Radiochemical proportional counter

Alpha counting is done with an internal proportional counter or a scintiUation counter. Beta counting is carried out with an internal or external proportional gas-flow chamber or an end-window Geiger-MueUer tube. The operating principles and descriptions of various counting instmments are available, as are techniques for determining various radioelements in aqueous solution (20,44). A laboratory manual of radiochemical procedures has been compiled for analysis of specific radionucHdes in drinking water (45). Detector efficiency should be deterrnined with commercially available sources of known activity. 

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. 

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). 

Gamma spectroscopy is a radiochemical measurement method that allows identification and quantitative determination of activity of radionuclides, which emit gamma radiation or x-rays. The equipment used in gamma spectroscopy includes an energy-sensitive radiation detector, such as semiconductors, scintillators or proportional counters, and a multichannel analyzer. The energies and the photon yields are characteristic for specific nuclides. 

Sr deserves special attention since it is released by nuclear power facilities, accumulates in organisms and replaces Ca (mainly in bones) and has a long half life. °Sr is a pure beta emitter and before measurement (liqiud scintillation counting, proportional counters, etc.) a laborious and time-consuming procedure for radiochemical separation has to be performed (IAEA, 1989). This is practically the best way for reliable determination of this radionuclide (IAEA, 1989). 

All chromatographic methods (liquid, liquid-liquid, liquid-solid, gas, gas-liquid, and gas-solid chromatography) are used in radiochemical analysis of food and environmental samples. Equipment for the evaluation of paper and thin-layer radiochromatograms is commercially available. The radiation detectors used vary according to the type and energy of the radiation emitted by the measured radionuclides. Use is made of ionization chambers, proportional counters, and Geiger-Miiller, scintillation, and semiconductor detectors. 

Since some detectors are unable to discriminate between radioisotopes, e.g. proportional counters, and others require a sample matrix clean-up, sample pretreatment is very often unavoidable. Thus, radionuclides are isolated before measurement to avoid the presence of other radionuclides and interfering elements, and therefore radiochemical methods usually include separation steps in their protocols. Apart from a limited number of radionuclides that can be analyzed by high-resolution gamma spectrometry, e.g. " Mn, °Co, " Cs, Cs, " Eu, Eu, and Sb, direct analysis is impossible, that is the sample needs to be previously chemically processed. Furthermore, if samples to be analyzed have low activities, e.g. environmental samples, a preconcentration procedure is mandatory. 

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