Gas flow counting

Gas-flow counting is a method for detecting and quantitating radioisotopes on paper chromatography strips and thin-layer plates. Emissions are measured by interaction with an electrified wire in an inert gas atmosphere. AH isotopes are detectable however, tritium is detected at very low (- 1%) efficiency. 

Assay of Radioactive Compounds. The radioactive samples were counted on steel planchets in a Nuclear Chicago Model D-47 low-background gas-flow counting chamber with an absolute counting efficiency (estimated by comparison with a standard) of about 20%. 

Gas-flow counting -for radioactive tracers [RADIOACTIVE TRACERS] (Vol 20)... 

Figure 3-27. Planchet—the device used to hold samples for gas flow counting. Plan-chets may be made of copper, aluminum, or stainless steel and are usually 3 cm in diameter and 0.3 cm deep. (Courtesy of Sigma Chemical Co.)...

Figure 3-41. Determination of the optimal operating voltage (arrow) within the Geiger counting region for gas flow counting.

Figure 3-42. Correction of experimental data obtained by gas flow counting for the dead time of the instrument.

The detection and quantification of y-radiation is accomplished by well coimting in a scintillator, the sample being contained in a well or drilled hole. y-Radiation is often measured by y-ray scintillation or semiconductor spectrometry. Gas flow counting is a method for detecting and quantifying radionuchdes on paper chromatographic strips and thin-layer plates. 

The low energy j3-rays emitted by can be counted in a number of different ways, including gas flow counting, liquid scintillation counting and autoradiography on photographic emulsions. The particular method chosen depends on the nature of the sample. 

Autoradiography is particularly well suited for determining the distribution of radioactivity in tissue. In principle, the distribution of radioactivity in a tissue could be assessed by gas flow counting, if the tissue was dissected, its parts weighed and uniformly spread as a dry film on a degassed planchet. However, very often it is difficult to identify exactly the part of the tissue that has been dissected. Furthermore, the fluids which surround the tissue in the body may often be very highly labelled and will contaminate the dissected specimen. The use of autoradiography readily overcomes these difficulties . 

Radioactivity on paper or thin-layer plates can be determined by gas-flow counting in scanners, by direct counting of paper pieces or silica gel in liquid scintillation counters, or by autoradiography (12). 

Figure 8.28 shows how the X-rays fall on the solid or liquid sample which then emits X-ray fluorescence in the region 0.2-20 A. The fluorescence is dispersed by a flat crystal, often of lithium fluoride, which acts as a diffraction grating (rather like the quartz crystal in the X-ray monochromator in Figure 8.3). The fluorescence may be detected by a scintillation counter, a semiconductor detector or a gas flow proportional detector in which the X-rays ionize a gas such as argon and the resulting ions are counted.

Alpha counting is done with an internal proportional counter or a scintiUation counter. Beta counting is carried out with an internal or external proportional gas-flow chamber or an end-window Geiger-MueUer tube. The operating principles and descriptions of various counting instmments are available, as are techniques for determining various radioelements in aqueous solution (20,44). A laboratory manual of radiochemical procedures has been compiled for analysis of specific radionucHdes in drinking water (45). Detector efficiency should be deterrnined with commercially available sources of known activity. 

Early measurements of " Th were on seawater samples and Th was co-precipitated from 20-30 L of seawater with iron hydroxide (Bhat et al. 1969). This procedure may not recover all of the " Th in the sample, and an alpha emitting Th isotope (e g., °Th or Th) is added as a yield monitor. Following chemical purification of the Th fraction by ion exchange chromatography, the Th is electrodeposited onto platinum or stainless steel planchets. The planchets are then counted in a low background gas-flow beta detector to measure the beta activity and subsequently with a silicon surface barrier detector to determine the alpha activity of the yield monitor. The " Th activity is thus determined as ... 

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%. 

Radioactivity of uranium can be measured by alpha counters. The metal is digested in nitric acid. Alpha activity is measured by a counting instrument, such as an alpha scintillation counter or gas-flow proportional counter. Uranium may be separated from the other radioactive substances by radiochemical methods. The metal or its compound(s) is first dissolved. Uranium is coprecipitated with ferric hydroxide. Precipitate is dissolved in an acid and the solution passed through an anion exchange column. Uranium is eluted with dilute hydrochloric acid. The solution is evaporated to near dryness. Uranium is converted to its nitrate and alpha activity is counted. Alternatively, uranium is separated and electrodeposited onto a stainless steel disk and alpha particles counted by alpha pulse height analysis using a silicon surface barrier detector, a semiconductor particle-type detector. 

Elemental composition U 88.15%, O 11.85%. The compound is digested in nitric acid and alpha activity is measured by a gas-flow proportional counter, alpha scintillation counter or other counting instrument. 

The plutonium solution concentration was determined by alpha counting 0.5-ml. aliquots evaporated to dryness on planchets. Gas flow internal proportional counting was used with 90% argon-10 % methane gas and Nuclear Measurement Corp. counters. 

The gas flow proportional counter shown in Figures 13, 14, and 15 was developed primarily for use in an x-ray-7 coincidence system. The counting chamber of the detector is 2-inch o.d. X 5-inch long and is used... 

Figure 15 ICP-MS background spectra (plotted on a logarithmic scale) for a sample containing 0.1% nitric acid (a) Conventional plasma conditions (1000 W, 0.77 L/min nebulizer gas flow rate). The Ar+ and 0+ signals are saturating the detector, so their signals are greater than 2 x 109 counts/sec. (b) Cold plasma conditions (600 W, 1.08 L/min nebulizer gas flow rate). Note that full scale in (a) is 10l° whereas in (b) it is 108. (From Ref. 147.)...

In Part 2A, the student will calibrate a gas-flow, end-window, anti-coincidence proportional counter for beta-particle counting efficiency as function of energy with certified standard solutions, and perform quality assurance (QA) counting tests. 

Beta particle calibration sources span energies from about 100 to 3,000 keV for proportional counters, and down to a few keV for liquid scintillation counters. In this experiment, a low-background, gas-flow, end-window proportional counter with automatic sample changer for alpha- and beta-particle counting is calibrated. Beta-particles sources are counted with pulse-height discrimination to eliminate interference from alpha particles the discriminator may be turned off when no alpha particles are present. 

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