Corrections for radioactive decay

All measured activities must be related to a point in time. When generating calibration data, whether they be used 

Activation analysis can involve the measurement of nuclides with half-lives as short as a few seconds, and half-lives of a few minutes to hours are common. As we saw in Chapter 5, under these circumstances the optimum count period is one to two half-lives. Obviously during the count period the activity of the source will decrease by a factor of somewhere between two and four and this must be accounted for. 

It is sometimes advised that the correction for decay during counting can be neglected if the mid-point of the count, instead of the start time, is used when making the normal decay correction. If the count period is short compared to the half-life, the error introduced by doing this is indeed small. However, the error after a one half-life count period is 2%. It would not seem worthwhile accepting an unnecessary error of this magnitude for the sake of a small amount of calculation. I would not advocate it. 

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The C content of a sample is described in a similar manner. The basis for Rq is an oxalic acid standard of the US National Bureau of Standards normalized for C fractionation and corrected for radioactive decay since a reference date January 1, 1950 (Stuiver and Polach, 1977). The absolute value of Rq is 1.176-10 (Stuiver et al, 1981). 

If we choose a much larger than 1 (thin samples d<0.5L) or h pL (thick samples d>>L), the final steady-state exhalation deviates very little from the free exhalation rate and we do not need to know the reshaping time or use Equation 2 for corrections. An air grab sample taken at any time (and corrected for radioactive decay if necessary) after closure, will yield the free exhalation rate to a good approximation, provided that the can is perfectly radon-tight. 

Pulmonary retention functions are given as fractions of the initial pulmonary burden and are not corrected for radioactive decay of lMCe Ti/> = 285 d). 

Table 2.8. Relative activities in air filters (corrected for radioactive decay to 26.4.86)... 

Figure 3.2 shows results of the experiment in which the initial concentration of stable I2 was 0.013 /ug m-3. The scrubber was not in operation. All results are corrected for radioactive decay of 132I. The airborne concentration rose initially as the source was mixed in the air within the containment shell. The activity on the charcoal-loaded papers was due mainly to inorganic iodine, and this declined with a half-life of about 30 min, as 132I was adsorbed on surfaces. Plaques of various materials were exposed periodically to monitor the deposition. In other experiments with more iodine carrier, loss by deposition was more rapid. 

Begemann Libby (1957) estimated that 1.1 kg of T was released to atmosphere for each megatonne (MT) thermonuclear explosion. The tests between 1954 and 1963 had a fusion yield of 320 MT. Allowing for radioactive decay, the global inventory in 1963, including tritium in the atmosphere, groundwater and oceans, was about 330 kg. French and Chinese thermonuclear tests between 1968 and 1977 may have added another 20-30 kg. In 1972, by which time most of the pre-1963 tritium had returned to the earth s surface, a world-wide survey of oceanic waters gave a total of 164 kg (Ostlund Fine, 1979). Corrected for radioactive decay, this is equivalent to an inventory of 270 kg in 1963. 

In this experiment, the Ge detector with spectrometer is calibrated for its efficiency, s, with a standard that emits a set of gamma rays at energies that span the range of interest, usually from a few keV to 1.5 MeV. The counting efficiency is calculated from the ratio of the net count rate to the reported disintegration rate at each full-energy peak in the spectrum. A correction for radioactive decay is needed. 

In the ceruloplasmin experiments appropriate aliquots of specimens were placed in small test tubes and assayed for Cu64 activity in a well-type scintillation counter. Analyses were made with reference to a Cu64 standard prepared and assayed at the outset and whenever other samples were assayed. This permitted correction for radioactivity decay of this 12.8-hour half-life isotope. 

Cyanombalamln Radioactive Cobalt ( Co) Capsules, USP. Cyanocobalamin Co capsules contain cyanocobaJ-amin in which some of the molecules contain radioactive cobalt ( Co), Each microgram of this cyanocobalamin preparation has a specific activity of not less than 0.02 MBq (0.. i Ci). The USP cautions that in making dosage calculations one. should correct for radioactive decay. The radioactive half-life of Co is 270 days. 

Of the total available material (i.e. cumulative discharges corrected for radioactive decay and/or ingrowth) the fractions retained in the Irish Sea for Cs, 239.240py approximately 10%, 90% and 95%... 

Assuming this percentage was consistent in past years, the erosional contribution was estimated from the accumulated Cs in the ground which was calculated as an integrated fallout until the year of interest, with a correction for radioactive decay. Since Lake Shinji has a channel of outflow, some portion of the fallout on the lake might have escaped. Two cases were considered to account for the effect of escape on the direct fallout contribution. The first is an extreme case, in which there is no escape (100% accumulation. Case 1). The second is based on an analysis of the Chernobyl accident, although the Chernobyl contribution was minimal in this area. The atmospheric fallout of Cs and the accumulation in lake bottom sediment due to the accident was evaluated from the records of atmospheric fallout and the dated sedimentation profile of Cs. Comparison of these values showed that about 40% of the Chernobyl fallout on the lake remained in the bottom sediment (40% accumulation. Case 2). It was considered that Case 2 would be more probable than Case 1. 

Studies that have included measurement of urinary and fecal excretion of radioactivity have helped to confirm these studies of total volumes of distribution of the thyroid hormones. When excreted radioactivity is subtracted from injected radioactivity and the result divided by plasma radioactivity (all in the same units and corrected for radioactive decay), the quotient is taken as the volume of distribution. The problem with this approach is that it is difficult to ensure complete collection of excreta. 

As was mentioned earlier, honeydew honey can be used as an indicator of pollution over a very long time. Honey is a stable product, lasting for decades or even hundreds of years, if properly stored. By measuring in archived samples of collected honey, it was thought it would be possible to retrospectively detect radioactive contamination events. Numerous honey samples that had been collected and kept by beekeepers over a period from 1952 to 1995 were found for locations in Central European countries located between the North sea and the Adriatic sea (Austria, Germany, and Slovenia). Analysis of these honey samples detected Cs activities, which were corrected for radioactive decay and recalculated on July 1 of each year of the sample s collection. The Cs determination in Austrian spruce honeydew honey samples collected in the Alps region from 1952 to 1994 (mainly in a circle of radius 30 km around Lunz am See)... 

Fe beta particles are counted with a proportional detector or its gamma rays are analyzed with a Ge detector and spectrometer. The sample is then measured for Fe content with a thin Ge detector and spectrometer or xenon-filled X-ray proportional detector with a thin (e.g., 140 mg cm ) beryllium absorber. The Fe count rate is adjusted for background, the Fe contribution, self-absorption in the plated sample, and the chemical yield, and converted to the disintegration rate. The activity of both radioisotopes is corrected for radioactive decay from the sampling date. 

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