Beta counting

Self-absorption. Beta particles have a continuous energy spectrum extending from zero energy up to a maximum energy max. K a flat beta source is measured with a thin absorber of thickness L between the source and detector, the count rate / is very closely represented by 

If filters have to be measured with a suspended load that is at least comparable to the filter mass, , has to be calculated with Eq. (13-2). The parameter aL has to be measured for each individual sample from measurements of a (strong) beta source with and without the filter in-between, using Eq. (13-1). As the Eqs. (13-1) and (13-2) only hold for the energy distribution of a single beta decay, all beta measurements have to be made with a cover removing the weak betas of Th. A cover of 30mg/cm, approximately equivalent to two overhead sheets, transmits 82 % of 234mpg jjjy 4 % qj Th betas. 

However, if the absorber has reproducible geometry and thickness, as in the case of a carefully folded filter with particle load filter mass, or in the case of a filter with a reproducible load of Mn02 precipitate (see Section 13.6.3.2), , will have a constant value, which can be determined with standard filters. Measurements can be made with a thin cover, improving counting statistics. 

Procedure. 20-50 L are filtered over 142 mm polycarbonate filters with 1/tm pore size. The filters are drained by suction, folded twice in two, air dried and carefully folded four more times to produce a 18 x 18 mm, 64 sheet thick package which is wrapped in thin (e.g., 0.01 mm) plastic (polyester or polyethylene) foil, and counted directly in a beta counter (count rate 7p). 

Blanks. Polycarbonate (Nuclepore) filters contain some Cs, which contributes to the blank. The count rate /u of blank filters (including instrument background) is typically around 0.S cpm, but can vary widely between batches and between pore sizes and definitely has to be checked in advance. 

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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. 

The counting techniques described in this paper are also readily applicable to studies of "hot radioactive waste (z.e.j radioactive waste from reprocessed nuclear fuel). With this type of material, the cesium can be analyzed as 30-y (662-keV y), the RE as 13-y Eu (964-keV and 1408-keV y), strontium as 28-y Sr (after chemical separation and beta counting), and the actinides by group separation and alpha counting. 

Harvey, C. O., Separation of technetium by cation exchange and solvent extraction prior to measurement by beta counting. PNNL Technical Procedure pnl-alo-432, 1993, Pacific Northwest National Laboratory, Richland, WA. 

Water (for Precipitate with barium sulfate Beta counting No data 94.2% APHA 1985c... 

The determination of Pn values is based on the beta saturation counting rate (cP ), the neutron saturation counting rate (C11 ), the beta-neutron coincidence saturation counting rate (dP ), the beta counting efficiency (eg), and the neutron counting efficiency (en). The usual relation for the delayed neutron emission probability is... 

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

Beta Count Rate Planchet under Setting (a)... 

Beta Count Rate Beta Count Rate... 

The different terms of Equation 1 were obtained as follows— /ce, formal potential of the Ce(IV)-Ce(III) couple in the medium, was taken from publications [Ce(IV)]a and [Ce(IV)]o have been measured by direct absorption spectrophotometry [Ce(III)] was calculated by difference between total cerium, titrated by potentiometry, and tetravalent cerium [Bk(IV)]a was calculated from the solvent beta counting, allowing for the measured distribution coefficient of Bk(IV) [Bk(III)] was determined by subtracting the [Bk(IV)]a value from the aqueous counting in all cases [Ce(III)]o and [Bk(III)]o were found to be negligible. 

Because the reduction rate of cerium (IV) in these solvents is not negligible, it is necessary to take samples for beta counting and spectro-photometric analysis at the same time. The concentration of cerium (IV) was determined by direct absorption spectrophotometry at 380 m/x, both in aqueous and in organic solutions. This wavelength was chosen in order to avoid any interference by the reagents. 

The verification of Lamber Beer s law is shown in Figure 4. Organic solutions of cerium were prepared by extracting cerium (IV) from titrated aqueous solutions and standardized by beta counting of Ce tracer. Standardization curves were plotted from three values for further spectrophotometric determinations of cerium (IV). 

The berkelium (IV) extraction coefficients have been determined by stripping solvents previously loaded with tetravalent cerium and berkelium in the presence of sodium bismuthate. Sodium bismuthate has been found to be an efficient oxidizing agent for trivalent cerium. Because of its small solubility it does not affect the distribution coefficients of tetravalent cerium. These two properties have been demonstrated by comparing the distribution coefficients of cerium (IV) measured by spectrophotometry with those of cerium oxidized by sodium bismuthate and measured by beta counting of the cerium isotope tracer. The data are summarized in Table I and indicate no real difference in the distribution coefficients of cerium obtained by these two methods when using trilaurylmethylammonium salts-carbon tetrachloride as solvent. 

Apparatus. The following apparatus was utilized for this study a PRT 2000 type potentiostat (Tacussel) for the electrolysis a Graphi-spectral spectrophotometer (Jouan) for the absorption measurements a TS 6 type millivoltmeter (Tacussel) for potentiometric analysis a 2ir windowless flow gas counter (S.A.I.P.) for measuring the soft beta from and a bell type Geiger counter for Ge and Ge beta counting. 

Reagents. Berkelium. An amount of 5/x Gi of berkelium 249 was supplied by Euratom. Its purity was checked by alpha and beta counting and by alpha and gamma spectrography. The yield of soft beta was greater than 98%. The sample was dissolved in 6N nitric acid, and the conversion to sulfuric acid medium was made by fuming down one aliquot three times and dissolving the residue in the appropriate solution. 

Procedure. Aqueous phases were prepared from samples of cerium (IV), cerium (III), berkelium, and acid and diluted by distilled water to the proper concentrations. Samples of cerium were chosen in order to obtain dijSerent cerium (IV)/cerium (III) ratios. The solutions were allowed to stand for six hours to reach the oxidation equilibrium. A 2 cc. sample of the solvent was added to the same volume of aqueous solution and mixed for 15 minutes. After separation by a centrifuge, samples of both phases were taken for the beta counting of berkelium and the spectrophotometric determination of cerium (IV). In addition, one aliquot of the loaded solvent was taken for determining the distribution coefficient of berkelium (IV). 

The scattering of the results depends upon the soft beta counting of which is sensitive to the self absorption of the source the spectro-photometric determination of unstable cerium (IV) and the great number of experimental determinations needed for calculating the potential. 

In average sea water the " Ra content is about 0.2 dpm/1 and (or is about 2 dpm/1. The uranium usually presents no problem, but a good separation of the final from Ra and its daughters is important, especially for deep ocean water. During the separation steps, this procedure effectively removes all radionuclides that are chemically similar to yttrium or rare earths from the Sr fraction and, after the establishment of Sr/ Y radioactive equilibrium, the final purification steps further remove any interfering radionuclides in the ingrowth Y fraction before measurement by beta counting. 

For many samples one collects, the concentration of Cs is determined directly by gamma spectrometry. Only water samples and a small percentage of the soil, sediment, and biota samples require preconcentration of Cs and measurement by beta counting and/or in some cases in low background well-type Ge detectors. 

The radiochemical procedure for the determination of Cs in aqueous samples is based on the batch extraction of caesium onto a microcrystalline cation exchanger, ammonium molybdophosphate (AMP), and subsequent purification from potassium and rubidium activities by ion-exchange separation using a strongly acidic cation exchange resin (BIO-REX-40). Natural K and Rb have radioactive isotopes that interfere with the beta counting of Cs. The purification of caesium is also necessary to determine the chemical recovery. 

Evaporate the caesium eluate to dryness and prepare the Cs for beta counting as described in the next section. 

Afterwards Cs samples should be prepared for beta counting following the steps ... 

Thermal Ionization Mass Spectrometry, TIMS Alpha Spectrometer Systems, ASS Beta Counting Systems, BCS Gamma Spectrometer Systems, GSS... 

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