property makes it possible, in favourable circumstances, for the laser radiation to be absorbed selectively by a single isotopic molecular species. This species is then selectively dissociated resulting in isotopic enrichment both in the dissociation products and in the undissociated material. 

Most chemical iavestigations with plutonium to date have been performed with Pu, but the isotopes Pu and Pu (produced by iatensive neutron irradiation of plutonium) are more suitable for such work because of their longer half-Hves and consequendy lower specific activities. Much work on the chemical properties of americium has been carried out with Am, which is also difficult to handle because of its relatively high specific alpha radioactivity, about 7 x 10 alpha particles/(mg-min). The isotope Am has a specific alpha activity about twenty times less than Am and is thus a more attractive isotope for chemical iavestigations. Much of the earher work with curium used the isotopes and Cm, but the heavier isotopes... 

Yearly consumption of isotopes for production of tracer chemicals varies depending on the radionucHde. Eor example, about 1,100 GBq (30 Ci) of P was expected to be utilized worldwide in 1995 compared with ca 7,400 TBq (200,000 Ci) of tritium. 

Although performance varies with the isotopes for which they are intended, and with the balance in the design between resolution and efficiency, the overall sensitivity of a y-camera collimator is on the order of 5000 counts/(MBqmin) (several hundred counts/(/iCi-min)). In terms of photons detected per photon emitted, this is equivalent to about 2 x lO ". In other words, about two photons out of 10,000 emitted arrives at the crystal. This necessitates exposure times that range from several minutes to the better part of an hour. Fortunately, the large number of photons available from a modest injected radioactive dose more than offsets the poor detector sensitivity. The camera s abiUty to resolve small objects, however, is ultimately limited by the collimator inefficiency. 

A SSIMS spectrum, like any other mass spectrum, consists of a series of peaks of dif ferent intensity (i. e. ion current) occurring at certain mass numbers. The masses can be allocated on the basis of atomic or molecular mass-to-charge ratio. Many of the more prominent secondary ions from metal and semiconductor surfaces are singly charged atomic ions, which makes allocation of mass numbers slightly easier. Masses can be identified as arising either from the substrate material itself from deliberately introduced molecular or other species on the surface, or from contaminations and impurities on the surface. Complications in allocation often arise from isotopic effects. Although some elements have only one principal isotope, for many others the natural isotopic abundance can make identification difficult. 

Consider a reactant molecule in which one atom is replaced by its isotope, for example, protium (H) by deuterium (D) or tritium (T), C by C, etc. The only change that has been made is in the mass of the nucleus, so that to a very good approximation the electronic structures of the two molecules are the same. This means that reaction will take place on the same potential energy surface for both molecules. Nevertheless, isotopic substitution can result in a rate change as a consequence of quantum effects. A rate change resulting from an isotopic substitution is called a kinetic isotope effect. Such effects can provide valuable insights into reaction mechanism. 

All frequency calculations include thermochemical analysis of the system. By default, this analysis is carried out at 298.15 K and 1 atmosphere of pressure, using the principal isotope for each element type. Here is the start of the ermochemistry output for formaldehyde ... 

You can specify a different temperature, pressure, and/or set of isotopes for the thermochemical analysis by specifying the Readlsolopes option to the Frecj keyword in the route section. Values for all parameters must then be specified in a separate input section following the molecule specification—and separated from it by a blank line. 

The ratios of stable isotopes (red dots) fall within a narrow range, referred to as the "belt of stability." For light isotopes of small atomic number the stable ratio is 1 1. For heavier isotopes the ratio gradually increases to about 1.5 1. Isotopes outside the band of stability are unstable and radioactive. There are no stable isotopes for elements of atomic number greater than 83 (Bi). 

Fig. 8. A curve of proton number Z as a function of neutron number N, calculated as described in the text. The horizontal lines show the ranges of stablg isotopes for alternate Z-even elements (for large Z the four most stable isotopes).

White, C.D. and Schwarcz, H.P. 1994 Temporal trends in stable isotopes for Nubian mummy tissues. American Journal of Physical Anthropology 93 165-187. 

Farmer, A., Cade, B.S., Torres-Dowdall, J. (2008). Fundamental limits to the accuracy of deuterium isotopes for identifying the spatial origin of migratory animals. Oecologia, Vol. 158, pp. 183-192. (http //dx.doi.org/10.1007/s00442-008-1143-6)... 

Two atoms with the same number of protons but different numbers of neutrons are called isotopes. For example, every chlorine atom has 17 protons in its nucleus, but whereas most chlorine nuclei contain 18 neutrons, others contain 20 neutrons. Naturally occurring chlorine contains both isotopes, so any sample that contains chlorine includes atoms with two different masses. 

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