Counters windowless

In these gas-flow proportional counters, the windows are exceedingly thin, fragile,. and unavoidably leaky. Such a window is satisfactory only if a steady flow of. the filling gas is maintained at minimum pressure differential against the helium atmosphere in the optical path. The purging of impurities from the counter is an incidental benefit derived from the gas flow. Hendee, Fine, and Brown20 look upon the gas-flow proportional counter as a steppingstone on the road toward a windowless counter. 

Wavelength shift of K absorption edge of sulfur, 37, 38 Weight-fraction , use, 164, 171 Window absorption, 45 Windowless counter tube, 55, 222 Windows, for electron beam, 177 for proportional counter, 55... 

By the late 1940s methods of ionization counting had been developed for the estimation of 14C. Thin layers of suitably prepared material could be counted in windowless counters, with the emissions being detected by the probe in a flow of inert gas. Otherwise 14C-containing material could be combusted to 14C02 and this drawn over the probe. Neither method was very convenient to use and the instruments were costly. Early experiments with 14C were mainly performed in the U.S. (see below). 

With respect to the influence of the factors S, B and W in eq. (7.3), determination of the absolute activity A of a and emitters may cause problems, and thin samples and windowless counters are preferable for these radiations. Thin samples are obtained by electrical or vapour deposition on thin metal sheets or thin polymer foils. By use of windowless 2n- or 47c-proportional counters the influence of W can be neglected. Determination of absolute activities of y-ray emitters involves fewer problems, because the influence of the factors S, B and IF is, in general, negligible. 

A widely used instmment for air monitoring is a type of ionization chamber called a Kaimn chamber. Surface contamination is normally detected by means of smears, which are simply disks of filter paper wiped over the suspected surface and counted in a windowless proportional-flow counter. Uptake of tritium by personnel is most effectively monitored by urinalyses normally made by Hquid scintillation counting on a routine or special basis. Environmental monitoring includes surveillance for tritium content of samples of air, rainwater, river water, and milk. 

Copper-zinc alloy, analysis, 177-179 Corrosion, of Monel metals, study by x-ray emission spectrography, 231 of stainless steels, study by x-ray emission spectrography, 230, 231 Counter, windowless, 55, 222 Counterfeit bank notes, histograms from, 225-227 Counters, see Detectors Counting, electronic, 46 Counting error, 65, 66 equation, 66, 278... 

Nilsson, G. (1957) The adsorption of tritiated sodium dodecyl sulfate at the solution surface measured with a windowless, high humidity gas flow proportional counter. /. Phys. Chem., 61, 1135—42. 

Windowless proportional counters are well suited to measure a and low-energy fi radiation. High and well-defined values of G in eq. (7.3) are obtained with 2n and An counters, which are shown in cross-section in Fig. 7.11. 

Apparatus. The following apparatus was utilized for this study a PRT 2000 type potentiostat (Tacussel) for the electrolysis a Graphi-spectral spectrophotometer (Jouan) for the absorption measurements a TS 6 type millivoltmeter (Tacussel) for potentiometric analysis a 2ir windowless flow gas counter (S.A.I.P.) for measuring the soft beta from and a bell type Geiger counter for Ge and Ge beta counting. 

Fig. 5.24. Diagram of windowless gas flow proportional counter used for a and P counting. The sample is...

The following data were obtained with a windowless, gas-flow planchet counter. 

This can be effected for example by evaporating a larger quantity of water. This does however involve the loss of the volatile a-emitters such as radon 222. The dry residue of a larger water sample can then be measured, for example using a windowless proportional counter, a scintillation counter or, to advantage, a large-surface counter. 

Proportional counter. Thin-window or windowless gas-flow proportional counter using CH4 or CH4 + Ar at atmospheric pressure as a counter gas is another choice for tracers emitting low-energy P rays. In the windowless one, the sample is placed inside a counter and the counter is operated after flushing by a counting gas. With sufficiently thin samples, the counting rate is close to 50% (2tc counter). The dead time of proportional counters is much shorter than that of the GM counters and the loss of counts is not serious either. 

Alpha rays are counted with the use of an ioniration chamber, a windowless gas-flow proportional counter, a liquid scintillation counter, or a Si barrier detector. Autoradiography is also effective for a particles. However, a-emitting nuclides are rarely used in ordinary tracer works. 

A convenient and fast method of locating S-labelled spots on thin-layer plates or on paper chromatograms is to pass the chromatogram under a windowless gas flow counter - . Radiochromatogram scanners of this type are commercially available and their design have been described - . However, for accurate determination of radioactivity the spot must be counted in a liquid scintillation counter. 

The above limitation of conventional tubes can be addressed by what is called a flow tube. Such a tube has entrance and exit ports through which gas is made to flow continuously. Even the window here is eliminated for greater sensitivity. This is done to allow measurement of those P-particles which have very low energies and cannot enter even a thin window. Moreover, other P -particles may also lose sufficient energy in penetrating the window. This will also not occur if the window is removed and the measurement will have great sensitivity. This modification is known as the windowless flow counter. 

Although, gasJlow counters which are windowless, can be used to measure soft P-emitters, scintillation counters are preferred for such isotopes. 

---