Beta proportional counting

Rosner, G., H. Hotzl, and R. Winkler. 1992. Determination of 241Pu by low level beta proportional counting, application to Chernobyl fallout samples and comparison with the 241Am build-up method. J. Radioanal. Nucl. Chem. 163 225-233. 

The Tc used in this work was analyzed directly by beta scintillation counting. Analyses of samples were carried out using a radiochemical separation procedure, with perrhenate as carrier, and beta proportional counting. Samples, which were always run in duplicate, generally showed a spread of <10%. 

In Part 2A, the student will calibrate a gas-flow, end-window, anti-coincidence proportional counter for beta-particle counting efficiency as function of energy with certified standard solutions, and perform quality assurance (QA) counting tests. 

Beta particle calibration sources span energies from about 100 to 3,000 keV for proportional counters, and down to a few keV for liquid scintillation counters. In this experiment, a low-background, gas-flow, end-window proportional counter with automatic sample changer for alpha- and beta-particle counting is calibrated. Beta-particles sources are counted with pulse-height discrimination to eliminate interference from alpha particles the discriminator may be turned off when no alpha particles are present. 

Scheme 2. Count the sample immediately with an a and (3 counter (e.g., the proportional counter) for 200 minutes. Repeat the count each day for 14 days or until the count rate equals or nearly equals the background. Obtain background counts for both alpha-particle and beta-particle counting modes. Subtract respective backgrounds for each count period and record in Data Table 8.7...

As discussed in Chapter 7, a source may also be deposited by volatilization or electrodeposition for alpha or beta particle counting by Si diode or proportional counter, or added as a solution or suspension to scintillation cocktail for liquid scintillation counting. For electrodeposition, as indicated by Eq. (3.20), the... 

The typical purification method for rare earths is coprecipitation with ferric hydroxide, dissolution in dilute acid, precipitation as fluoride in strong mineral acid solution, dissolution in strong nitric acid with boric acid to complex fluoride, and precipitation for counting as the oxalate in dilute acid solution (Stevenson and Nervik 1961). Because Pm has no stable isotope, another rare earth (such as lanthanum) is added as carrier. The " Pm precipitate can be counted with a proportional counter, or can be dissolved and measured with an LS counter because of the low beta-particle energy. If small amounts of the other rare earth radionuclides are detected by gamma-ray spectrometric analysis, the beta-particle count rate of Pm can be calculated by difference. 

The choice of the sample size is governed by the magnitude of the concentration of Pu activity and of the beta and gamma ray emitters in the original solution. The presence of high levels of radioactivity must be considered because of the health hazard involved and because a beta ray counting rate of over 10 counts per minute will interfere with the alpha counting on the Simpson proportional alpha counter. 

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. 

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

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. 

Gross alpha and gross beta activity can be determined by various radioactive counters, such as internal proportional, alpha scintillation, and Geiger counters. Radium in water can be measured by co-precipitating with barium sulfate followed by counting alpha particles. Radium-226 can be measured from alpha counting of radon-222. Various methods are well documented (APHA, AWWA, and WEF 1998. Standard Methods for the Examination of Water and Wastewater, 20 ed. Washington DC American Public Health Association). 

Low background gas-flow, end window proportional counter with automatic sample changer for alpha and beta counting (or equivalent counting system)... 

Step 1. Place 10 blank planchets in the proportional counter system and count each for 50,000 s at settings (a) and (b) to determine the beta-particle background count rate. 

Step 4. Disassemble the filtering apparatus and remove the filter with forceps. Fix the filter to a planchet with 2-sided tape. Count the sample three times with a proportional counter for alpha particles and beta particles for 3,000 s. Record mid-point of counting time. Record counting data in Data Table 7.1. Also measure detector background data for at least the same period and record in Data Table 7.1. 

Determine counting efficiency of the proportional detector in Step 5 for three 3,000-s periods to measure alpha particles and beta particles. Record in Data Table 7.2. Also perform overnight count (50,000 s) for alpha-particle spectral analysis of the planchet to identify the uranium isotopes and any other radionuclides and to determine their relative amounts from their alpha-particle energy spectra and record results in Data Table 7.2. Count alpha- and beta-particle background in proportional counter and alpha-particle spectral background in spectrometer for at least the same periods. 

Average proportional counter net a-particle and beta-particle rate (R) of three counts (alpha) cps (beta) cps... 

Step 7. Count the sample three times for 3,000 s each with a proportional counter for alpha and beta particles and record in Data Table 7.4. Record the time. 

Step 8. Evaporate 10 ml 0.5 M oxalic acid to about 2 mL and pour onto the second planchet with dried 100-A. sample of the initial uranium solution (see Part 7A, Step 5). Evaporate to dryness under the heat lamp. Flame the planchet as in Step 6. Count three times for 3,000 s each with proportional counter for alpha and beta particles. Record in Data Table 7.5... 

Step 13. Count the sample for beta particles with proportional counter for 6,000 s and record results in Data Table 10.1 Measure the background count rate for 6,000 s immediately before or after the sample measurement. To check radioactive decay rate, repeat counting under identical conditions every second or third day for at least two weeks. 

Solvent Extraction. The procedures used are described elsewhere (9, 10). Although preliminary experiments showed that equilibrium was established within two hours, all samples were mixed for 24 hours. Duplicate 1-ml. aliquots of the organic phase were counted either in a well counter (for gamma emission) or as dry samples in a 2tt proportional counter (for beta emission). 

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