Radioactive nuclide decay techniques

Mass-spectroscopic technique has also been used with non-fissile targets after pile or cyclotron bombardment to determine the mass-numbers of radioactive nuclides. In one case, the branching ratios of certain isotopes for and electron capture decay (where different elements are produced by the two routes) were determined from the amount of the stable end-products of radioactive decay, using the mass-spectrometer to identify the isotopes concerned and to correct for any stable impurities of the elements concerned (98). For some purposes, mass-spectroscopic separations could be very valuable technically such as the... 

In their attempts to establish the geologic history of the earth, geologists have made extensive use of radioactivity. For example, since decays to the stable Pb nuclide, the ratio of glPb to in a rock can, under favorable circumstances, be used to estimate the age of the rock. The radioactive nuclide 7iLu, which decays to Hf, has a half-life of 37 billion years (only 186 nuclides out of 10 trillion decay each year ). Thus this nuclide can be used to date very old rocks. With this technique, scientists have estimated that the earth s crust formed 4.3 billion years ago. 

A specialized imaging technique known as positron emission tomography (PET) employs positron-emitting nuclides, such as fluorine-18, synthesized in cyclotrons. The fluorine-18 is attached to a metabolically active substance such as glucose and administered to the patient. As the glucose travels through the bloodstream and to the heart and brain, it carries the radioactive fluorine, which decays with a half-life of just under 2 hours. When a fluorine-18 nuclide decays, it emits a positron that immediately combines with any... 

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). 

Because of interference from the radioactive decay of other nuclides (which are typically formed with much higher yields), extraction systems with relatively high decontamination factors from actinides, Bi, and Po must be chosen, and the transactinide activity can only be measured in the selectively extracting organic phase. For this reason, measurement of distribution coefficients is somewhat difficult. By comparing the Rf or Db detection rate under a certain set of chemical conditions to the rate observed under chemical control conditions known to give near 100% yield, distribution coefficients between about 0.2 and 5 can be determined. If the control experiments are performed nearly concurrently, many systematic errors, such as gas-jet efficiency and experimenter technique, are cancelled out. Additionally, extraction systems which come to equilibrium in the 5-10 second phase contact time must be chosen. 

Mass spectrometric techniques play a dominant role for the determination of transuranium elements in bulk samples as well as in microparticles. The radioactive element most frequently investigated by inorganic mass spectrometry is uranium. The determination of the concentrations and the precise isotopic analysis of naturally occurring radioactive elements (e.g. Th and the decay nuclides) by inorganic mass spectrometry as terrestrial... 

Emanation techniques are based on the production of radioactive noble gases by decay of mother nuclides or by nuclear reactions. The emanating power has been defined by Hahn as the fraction of radioactive noble gas escaping from a solid relative to the amount produced in the solid. It depends on the composition of Ihe solid, ils lallice structure and its spccihc smTace area. Reactions in the solid have a major inlluence. Further factors affecting the emanating power are the half-life of the noble gas radionuclide, its recoil energy and the temperature. 

In the meantime, a reasonably fast radioactive decay makes a different technique for obtaining data on fjp feasible, even with rather low activities. One can measure the fraction r]c of the nuclei introduced into the IC column which survives at its exit. This principle can be used in on-line experiments with the nuclides which have mean lifetimes much less than the nominal duration of the run in practice, it means the range from seconds to hours. For a nuclide with the particular tx, two or more measurements at different temperatures must be done. At least one at a temperature when t -C tx to find the production rate of the detectable activity, as well as one when r c is of the order of lx and so the surviving fraction is in the range 0 < rf < 1. From the point of view of the statistics, most desirable is to aim at rjc near 0.5. Obviously, r = exp -t r/tx), hence ... 

The time dependence of radioactive decay is expressed in terms of the half-life which is the time required for one-half of the radioactive atoms in a sanq>le to undergo decay. In practice this is the time for the measured radioactive intensity (or simply, radioactivity of a sanq>le) to decrease to one-half of its previous value (see Fig. 1.1). Half-lives vary from millions of years to fractions of seconds. While half-lives between a minute and a year are easily determined with fairly simple laboratory techniques, the determination of much shorter half-lives requires elaborate techniques with advanced instrumentation. The shortest half-life measurable today is about 10 s. Consequently, radioactive decay which occurs with a time period less than 10 s is considered to be instantaneous. At the other extreme, if the half-life of the radioactive decay exceeds 10 y, the decay usually cannot be observed above the normal signal background present in the detectors. Therefore, nuclides which may have half-lives greater than 10 y are normally considered to be stable to radioactive decay. However, a few unstable nuclides with extremely long half-lives, > 10 y, have been identified. It should be realized that 10 y is about 10 times larger than the age of the universe. 

As the detection technique for radioactivity has been refined, a number of long-lived radionuclides have been discovered in nature. The lightest have been motioned in 5.1. The heavier ones, not belonging to the natural radioactive decay series of uranium and thorium, are listed in Table 5.2. is the nuclide of lowest elemental specific activity ( 0.(XX)1 Bq/g) while the highest are Rb and Re (each —900 Bq/g). As our ability to make reliable measurements of low activities increases, the number of elem ts between potassium and lead with radioactive isotopes in nature can be expected to increase. 

In activation analysis advantage is taken of the fact that the decay properties such as the half-life and the mode and energy of radioactive decay of a particular nuclide serve to identify uniquely that nuclide. The analysis is achieved by the formation of radioactivity through irradiation of the sample either by neutrons or charged particles. Neutron irradiation is by far the more common technique, and hence this method is often referred to as neutron activation analysis, NAA. A major advantage in activation analysis is that it can be used for the simultaneous determination of a number of elements and complex samples. If the counting analysis of the sample is conducted with a Ge-detector and a multichannel analyzer, as many as a dozen or more elements can be measured quantitatively and simultaneously (instrumental NAA, or INAA). 

Radionuclide techniques often have higher sensitivity than other analytical methods. The amounts of nuclides, correlated to an activity of 1000 Bq (see Table 1), can be derived from the law of radioactive decay. The amounts vary considerably, corresponding to the wide range of half-lives. For 90% of the commonly used nuclides, half-lives range from several minutes to several years, so the corresponding masses are extremely low. 

In chronometry, the age of the sample is defined not in terms of the decay of a parent nuclide, but rather as the in-growth of a daughter activity. Radionuclides that are linked to one another by the processes of radioactive decay have relative concentrations that can be calculated with the Bateman equations, which express the simple laws of radioactive decay and ingrowth. If there exists a time at which all the descendant radionuclides have been removed from the mother material, that time can be determined through the measurement of the relative concentrations of the mother and daughter nuclides at a later time. The time interval between the purification of the sample and the subsequent analysis of the sample is defined as the age of the material at the analysis time. The technique does not apply when the half-life of the daughter nuclide involved in the determination is significantly shorter than the elapsed time. 

The ratio of the concentrations of nuclides that are related by radioactive decay is interpreted as an age through the application of the rate laws, which are formulated in terms of characteristic decay constants, which are related to half-lives as discussed above. Radioactive decay is a statistical process, and in the limit of a small number of decays of atoms, the rate laws break down and the statistics of small numbers dominates (Schmidt et al. 1984). (See O Fig. 9.10 in Chap. 9, Vol. 1.) Measurement of the product of a small number of decays and interpretation of that measurement as an age is the fundamental limitation of the technique at short times. 

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