Lead, radioactivity measurement

Radioactivity measurement The radioactivities of lead isotopes and their decay products were measured with TRICARB 3380 liquid scintillation counter (Packard Inc.). The radioisotopes concerned, and their decay charateristies are shown in Figure 2. In the case of the direct method, the absolute radioactivity can be obtained by the integral... 

Since we know that A = eAN where A is the measured radioactivity, A is the decay constant, N is the number of radioactive nuclei present, and e is a constant representing the detection efficiency, we know that just a few radioactive nuclei need to be present to give measurable activities. Use of activation analysis can lead to measurement of elemental abundances of the order of 10-6-10-12 g. The actual detection... 

Each mixture of resin and neptunium sulfate solution was mixed by mechanical shaking and was allowed to equilibrate overnight. This time interval is long, compared to the duration of an analytical procedure. The liquid was then filtered away from the resin, and the residual radioactivity of a measured aliquot of the liquid was compared to an identical solution that had not been added to any anion exchange resin. The radioactivity measurements were performed in an end-window Nal crystal counter shielded with lead. The acidity of each solution was measured by titration with a standard base. 

As it is impossible in most cases to establish a quantitative differentiation between radioactivity and chemiluminescence pulses measured at the same time, this often leads to wrong results in radioactivity measurements. This applies particularly to measurements of tritium because, as will be shown below, the pulses from chemiluminescence reactions are... 

Lead isotope ratios can provide analytical information about sources of lead contamination whereas lead concentration measurements cannot. Due to this fact, studies of the isotopic composition of lead are commonly used in environmental science as well as in geological and anthropological studies. Among all of the naturally occurring lead isotopes only °" Pb is nonradiogenic, whereas ° Pb, ° Pb, and ° Pb are daughter products from the radioactive decay of and Th, respectively. This fact... 

Also of far-reaching consequences was the idea (Hevesy and Paneth 1913) to use radioisotopes as indicators or tracers in chemical, physical, and biological studies because radioactive and stable isotopes are chemically indistinguishable. Hevesy and Paneth determined the solubility of sparingly soluble lead salts by labeling the samples with radioactive lead and measuring the radioactivity in the solution after equilibration. The result can be obtained by other techniques too, but not in such a simple way as with radioactive indicators. This holds for most indicator applications. 

Some tonnes of raw material had been treated. The amount precipitated by electrolysis was 0.002 g, with an estimated quantity 0.0001 g of a new element. This new element was given the name polonium (Po) after Marie s native country. Polonium was the first element whose discovery was based on radioactive measurements. The isotope found in Curie s and Debierne s investigations was °Po with a half-life of 138 days. In nature polonium exists only in equilibrium with its mother substances in different decay series. If separated from its environment, it disintegrates to lead. 

For the naturally occurring elements, many new artificial isotopes have been made, and these are radioactive. Although these new isotopes can be measured in a mass spectrometer, this process could lead to unacceptable radioactive contamination of the instrument. This practical consideration needs to be considered carefully before using mass spectrometers for radioactive isotope analysis. 

In NAA the sample is made radioactive by subjecting it to a high dose (days) of thermal neutrons in a reactor. The process is effective for about two-thirds of the elements in the periodic table. The sample is then removed in a lead-shielded container. The radioisotopes formed decay by B emission, y-ray emission, or X-ray emission. The y-ray or X-ray energies are measured by EDS (see Chapter 3) in spe-... 

The sequences of radioactive decays that lead to lead are well-known and the rates of decay have been carefully measured. We shall consider the sequence based upon the relatively slow decomposition of the most abundant uranium isotope, mass 238 (natural abundance, 99%) ... 

The role of radionuclides as tracer of the chemical transport in river is also reinforced by the fact that each of the U-Th-Ra elements has several isotopes of very different half-lives belonging to the U-Th radioactive series. Thus, these series permit comparison of the behavior of isotopes of the same element which are supposed to have the same chemical properties, but very different lifetimes. These comparisons should be very helpful in constraining time scales of transport in rivers. This was illustrated by Porcelli et al. (2001) who compared ( " Th/ U) and ( °Th/ U) ratios in Kalix river waters and estimated a transit time for Th of 15 10 days in this watershed. The development of such studies in the future should lead to an important progress in understanding and quantifying of transport parameters in surface waters. This information could be crucial for a correct use of U-series radioactive disequilibria measured in river waters to establish weathering budgets at the scale of a watershed. 

After each series of experiments with beams of various intensity the section plate would be removed from the cell and disassembled, with radioactive silver washed out by nitric acid. Radioactivity of the solutions obtained was measured by a multichannel spectrometric scintillation y-counter with sensitivity of up to 10 G, i. e. around 10 of atoms which, according to calculations, is 10 times lower than sensitivity of ZnO sensor 10 G or 10 of Ag atoms respectively [28]. This difference in sensitivity lead to great inconveniences when exposing of targets was used in above methods. Only a few seconds were sufficient to expose the sensor compared to several hours of exposure of the scintillation counter in order to let it accumulate the overall radioactivity. It is quite evident that due to insufficient stability during a long period of exposure time an error piled up. 

Many scientists thought that Earth must have formed as long as 3.3 billion years ago, but their evidence was confusing and inconsistent. They knew that some of the lead on Earth was primordial, i.e., it dated from the time the planet formed. But they also understood that some lead had formed later from the radioactive decay of uranium and thorium. Different isotopes of uranium decay at different rates into two distinctive forms or isotopes of lead lead-206 and lead-207. In addition, radioactive thorium decays into lead-208. Thus, far from being static, the isotopic composition of lead on Earth was dynamic and constantly changing, and the various proportions of lead isotopes over hundreds of millions of years in different regions of the planet were keys to dating Earth s past. A comparison of the ratio of various lead isotopes in Earth s crust today with the ratio of lead isotopes in meteorites formed at the same time as the solar system would establish Earth s age. Early twentieth century physicists had worked out the equation for the planet s age, but they could not solve it because they did not know the isotopic composition of Earth s primordial lead. Once that number was measured, it could be inserted into the equation and blip, as Patterson put it, out would come the age of the Earth. ... 

The refrigerator itself is shielded with a 20 cm thick low activity lead and a 10 cm thick borated PET. Nitrogen is fluxed between the external lead shield and the cryostat to avoid any Rn contribution to the detector background. Measurements of residual radioactivity have been carried out on several Pb samples [103], For the roman lead, a contamination lower than 4 mBq/Kg (in 210Pb) is to be compared with 250 mBq/Kg of a modem lead (Johnson Mattey). 

Vitamin Bi2 catalyzes incorporation of single carbon units, e.g., the S-linked methyl of methionine and the C-methyl of thymine it also participates in the redox reactions leading to deoxyribose (S10). The many techniques for measuring vitamin Bi2 activity fall into 2 classes (a) microbiological, and (b) radioactive. 

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