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6.3 The containehoice should be plastic rather than gT~ss to preven t <br />loss due to breakage during transportation and handling. <br />7.0 PROCEDURE <br />7.1 Calibration <br />7.1 1 For absolute gross alpha and gross beta measurement. th e <br />det ectors must be calibrated to obtain the ratio of count rate t o <br />d~si ntegrat~on rate. Americium-241 (used for alpha activity in th e <br />collaborative test of this method) has higher alpha particle energy an d <br />radium-226 radionuclides but is close to the energy of the alpha particles <br />emitted by naturally occurring thorium-228 and radium-224. Standards shoul d <br />be prepared in the geometry and weight ranges to be encountered in then e <br />gross analyses. It is, therefore, the prescribed radionuclide for gros s <br />alpha calibration. NBS or NBS-traceable americium-241 is available fro m <br />Stan dard Reference Materials Catalog. NBS Special Publications 260. U.S <br />Department of Commerce (1976) and from Quality Assurance Branch, EMSL LV <br />P.O. Box 15021. Las Vegas. Nevada 89114. <br />7.1.2 Str ontium-90 and cesium-137 have both been used quit e <br />extensively as standards for gross beta activity. Standard solutions o f <br />each of these radionuclides are readily available. Cesium is volatile at <br />eleva ted temperatures (above 450 °C). Some water supplies have dissolve d <br />solids (salts) that. when converted to nitrate salts. are quite hygroscope c <br />and need to be converted to oxides by he at~ng to red heat to obtain sample <br />aliquots that are weight-stable. Sample weight stability is essential t o <br />gross alpha and gross beta measurements to ensure the accuracy of the self - <br />absor ption counting efficiency factor to be used for the samples <br />Strontium-90 in equilibrium with its dau ghter yttrium-90 is the prescribed <br />rad~onucl~de for gross beta calibrations <br />7.1.3 For each c punting instrument to be used. the analyst should <br />prepare separate alpha and beta particle self-absorption graphs showin g <br />water sample residue weight (mg) vs. the efficiency factor (cpm/dpm), usin g <br />standard alpha and beta emitter solutions and tap water. For the alph a <br />graph standard. alpha activity is added to varying sizes of aliquots of to p <br />water such that the aliquot residue weight is varied between 0 and 100 mg <br />(for a 2-in. counting planchet). A similar graph is prepared with standar d <br />beta activity and tap-water aliquots, va vying the residue weight between 0 <br />and 300 mg (for a 2-in. planchet). If it is planned to use water-sampl e <br />aliquot volumes that always contain 100 mg of dried water solids. then onl y <br />the efficiency factor for that residue weight needs to be established. <br />7.1.4 Tap water aliquots. with added americium-241 or strontium 9 0 <br />standard. should be acidified with a few mL 16 N HNO ,. evaporated to a smal 1 <br />volume in a beaker on a hot plate. transferred quantitatively in 5-m L <br />portions or less to a tared counting planchet, evaporated to dryness. <br />and finally dried at 105 °C for 1 hr (or flamed to a red heat if drie d <br />9310 - 4 <br />CD-ROM Revision 0 <br />Date September 1986 <br />