Gas-flow counters

Radiometric analysis using a plastic scintillator connected to coincidence and anticoincidence circuits (e.g. a Picobeta counter) can be used to detect "Tc. The detection limit is estimated to be about 10 mBq. A slightly better detection limit (1-5 mBq) is reported for a gas flow counter with an anticoincidence shield [8],... 

Ballestra et al. [32] described a radiochemical measurement for determination of "technetium in rain, river, and seawater, which involved reduction to technetium (IV), followed by iron hydroxide precipitation and oxidation to the heptavalent state. Technetium (VII) was extracted with xylene and electrode-posited in sodium hydroxide solution. The radiochemical yield was determined by gamma counting on an anticoincidence shield GM-gas flow counter. The radiochemical yield of 50 to 150 litre water samples was 20-60%. 

Surface radioactivity was determined with an end-window, gas-flow counter equipped with a 100-200 nm thick Formvar window supported by a thin, perforated aluminum sheet as described earlier (12). The counter was calibrated with a spread monolayer of 3H oleic acid, the radioactivity of which was determined also in bulk. 

The determination of the volatile radioactivity in urine, feces and expired air can be performed within the framework of the radiokinetic study (for instance with the animals provided for the 24 h sample). The air of the metabolism cage is continuously extracted, therefore, (for instance with 3.5 L/min) and measured using a gas flow counter (Exhalameter, Raytest, Strauchenhardt, Germany). The study period is approx. 24 hours. 

C-Carbon is now utilized more often in accordance with the development of new labeling techniques which involve so-call i natural labeling, i.e. replaces by isotopic exchange in molecules of the proteins to be labeled. The application of is more convenient than tritium labeling. Radiocarbon may be measure by various gas flow counters and semiconductor detectors, but liquid scintillation counting is still preferred. 

In practice the gas is allowed to react with base and the carbonate formed is precipitated as barium carbonate. This precipitate may be collected by filtration and its radioactivity determined by means of a gas flow counter. 

Figure 3-23. A gas flow counter. (Courtesy of Amersham Searle Corp.)... 

The reactants enter in the annular space between an outer insulated tube and an inner tube containing the catalyst. No reaction takes place in the annular region. Heat transfer between the gas in this packed-bed reactor and the gas flowing counter currently in the annular space occurs along the length of the reactor. The overall heat-transfer coefficient is 5 W/ K. Plot the conversion and temperature as a function of reactor length for the data given in... 

All samples were counted with a Berthold LB 770 gas-flow counter in the alpha/beta mode, meaning both alpha and beta activities were measured in the same run. 

The gross-alpha and gross-beta activities were measured separately with a gas flow counter there were no alpha-to-beta spill-over issues. 

All samples were evaporated to dryness on a stainless steel planchette and counted with a gas flow counter. 

The most often used methods include the 2n a-counting method, 47t P-counting with a 47t gas flow counter, 4jt P-counting with a liquid scintillation spectrometer, and the 471 P-y coincidence counting method. 

Radioactive counting Fractions containing or label may be counted either in a low-background gas-flow counter or in a scintillation counter. Only the latter method is sufficiently sensitive to deal with the weak )3-emission of compounds. 

Slices of gel can be dried on adhesive labels and transferred to planchets for counting in a gas-flow counter. Loening (1968) describes a method for processing a large number of samples. Slices are dried on 16 mm cine film, placing one slice on every other frame. The film is fed under a Geiger tube using a cine camera or projector mechanism and the counts for each slice are thus printed out. 

Some counters have two windows in line. The beam from the crystal passes through a gas-flow counter, with two side windows opposite one another, and then strikes a scintillation counter. By collecting pulses from both counters, high sensitivity to both long and short wavelengths is achieved. 

First, gas flowing counter-currently to the feed material in the nip causes particulate solids to alternately fluidize and flow. Figure 247 shows " schematically the phenomena air escaping upwards aerates the still loose material above the nip so that the powder s condition becomes similar to that... 

Methods. Surface pressure was measured on a Cahn RG electrobalance using the Wilhelmy plate method. The surface radioactivity was measured with a gas flow counter (2, 3). Oligomers were spread with an Agla syringe from a benzene solution. Buffered solutions (tris HC1 10 1M) were used to prepare the subphase, and the enzyme was dissolved in the support. Adsorption was studied without stirring the bulk phase. The Ca2+ (CaCl2) concentration in the subphase, essential for a correct enzymic activity (18), was fixed at 5 mM. 

Radioactivity of samples relative to P-radiation was measured by gas-flow counter, MCT-17. Measurement precision was + 5% and the data reproducibility 30%. 

Gas counters of this type are called gas-flow counters. Their name comes from the fact that the gas flows continuously through the counter during operation. This is necessary because the detector cannot be sealed if the source is placed inside the chamber. 

Counting with gas-flow counters involves the following steps ... 

Gas-flow counters may operate as proportional or GM counters. In fact, there are commercial models that may operate in one or the other region depending on the voltage applied and the gas used. In a proportional gas-flow counter, the gas is usually methane or a mixture of argon and methane. In the GM region, the gas is a mixture of argon and isobutane. 

In some gas-flow counter models, there is provision for placing a very thin window between the sample and the sensitive volume of the counter to reduce the effects of slight contamination of the sample well or of static charges that interfere with the measurement. In the counter of Fig. 5.24, the thin window will be placed on top of the sample well. A different arrangement is shown in Fig. 5.25. 

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