Flow proportional counters

It has always been difficult to do quantitative work with the characteristic x-ray lines of elements below titanium in atomic number. These spectra are not easy to obtain at high intensity (8.4), and the long wavelength of the lines makes attenuation by absorption a serious problem (Table 2-1). The use of helium in the optical path has been very helpful. The design of special proportional counters, called gas-flow proportional counters,20 has made further progress possible, and it is now possible to use aluminum Ka (wavelength near 8 A) as an analytical line (8.10). 

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. 

The tubes run cool when operated at capacity. No contaminant gives a line at intensity exceeding 2% of the strongest characteristic line from the target. (2) For reasons given in Chapter 2, the preferred detectors are scintillation and gas-flow proportional counters. (3) The com-... 

Gas-flow proportional counter, 55 in aluminum analysis, 217 in comparison with other detectors, 65-67... 

The basic function of the spectrometer is to separate the polychromatic beam of radiation coming from the specimen in order that the intensities of each individual characteristic line can be measured. In principle, the wide variety of instruments (WDXRF and EDXRF types) differ only in the type of source used for excitation, the number of elements which they are able to measure at one time and the speed of data collection. Detectors commonly employed in X-ray spectrometers are usually either a gas-flow proportional counter for heavier elements/soft X-rays (useful range E < 6keV 1.5-50 A), a scintillation counter for lighter elements/hard X-rays (E > 6keV 0.2-2 A) or a solid-state detector (0.5-8 A). 

Gas ionization detectors are widely used in radiochemistry and X-ray spectrometry. They are simple and robust in construction and may be employed as static or flow detectors. Flow studies have received attention in the interfacing of radioactive detectors with gas chromatographs. A radio-gas chromatograph (Figure 10.9) uses a gas flow proportional counter to monitor the effluent from the gas chromatography column. To achieve... 

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. 

Figure 11. fi-y Coincidence spectrometer utilizing gas flow proportional counters and 8-inch X 4-inch opposed Nal(Tl)... 

Systems Using Gas Flow Proportional Counters. Two types of gas flow proportional counters have been used with large area samples to... 

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... 

Detector copper—xenon-sealed proportional counter silver, gold, and lead—flow proportional counter using 10% methane and 90% argon. 

Figure 18.5 (a) Diagram of a 2tt gas flow proportional counter. The sample is introduced by... 

Gas-flow proportional counter (beta and alpha particles) 2,4, 6, 13 and 14... 

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. 

An important factor in the interaction of foreign surfaces with blood is the rapid adsorption of plasma proteins onto such surfaces when they are exposed to blood (4). For this reason the adsorption of radioactively tagged blood components on heparinized and unheparinized surfaces was measured. Proteins were dissolved in approximate physiological concentrations in a buffered (pH 7.35) physiological saline solution and the solutions were exposed to the test surfaces for 2 hours at 37 °C. in a static system. After the exposure, the surfaces were rinsed with physiological saline and distilled water and then dried. The amount of protein on the surfaces was determined in a 27r-gas flow proportional counter (7). As shown in Table III, although both heparinized surfaces were nonthrombogenic, there is no consistent pattern of either increased or decreased adsorption of the proteins caused by the heparinization. In-... 

The radiochemical method quantifies gross a activity utilizing either a gas flow proportional counter or a scintillation detection system following chemical separation. In the EPA radiochemical method, the uranium is co-precipitated with ferric hydroxide, purified through anion exchange chromatography, and converted to a nitrate salt. The residue is transferred to a stainless steel planchet, dried, flamed, and counted for a particle activity (Krieger and Whittaker 1980). 

The X-ray photons selected by the monochromator are detected and counted by a gas flow proportional counter and a measurement chain designed to amplify the impulses supplied by the counter and shape them for subsequent manipulation. 

The recommended procedure RP515 from U.S. D.O.E. proposes to count the membrane by gas flow proportional counter or by liquid scintillation. This approach was not followed. It was preferred to eluate the fixed strontium fi-om the disk using a solution of disodic EDTA. The reason of this approach was a concern for an eventual interference of the solid matrix (filter) in the scintillating cocktail. 

A specific type of proportional counter that is used for accurate counting of a and P activity on smear samples, is the gas flow proportional counter. In some types of gas flow proportional counters, the sample is put inside the detector for greater sensitivity—there is no structural material to absorb the radiation before it can be detected. Figure 5.24 shows a diagram of the detector of such a gas flow proportional counter. Other gas flow proportional counters have very thin Mylar windows sealing off the detector s gas chamber. During counting, samples are positioned very close to the window. 

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

In cases of radioactivity measurements of an a-emitter such as Am or uranium source electrodeposited onto a metal disk, a simple 2k a counting with a gas flow proportional counter is practical, and a reasonable accuracy can be expected. If the applied potential to the proportional counter is set at the a-plateau region, the counter responds only to a-particles. Since the backscatter of a-rays is small (a few percent) compared with that of P-rays, and the value can be estimated as a function of the atomic number of the backing material, the radioactivity, (, can be readily obtained from the observed counting rate, n, by the following simple relation ... 

The classical method of 4ji p counting with a 4jt gas flow proportional counter is still useful for the absolute measurements of P-emitting nuclides provided that good sources with small self-absorption can be prepared. From the observed counting rate, after fundamental corrections for background and counting loss due to dead time, the radioactivity, n, can be calculated as... 

In x-ray spectroscopy (Chap. 15), but not in diffraction, there is a need to measure soft x-rays of wavelength about 5-20 A. Because ordinary windows would almost totally absorb such radiation, thin sheet plastic is used as a window, so thin that it leaks. To allow for this, a stream of counter gas is continuously passed through the counter, which is then called a gas-flow proportional counter [G.29, 7-10]. 

The scintillation counter (Sec. 7-7) and the sealed gas proportional counter (Sec. 7-5) are both used in spectrometry. The scintillation counter is better for the very short wavelength region because of its greater efficiency there (Fig. 7-12) in the 1 A-2 A range either counter is suitable. In the long-wavelength region a gas-flow proportional counter is required, because of its thin low-absorption window. 

We applied this technique to the study of photolytic reactions in solid potassium tris(oxalato)ferrate(111) (12). Figure 4 compares the Mflssbauer spectra at 293 K of photoirradiated Kj[5 Fe(C204)3] 3H2O obtained by three types of MOssbauer measurements y-ray transmission spectrum, integral conversion electron spectrum(with the He-CH4 gas flow proportional counter), and depth-resolved conversion electron spectrum with the above spectrometer for 7.2-keV electrons. The spectra in Figs. 4a, b, and c charac-... 

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