Half-life of radionuclides

The main tools of the radioanalytical chemist for identifying and quantifying a radionuclide are chemical separation and radiation measurement, especially by spectral analysis. The former separates radionuclides by element, i.e., Z value, and removes potentially confusing contaminants. The latter determines the type of radiation, its energy, and relative intensity at various energies. These measurements are repeated during the period of study to determine the half-life of radionuclides that are sufficiently short-lived to observe a decrease in the count rate. 

Accurate values of deposition of radionuclides near the sampling locations are required to determine the mean residence time or effective half-life of radionuclides in lichens. Liden and Gustafsson (1967) obtained in Cladonia lichens the value of 17 4 years for T y and 11 years for Teff. They made Cs determinations in Swedish Lapland in 1962-1965. Near the sampling location there was an observation station where Cs has been... 

Decay products of the principal radionuclides used in tracer technology (see Table 1) are not themselves radioactive. Therefore, the primary decomposition events of isotopes in molecules labeled with only one radionuclide / molecule result in unlabeled impurities at a rate proportional to the half-life of the isotope. Eor and H, impurities arising from the decay process are in relatively small amounts. Eor the shorter half-life isotopes the relative amounts of these impurities caused by primary decomposition are larger, but usually not problematic because they are not radioactive and do not interfere with the application of the tracer compounds. Eor multilabeled tritiated compounds the rate of accumulation of labeled impurities owing to tritium decay can be significant. This increases with the number of radioactive atoms per molecule. 

For a radionuclide to be an effective oceanic tracer, various criteria that link the tracer to a specihc process or element must be met. Foremost, the environmental behavior of the tracer must closely match that of the target constituent. Particle affinity, or the scavenging capability of a radionuclide to an organic or inorganic surface site i.e. distribution coefficient, Kf, is one such vital characteristic. The half-life of a tracer is another characteristic that must also coincide well with the timescale of interest. This section provides a brief review of the role of various surface sites in relation to chemical scavenging and tracer applications. 

Figure 4-4b illustrates exponential decay. A simple example could be the reservoir of all on Earth. The half-life of this radionuclide is... 

A large neutron cross section of 235U for fission (5.8 x 10 26 m2), a high fission yield (6%) for "Tc, and a long half-life of the resulting "Tc (2.1 x 105 yr) make this radionuclide one of the principal nuclear wastes. Fig. 1 shows radioactivity of nuclear wastes plotted against cooling time in years. Tc activity is very important in the time interval 104-106 years. 

Radiotherapy generally involves cell destruction, requiring some form of particle emission on decay and a half-life between 1 and 10 days. The choice of a particular therapeutic application determines the type of particle emission (a, ft, or Auger e ), and the energy and half-life of the radionuclide to be used. Considerations include time for delivery of the radiopharmaceutical to its in vivo target, location of the target (tumor surface, tumor cell cytoplasm, tumor cell nucleus) and size of the tumor. The reader is directed to a number of reviews on this subject.15-22... 

Pb is a ft emitting radionuclide with a half-life of 10.64 h. It is the parent radionuclide of the a-emitting 212Bi. The decay scheme is shown below ... 

Sr, as the strontium ion (Sr2+), is used for pain palliation in patients with metastatic bone disease. The strontium ion is a calcium ion mimic, being taken up in metabolically active bone such as cancer. 89Sr is a therapeutic radionuclide with a half-life of 50.53 days, emitting a 1.49 MeV [3 particle on decay. Several recent reviews discuss the use of radionuclides and their complexes as pain palliation agents in metastatic bone disease.18,212-215... 

Radon-222, a decay product of the naturally occuring radioactive element uranium-238, emanates from soil and masonry materials and is released from coal-fired power plants. Even though Rn-222 is an inert gas, its decay products are chemically active. Rn-222 has a a half-life of 3.825 days and undergoes four succesive alpha and/or beta decays to Po-218 (RaA), Pb-214 (RaB), Bi-214 (RaC), and Po-214 (RaC ). These four decay products have short half-lifes and thus decay to 22.3 year Pb-210 (RaD). The radioactive decays products of Rn-222 have a tendency to attach to ambient aerosol particles. The size of the resulting radioactive particle depends on the available aerosol. The attachment of these radionuclides to small, respirable particles is an important mechanism for the retention of activity in air and the transport to people. 

The development of PET radiopharmaceuticals labeled with generator-produced radionuclides has facilitated greater use of PET in clinical nuclear medicine. The 68Ge/68Ga parent/daughter pair is ideal as a source of PET radiopharmaceuticals as a result of the favorable half-lives of both the parent and daughter radionuclides (43-45). The 271 days half-life of the 68Ge parent... 

The half-life of a radionuclide represents the amount of time required for half of the sample to decay. Relative stabilities of radionuclides are indicated by their half-life values. The shorter the half-life, the less stable is the radionuclide. 

A second important parameter is the decay half-life of the radionuclide. It must be long enough to give sufficient time for pharmaceutical preparation and in vivo accumulation in the target, but not so long as to impose a prolonged hazard to other personnel or cause an environmental... 

Sr, caribou, muscle vs. bone Various species, biological half-life of selected radionuclides 241 Am BCF of 0.02 vs. 7.0 24... 

The amount of time ,/2 required to reduce by one-half the initial number A o of radionuclides is called the half-life of the species ... 

As shown in figure 11.8B, because the ratio A1/A2 becomes constant, the slopes of the combined decay curves of the two radionuclides attain a constant value corresponding to the half-life of the longer-lived term (curves a and b in figure 11.8B). Moreover, assuming identical detection coefficients for the two species, their radioactivity ratio also attains a constant value of... 

In equation 11.77, 4 is a numerical constant depending on the geometry and decay constant of the parent radionuclide. If the half-life of the parent is long in comparison with the cooling period, A takes a value of 55, 27, or 8.7 for volume diffusion from a sphere, cylinder, or plane sheet, respectively. If decay rates are faster, A progressively diminishes (see table 1 in Dodson, 1973, for numerical values). 

The chemical reactions involving positron emitters have to be specially designed to take into account the short half life of the radionuclide, the limited number of labelled precursors and the sub-micromolar amounts of these precursors. Moreover, the reactions must be possible without any addition of the stable isotope (especially when ligands of receptors are synthesized). Several practical considerations that influence the design of positron-emitter radiotracers with a high specific radioactivity and their experimental handling have been reviewed [4,8,14-19]. 

The SI unit of activity is the becquerel (Bq) 1 Bq = 1 transformation/second. Since activity is proportional to the number of atoms of the radioactive material, the quantity of any radioactive material is usually expressed in curies, regardless of its purity or concentration. The transformation of radioactive nuclei is a random process, and the rate of transformation is directly proportional to the number of radioactive atoms present. For any pure radioactive substance, the rate of decay is usually described by its radiological half-life, T r i.e., the time it takes for a specified source material to decay to half its initial activity. The activity of a radionuclide at time t may be calculated by A = A° e ° rad where A is the activity in dps, A ° is the activity at time zero, t is the time at which measured, and T" is the radiological half-life of the radionuclide. It is apparent that activity exponentially decays with time. The time when the activity of a sample of radioactivity becomes one-half its original value is the radioactive half-life and is expressed in any suitable unit of time. 

Fluorine-18 is an artificial radionuclide, discovered in 1937. It decays with a half-life of 109.8 min for 97% by positron emission and for 3% by electron capture to the stable isotope oxygen-18. The maximum jS+-particle energy is 0.635 MeV [4],... 

The choice of the radionuclide and the position of the labelling are generally determined by the chemical structure of the target molecule to label and the ease of introduction of the radionuclide from a chemical point of view. The physical half-life of the radionuclide should however match the timescale of the studied process. For example, in repeated blood flow measurements, oxygen-15 in the form of p Ojwater is ideal, while carbon-11 and especially fluorine-18 are preferable in the study of slower processes. 

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