Measurement of Radioactivity

Since most of the radioisotopes used in biochemical research are j8 emitters, only methods that detect and measure /3 particles will be emphasized. Two counting techniques are in current use, scintillation counting and Geiger-Miiller counting. 

The scintillation process, in detail, begins with the collision of emitted j8 particles with solvent molecules, S (Equation 6.10). 

Contact between the energetic /3 particles and S in the ground state results in transfer of energy and conversion of an S molecule into an excited state, S. Aromatic solvents are most often used because their electrons are easily promoted to an excited state orbital (see discussion of fluorescence, Chapter 5). The / particle after one collision still has sufficient energy to excite several more solvent molecules. The excited solvent molecules normally return to the ground state by emission of a photon, S --- S + hv. Photons 

Diagram of a typical scintillation counter, showing coincidence circuitry. 

Energy is transferred from S to Fj, resulting in ground state S molecules and excited Fj molecules, F. F molecules are fluorescent and emit light of a longer wavelength than S (Equation 6.12). 

The amount of radiation that an individual encounters can be measured by a film badge. This badge contains a piece of photographic film in a light-proof holder and is worn in areas where radiation might be encountered. The silver grains in the film will darken when exposed to radiation. The badges are processed after a predetermined time interval to determine the amount of radiation the wearer has been exposed to. 

A scintillation counter is used to measure radioactivity for biomedical applications. A scintillator contains molecules that emit light when exposed to ionizing radiation. A light-sensitive detector counts the flashes and converts them to a numerical readout. 

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The modem ionization chamber, called a dose caUbrator in this appHcation, is capable of linear measurements of radioactivity having a precision in the range of several percent coefficient of variation over a range of 370 kBq (10 -lCi) to at least 370 GBq (10 Ci). This extraordinary range is the chief advantage of this instmment. It may only be used when the sample is known to have only a single isotope. It has no capacity to distinguish radiation from different isotopes. 

In addition to the main general methods of analysis outlined above there are also certain specialised techniques which are applied in special circumstances. Among these are X-ray methods, methods based upon the measurement of radioactivity, mass spectrometry, the so-called kinetic methods, and thermal methods. 

Radioactivity. Methods based on the measurement of radioactivity belong to the realm of radiochemistry and may involve measurement of the intensity of the radiation from a naturally radioactive material measurement of induced radioactivity arising from exposure of the sample under investigation to a neutron source (activation analysis) or the application of what is known as the isotope dilution technique. 

Much of what we learned about the U, U and Th decay chain nuclides as chronometers and process indicators we owe to these seminal studies based on the measurement of radioactivity. 

Watson WS, Sumner DJ. 1996. The measurement of radioactivity in people living near the Doumeay Nuclear Establishment, Caithness, Scotland. Int J Radiat Biol 70(2) 117-130. 

Methods Based on Measurement of Radioactive Radiation or Amount of Radioactive Isotope or Daughter Isotope in Materials ... 

SC-18 Standards and Measurements of Radioactivity for Radiological Use SC-24 Radionuclides and Labeled Organic Compounds Incorporated in Genetic Material... 

Often the expected change in 210Pb concentration with depth (obtained from the logarithmic plot of unsupported activity as a function of the overlying mass of dry sediment accumulated) shows variations which are outside the analytical errors expected from the measurement of radioactive decay. Possible reasons for these discrepancies are ... 

Prescribed Procedures for Measurement of Radioactivity in Drinking Water... 

Disintegrations per minute (dpm) The unit measurement of radioactive decay rates. 

Measurement of Radioactivity by Liquid Scintiiiation in Organic Soiutions... 

The only method that can be used routinely to identify organoas-tatine compounds is measurement of radioactivity based upon its distribution over two or more phases. Such techniques are gas-liquid chromatography (GLC), high-pressure liquid chromatography (HPLC), thin-layer chromatography (TLC), and electrophoresis. 

Compounds and materials, including mixtures, which can undergo radioluminescence and are used in the measurement of radioactivity. Characteristics of these substances include a high fluorescence quantum efficiency and a short fluorescence lifetime. 

In 1990, Nakamura et al. reported for the first time the complete biosynthetic origin of the whole carbon skeleton of carbazomycin B isolated from lower plants (378). Based on feeding experiments with and C-labeled compounds, followed by measurement of radioactivity and C-NMR spectroscopy, it was shown that L-tryptophan (408) contributes to C-3 and C-4 of the hexasubstituted aromatic ring, in addition to the indole ring, indicating tryptophan as the progenitor of carbazomycin B (261), in contrast to Chakraborty s proposal of 2-methylcarbazole. The indole part of L-tryptophan (408) is formed by incorporation of two carbons from phosphor-ibosyl diphosphate (430), with loss of the anthraniiic acid (397) carboxyl. The... 

Exposure to thorium can be determined by measurement of radioactive thorium and/or daughters in the feces, urine, and expired air. The primary route of excretion of thorium is in the feces following either inhalation or oral exposure. Fecal excretion is essentially complete in a matter of several days (Patrick and Cross 1948 Scott et al. 1952 Sollman and Brown 1907 Wrenn et al. 1981). The measurement of external gamma rays emitted from thorium daughters present in the subject s body and of thoron in the expired air many years following exposure can be used to estimate the body burden of thorium (Conibear 1983). 

The examples for radioactive labeling by phosphorus-32 ( P) and iodine-125 ( 1) in this chapter were chosen for two reasons on one hand, they are relatively easy to do, and on the other hand, the measurement of radioactivity is simple. is counted in water in a liquid scintillation counter by measuring the Cerenkov radiation and is measured in a gamma counter. Both isotopes may be detected also by autoradiography. A further advantage of both isotopes is their short half-life, which eases the disposal of nuclear waste. 

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