Gas flow proportional counting

This study emphasizes the measurement procedure of Sr activity in radioactive aqueous waste produced by the nuclear industry. Considering radioecological and radioprotective elements, there is a need for systematic measurements of Sr activity in rejected wastes. The classical methods for this determination are gas flow proportional counting, liquid scintillation counting or Cherenkov counting.These techniques require efficient concentration and purification steps before the measurement of radiostrontium itself. 

Reactor pre-operational and operational surveys can be performed including collecting and analysing air, water and swipe (smear) samples. The experiments can include the use of counting equipment such as gas flow proportional counting systems, Nal and Ge or detector systems to identify radioisotopes. If no real activity or contamination exists, the decay products of radon will show up in high volume air samples. 

Historically, measurement of technetium-99 in environmental matrices has been carried out by radiometric methods such as liquid scintillation counting (LSC) and gas flow proportional counting (GFPC). ln order to successfiilly quantify " Tc by these techniques, analytical methods involve protracted purification schemes to isolate Tc from other beta emitters. Additionally, the measurement of environmental levels of Tc requires long count times. 

In-house analysis by accredited method using gas-flow proportional counting. Characterised by third party laboratory. 

A COMPARISON OF ANALYTICAL METHODS FOR SCREENING GROSS ALPHA AND BETA RADIOACTIVITY IN WATER BY LIQUID SCINTILLATION COUNTING AND GAS FLOW PROPORTIONAL COUNTING... 

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.

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 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 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... 

The quantity of radioactive strontium is typically determined by gas-flow proportional, liquid scintillation, and Cerenkov counting techniques (Scarpitta et al. 1999). The standard EPA analytical... 

Gas flow proportional counters exhibit higher sensitivity for beta-emitters, but also pure gamma-emitters like Tc are measured with high resolution. When placing a chromatogram into the counting chamber the developed strip must be completely dry. The main advantages over a conventional TLC scanner are ... 

Material around the source and detector, notably the detector housing, cause scattering into the detector. The opportunity for scattering into the detector increases when the source is more distant. This scattering adds a few percent to the count rate for end-window Geiger-Mueller (G-M) detectors when the sample is 2 cm or more distant (Zumwalt 1950), but little for gas-flow proportional counters with the sample only about 0.3 cm from a relatively large window. Scattering, attenuation. 

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. 

Similarly, planchets for automated gas-flow proportional-counter systems are stacked in plastic holders. The bottom holder is moved beneath the detector for counting and then removed to a second stack for storage or recycling. 

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. 

The BTMs are instruments deigned to detect and measure the tritium present in a mixture of radioactive gases. The BTM uses a gas-flow proportional counter tube with a volume of 1.3 liters. The proportional counter tube uses P-10 as tiie counting gas. P-10 is a mixture of 90 percent argon and 10 methane gas. The BTM detects tritium by using the rise-time... 

A minimum flux of 1 x n cm s is probably necessary to demonstrate the possibilities in trace element analysis. Simple materials such as pure aluminium, elemental iodine, pure CaCOs or other material may be irradiated and the resultant radioactive half-life measured on an available counting system such as a GM counter, gas flow proportional counter, Nal or Ge detector system. Such an introductory experiment is most appropriate following a lecture and tour of the facility. The tour should emphasize the capabilities and possible usage especially as related to the discipline involved. Frequently, prior to arrival, someone in the discipline (perhaps the class instructor or the students themselves) should be consulted to determine the type of application that would be of interest. This allows the reactor representative to relate to a potential use. 

Neutron flux and neutron spectrum measurements require an assortment of neutron activation foils and threshold detectors (e.g., Au, In, Cd, Al) as well as counting facilities such as gas flow proportional counters, Nal or HPGe detectors to any level of sophistication desired. 

The sensitivity of these instruments for a particular radioisotope depends on the nature of the detector. Open methane gas-flow proportional counters give the highest count yields and have the additional... 

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