Gas-flow proportional counter, use

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

Gas flow proportional counters use a gas ionization detector that produces ionizations as the radiation penetrates the chamber. These are not portable detectors, but they can be used to measure some radiation energies and are able to distinguish between a and p radiation. 

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. 

This detector is a gas flow proportional counter using P-10 gas (90% Argon, 10% Methane). The detector uses a very thin window of 80 g/cm. The counter can accommodate samples up to 2" in diameter and up to 100 samples per batch. The operating instructions are posted near the counter. 

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). 

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. 

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... 

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. 

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. 

An absorption curve of a sample emitting p- and y-rays was taken wifli aluminum absorber using a gas-flow proportional counter. The data obtained were ... 

The radiation detection systems employed in radioanalytical chemistry laboratories have changed considerably over the past sixty years, with significant improvement realized since the early 1980s. Advancements in the areas of material science, electronics, and computer technology have contributed to the development of more sensitive, reliable, and user-friendly laboratory instruments. The four primary radiation measurement systems considered to be necessary for the modern radionuclide measurement laboratory are gas-flow proportional counters, liquid scintillation (LS) counters. Si alpha-particle spectrometer systems, and Ge gamma-ray spectrometer systems. These four systems are the tools used to identify and measure most forms of nuclear radiation. 

The most common detector is the gas-flow proportional counter, although ionization chambers and G-M counters can be usefully applied in the laboratory. Several types of gas-flow proportional counters are used in modern counting laboratories. 

The intensities in the x-ray spectrum were recorded with a gas-flow proportional counter at intervals of 0.07-0.05 X.U. A special attachment was used to cover the spectrum at these intervals (Fig. 2). Damage to the surface of a sample was avoided by moving it slowly (the electron probe was kept in a fixed position). 

Most of the radioisotopes used as isotopic labels in activation analysis decay with beta (positron and negatron) radiations and/or gamma rays. By convention, beta-emitting radionuclides are usually measured by gas-filled or gas-flow proportional counters or Geiger counters. Sometimes, liquid scintillation counters are used to complete a beta-ray measurement. The more conventional method for gamma-ray measurements involves the use of a gamma-ray spectrometer equipped with either a scintillation or solid-state detector. Stevenson (918) discusses the characteristics of radioactive decay and gives details on the methods and instruments used to detect emitted radiations. 

The escape peaks for the Si(Li) detector are generated by the same mechanism as in the gas-flow proportional counter and the NaI(Tl) scintillation detector. Section 4.2.3 should be consulted for a more detailed description of this mechanism. The Si(Li) detector has two important differences. First, since the detector is composed primarily of silicon, it is the escape of the silicon K x-rays that causes the escape peaks in the spectrum. Second, the pulse height spectrum is used for elemental analysis and usually contains a large number of lines. Each of these parent lines will have an escape peak associated with it. The escape peak from an element with high concentration can interfere with the analysis of trace amounts of an element of lower atomic number. 

A combination of the standard argon gas flow proportional counter and the scintillation counter is often used on general-purpose XRF instruments, using tandem operation to compensate for the reduced detection efficiency of each, individually, in the 6-10 keV region. 

WDXRF systems commonly use one or more of the following detectors gas flow proportional counter (flow counter [FC]), sealed gas-proportional counter, and scintillation counters (SC). 

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. 

The system used in conventional wavelength dispersive spectrometry generally consists of an X-ray tube, an analyzing crystal, and detector (scintillation or gas flow proportional counter) as shown in Fig. 1.9. 

---