Radionuclide decaying activity

Consider a radionuclide (decay constant k) with activity A Bq at time th Calculate the number of nuclei that decay between times t and t2. 

Due to preferential scavenging and lateral transport of a daughter radionuclide, the activity of daughter Ap can be greater than that of the parent Ap in sediments. The inputs of daughter radionuclides that are not directly from the in situ decay of the parent (supported) are termed unsupported or excess activity. The unsupported Ap is equal to the supported A ) minus the Ap, as shown in the theoretical radionuclide profiles in figure 7.3. Moreover, the curve for the unsupported Ap decreases with depth more than the supported Ap because it is not being produced in situ from the parent. Consequently, the excess activity of a radionuclide can be used to calculate the time elapsed since the particles with unsupported Ap were last at the surface, relative to a particular depth (A). However, to calculate this it must be assumed that the sedimentation rate and supply of unsupported Ap has remained constant over time. 

If particle mixing is assumed to be analogous to diffusion with sediment accumulation and radionuclide decay, the steady-state profile for excess activity of a nonexchangeable radionuclide is defined by an advective-diffusion equation. 

If the activity of each isotope discharged within a given period is divided by the activity in the reprocessed fuel, a factor is obtained which crudely represents the relative ease with which a species is discharged. This release factor will be specific to the Sellafield site, which passes the effluent through a variety of decontamination procedures prior to discharge including storage whilst short-lived radionuclides decay. 

Effective half-life Each radionuclide decays with a definite half-life, called the physical half-life, which is denoted by Tp or f1/2. When radiopharmaceuticals are administered to patients, analogous to physical decay, they are eliminated from the body by biological processes such as fecal excretion, urinary excretion, perspiration, etc. This elimination is characterized by a biological half-life (Tb) which is defined as the time taken to eliminate a half of the administered activity from the biological system. It is related to the decay constant Ab by... 

The specific activity of a radionuclide relates activity (A) (the number of radioactive atoms decaying) to the total number (N) or total mass of atoms present ... 

The radiation fraction per radionuclide decay is one of the factors for calculating radionuclide activity from measurement that are described in Section 8.2. The intensity of a characteristic peak measured by spectral analysis or of a count rate measured by proportional or LS counter must be divided by the decay fraction (among other factors) to obtain the activity of the radionuclide. Decay fraction values can be obtained from the above-cited decay scheme compilations. 

Activity calibration of SRMs must be performed by the National Institutes of Standards and Technology (NIST) or be traceable to NIST. If the radionuclide standard source is purchased from a supplier other than NIST, its accompanying certificate must confirm that the standard is traceable to NIST. Traceability requires reporting an unbroken chain of comparisons to stated references. NIST traceability is discussed at htq) //ts.nist.gov/traceability/ (December 2005). A terminal date for use of a specific standard is appropriate when the radionuclide decays to a small fraction of its original concentration, an initially minor interference has become major, or chemical decomposition occurs with time. 

The becquerel (Bq), the SI unit of activity, is the activity of a radionuclide decaying at a rate, on average, of one spontaneous nuclear transition per second. Thus 1 Bq = 1 s . The former unit, the curie (Ci), is equal to 3.7 x 10 Bq. The curie was originally chosen to approximate the activity of 1 gram of radium-226. 

Another situation occurs as a result of an (n,y) reaction, in which an intermediate radionuclide decays to the product of interest. This route is followed to make for example, with the Xe(n, y) Xe process. The neutron capture product Xe beta decays to with a 16.9 h half-life. Because the final product can be chemically separated from the target, specific activity may approach the theoretical value for the pure radionuclide. Obviously, the use of high chemical purity targets and processing reagents is necessary to avoid introducing stable nuclides of the same element as the product. In the example, this means that both the... 

As the term implies, a half-life is the period of time during which half of a given number of atoms of a specific kind of radionuclide decay. Ten half-lives are required for the loss of 99.9% of the activity of a radionuclide. 

Adsorption of Radionuclides. Other appHcations that depend on physical adsorption include the control of krypton and xenon radionuchdes from nuclear power plants (92). The gases are not captured entirely, but their passage is delayed long enough to allow radioactive decay of the short-hved species. Highly rnicroporous coconut-based activated carbon is used for this service. 

Figure 2. (A.) The radionuclides in an aquifer are divided into three reservoirs groundwater, the host aquifer minerals, and adsorbed onto active surfaces. Also shown are the processes adding to a daughter nuclide (closed circles) in the groundwater of weathering, advection, recoil from decay of parent atoms ( P ) in the aquifer minerals, and production by parent decay, the processes of losses of a radionuclide of advection and decay, and exchange between dissolved and adsorbed atoms.

As a noble gas, Rn in groundwater does not react with host aquifer surfaces and is present as uncharged single atoms. The radionuclide Rn typically has the highest activities in groundwater (Fig. 1). Krishnaswami et al. (1982) argued that Rn and all of the other isotopes produced by a decay are supplied at similar rates by recoil, so that the differences in concentrations are related to the more reactive nature of the other nuclides. Therefore, the concentration of Rn could be used to calculate the recoil rate for all U-series nuclides produced by a recoil. The only output of Rn is by decay, and with a 3.8 day half-life it is expected to readily reach steady state concentrations at each location. Each measured activity (i.e., the decay or removal rate) can therefore be equated with the input rate. In this case, the fraction released, or emanation efficiency, can be calculated from the bulk rock Ra activity per unit mass ... 

Since Ra and " Ra are both produced by recoil from the host mineral, it might be assumed that the production rates are equal. However, the relative recoil rates can be adjusted by considering that the parent nuclides near the mineral surface may not be in secular equilibrium due to ejection losses i.e., the activity of Th may be lower than that of Th due to recoil into groundwater of the intermediate nuclide Ra. Krisnaswami et al. (1982) calculated that the recoil rate of " Ra is 70% that of Ra if radionuclides are depleted along the decay chain in this way. 

Charged particle activation analysis (CPAA) is based on charged particle induced nuclear reactions producing radionuclides that are identified and quantified by their characteristic decay radiation. CPAA allows trace element determination in the bulk of a solid sample as well characterization of a thin surface layer. 

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