Counting gas

Tritium and argon-37 spectra are shown in Figures 11a and b. The spectra were collected using a 3 1 H2 CH4 counting gas mixture at... 

One of the best checks of the counting method is sample comparison with other laboratories and reproducibility in counting standards and background. Table III shows such comparison. The counting efficiency for the tritium window used is 76%. All samples and background were measured at 400 cm. Hg pressure, using a 3 1 H2 CH4 counting gas mixture. 

A gas proportional counter with an anticoincidence shielding counter has also been utilized. Ethane gas is commonly used as a counting gas. A water sample is reduced to hydrogen gas using Mg or Zn powder in a furnace. Hydrogen gas thus produced is added to dead ethylene or acetylene gas with a Pd catalyst to form ethane-counting gas. 

Figure 2-9. Ceiger-Muller counting tube. In the biochemistry laboratory, Ceiger-Muller counters are used mainly for counting high energy fS radiation. They consist essentially of a screened cylinder containing the counting gas, with a central anode which is a very fine wire, and the cathode is a metal layer on the inner...

Turn on the instrument and allow it to warm up for 20-30 minutes. Permit the sample chamber to purge with the counting gas mixture during this time. 

FIG. 8.8. Characteristics for GM and proportional counter tubes. The GM tube (a) is designed for flowing liquids. The proportional tube (b) uses a flowing counting gas. 

Counting gases consists usually of one of the noble gases mixed with a small amount of polyatomic gas. The latter makes the gas multiplication factor less dependent on applied voltage, and increases the speed of electron collection. Typical counting gas mixtures are... 

The high-voltage power supply, amplifier-discriminator, and scalers were homebuilt (5, 7). During the operation of the detector, propane counting gas was mixed with the helium carrier gas in a 4 1 ratio, and the total combined helium and propane flow rate was maintained at 125 cm min" (NTP). With 2 in. of lead shielding, the typical counter background level was 100-200 cpm. 

Tritium has also been counted as a gas or vapor mixed with the counting gas in a flow-through ionization or proportional counter. Flow-through detectors are suitable as effluent monitors for tritium when tritium concentrations in air are high relative to concentrations of other radionuclides. Discrimination between tritium and radionuclides with more energetic beta-particle groups is achieved by pulse-height control for the proportional counter. 

The first type is a manual detector, either with a thin (typically 80 qg/cm ) window beneath which the sample is placed, or without a window but with a sample tray that slides under the detector and becomes the detector bottom. Use of the internal proportional counter eliminates external attenuation of low-energy beta particles but introducing the source into the detector can cause contamination or reduce the potential difference to a value below the applied voltage. The detection volume may be cylindrical (typically about 1 cm high and 3 cm in radius), or may be a hemisphere in the classical 27rgroportional counter. For either a thin window or no window, continuous gas flow is maintained. A thicker window (typically 0.5 mg/cm ) can seal the counting gas in the detector. 

Modified versions of the gas proportional counter are used for spectral analysis of X rays in the energy region of a few kiloelectron volts. Detectors are constructed with a greater depth and a window with a low-Z material such as Be, and with a high-Z counting gas such as Xe at apressure of several atmospheres. The resolution is about 0.6 keV at 5 keV (Knoll 1989). 

Low-background alpha/beta particle gas proportional counters can be placed anywhere if a dedicated counting-gas cylinder is provided for each unit. If a central supply of counting gas is used, the units should be placed in direct access to the gas supply manifold. Liquid scintillation counters can be placed anywhere in the counting room because they can be operated as stand-alone instruments. 

A 98% conversion efficiency has been achieved at 1.5 atmospheric pressure of CO2 and 10% excess of water, while the yield was 85% only when stoichiometric amounts of water and carbon dioxide were used. No isotopic fractionation has been noted when 5 to 10% excess of water has been used in the reaction in equation 5. Doubly labelled methane has been used in simultaneous analysis of and both in the atmosphere and in environmental carbon dioxide as well as in analysis of and H-labelled organic compounds. Proportional counters filled with methane possess very good counting properties and there is no need to add any inactive counting gas. and tritium-labelled methanes have also been produced" by pyrolysis of sodium acetate in molten sodium hydroxide (equation 6) ... 

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