Measuring radioisotope activity

Any radionuclide, whatever the type of radiation emitted, is characterized by its half-life T (Table 17.1), which is the time taken for half of corresponding atom population in the sample to decompose (from initial time, t = 0). Calling A the radioactive decay constant, the law of radioactivity decay allows calculation of the number of atoms N present after time t for a population containing Nq atoms initially. The integrated form of this law is written as equation 17.4  

Isotope Half-life Type of emission Energy (MeV) 

A is in units of time In practice, it is not N (the number of atoms remaining) that is known but the activity A A = —dN/dt), expressed in becquerels Bq (1 Bq= 1 nuclear decay per second and 1 curie (Ci), a former unit of activity, is equal to 

7 X 10 °Bq). The activity, accessible through use of an appropriate detector, is directly related to the concentration of the radionuclide (17.6)  

A is related to Aq, the initial activity by an equation analogous to the law of radioactive decay  

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Figure 17.3—Counting system, a) Device used to measure the activity of a low-energy radioisotope using the method of two coincident detectors. A single ft emission can produce hundreds of photons. It is thus possible to measure photons in opposite directions using two photomultiplier tubes (PMT). Counting only occurs if both PMTs produce a signal that is not offset by more than a few nanoseconds b) device involving a PMT in a counting well used to measure luminescence produced by a sample that has been mixed with a scintillation cocktail (in aqueous or non-aqueous media).

The noble gases dissolved in water, as well as the halogens and hydrogen, carbon, nitrogen, and oxygen (as elements or in a number of compounds) can be flushed from the solution with an inert gas for collection. Radon is carried from previously sealed radium-containing solutions to measure the activity of its Ra parent (Curtiss and Davis 1943), as discussed in Section 6.4.1. Fission-produced xenon and krypton radioisotopes and neutron-activated Ar are flushed from samples of reactor coolant water, collected as the gas, and counted. Similarly, in the form of dissolved CO2 or CO is flushed with an inert gas through the distillation system and collected in NaOH or NaaCOs solution. Mercury salts can be reduced to the metal, processed by steam distillation, and collected as liquid mercury on a cool surface. 

Naturally occurring radioactive elements can be determined by measuring the activity of the appropriate radioisotopes. These measurements are particularly suitable for radioactive elements with short half-lives. 

When an element has more than one radioisotope, determinations and data analysis are generally more complex because the isotopes may differ in half-life, especially when a series is involved, e.g., radium, thorium, polonium, radon, actinium, protactinium, and uranium. One possibility is to make measurements after the decay of the short-lived radionuclides, but this may require long waiting times. In favorable cases, it is more convenient to measure the activity of decay products (e.g., radon, thoron ( Rn), actinon ( Rn)), or correct the measurements of the short-lived radioisotopes after determination of the isotopic composition. 

Natural radioactive elements can be determined by measuring the activity of appropriate radioisotopes. 

Apart from the activity ratios of the radon-222 decay product radionuclides, the residence time of tropospheric aerosols can be derived from the activity ratios of the fission product radionuclides released into the atmosphere during the explosions from nuclear weapons testing or nuclear reactor accidents, such as Sr/ Sr and " Ba/ Sr. These nuclide ratios are considered as nuclear clocks. The applicability of the radionuclide ratios depends on whether steady-state conditions hold at the time and place of measurement and on the kind of sample, whether surface air or precipitation (rain or snow), used for the radioisotope activity determination. 

Occupational exposure due to radioiodine occurs in the nuclear industry, in nuclear medicine and in research. One common exposure is due to a short lived radioisotope (half-life 8 d) which decays with the emission of both beta particles (average energy for main emission 0.19 MeV) and gamma radiation (main emission 0.36 MeV) [43], Iodine is rapidly absorbed into the circulation following inhalation or ingestion, is concentrated in the thyroid, and is excreted predominantly in urine [34, 36]. Thus, after an intake, may be detected directly by measurement of activity in the thyroid, or indirectly in urine samples. 

Radioactivity in environmental waters can originate from both natural and artificial sources. The natural or background radioactivity usuaUy amounts to <100 mBq/L. The development of the nuclear power industry as weU as other industrial and medical uses of radioisotopes (qv) necessitates the deterrnination of gross alpha and beta activity of some water samples. These measurements are relatively inexpensive and are useful for screening samples. The gross alpha or beta activity of an acidified sample is deterrnined after an appropriate volume is evaporated to near dryness, transferred to a flat sample-mounting dish, and evaporated to dryness in an oven at 103—105°C. The amount of original sample taken depends on the amount of residue needed to provide measurable alpha or beta activity. 

Another application involves the measurement of copper via the radioisotope Cu (12.6-hour half-life). Since Cu decays by electron capture to Ni ( Cu Ni), a necessary consequence is the emission of X rays from Ni at 7.5 keV. By using X-ray spectrometry following irradiation, sensitive Cu analysis can be accomplished. Because of the short range of the low-energy X rays, near-surface analytical data are obtained without chemical etching. A combination of neutron activation with X-ray spectrometry also can be applied to other elements, such as Zn and Ge. 

Thin layer activation (TLA) has a long experience in monitoring or measuring wear and erosion. A small quantity of radioisotope tracer is introduced into the metal surface which can be either a coupon or component. Metal loss due to corrosion (provided the corrosion product is non-adherent) can be detected remotely with high sensitivity . 

Other radioisotopes known to be produced by cosmic rays include Be, H, Na, Be, and Of these Be, P, and P have activities that are high enough to be measured in rainwater. In several instances, notably 0 and Be, these radioactive elements are useful as tracers. 

The radioligand should also have a high specific activity so that very small quantities of bound ligand can be accurately measured. The specific activity, simply defined as the amount of radioactivity, expressed in becquerels (Bq) or curies (Ci) per mole of ligand, is dependent on the half-life of the isotope used and on the number of radioactive atoms incorporated into the ligand molecule. A radioisotope with a short half-life decays rapidly so that many disintegrations occur in unit time,... 

Sharama et al. [479] compared results obtained in the determination of cobal-amins in ocean waters by radioisotope dilution and bioassay techniques. These workers showed that the isotopic methods measured both biologically active and inactive cobalamins indiscriminately when porcine factor was used as the B12-specific binder. 

A mixture is being assayed by radioisotope dilution analysis. 10 mg of the labelled analyte (0.51 pCi mg-1) was added. 1.5 mg of the pure analyte was separated and its specific activity measured and found to be 0.042 pCi mg1. What was the amount of analyte in the original sample ... 

Electrophoresis is one of many electromigrational separation techniques which include isotachophoresis, immunoelectrophoresis and isoelectric focussing that have been used to separate various species on the basis of their different mobility in an electric field. These techniques can be used not only to achieve separations but also it is possible to identify the ligand bound to the metal. This can be done by comparing the isoelectric points, immunological behaviours, extent of mobilities or step heights of the sample constituents with those of well-characterised standards. A difficulty, however, is in the determination of the metal constituent itself. Except in the case of radioisotopes, the activities of which can be easily measured, non-radioactive elements can be detected only after further separation steps. 

In these types of isotopic analysis, the same compound as that to be measured is used (element or molecule) where one of the atoms in it has been replaced by a radioisotope to allow radioactivity measurements. A small, precisely known quantity of the labelled compound, called the tracer, is added to the sample and, after homogenisation, an aliquot of the spiked sample is isolated by a fractionation technique such as recrystallisation or chromatography. The specific activity of the tracer is measured before and after fractionation. 

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