using a solution of lead-210 in equilibrium with its daughters , supplied by the radiochemical centre , Amersham , a source was prepared and counted through a series of aluminium absorbers of increasing weight . the curve of observed activity plotted against absorber thickness is shown in figure 2 . an aluminium absorber weighing 27 mg &amp;sol; cm2 was used in the following experimental work although this was thicker than necessary and reduced the efficiency of the Geiger counter from about 15 % to 11 % . reference standards were prepared by precipitating lead chromate from a hot dilute acetic acid solution containing a known quantity of lead-210 in equilibrium with its daughters . the calibrated solution of lead-210 ( about 10-2 mc &amp;sol; ml ) was supplied by the radiochemical centre , Amersham . lead chromate is accompanied by only about 75-85 % of the bismuth-210 and therefore time must be allowed for radio-equilibrium to be restored . the presence of 75 % of the activity of bismuth-210 is equivalent to ingrowth over two half-lives ( ten days ) . therefore after a further forty days , the bismuth daughter will be within 0.1 % of its final equilibrium value . if reference sources are required for use sooner than forty or fifty days after preparation , the lead-210 together with added lead carrier must be separated from the bismuth-210 daughter by ion exchange ( see analytical method , steps 4 , 5 ) before precipitating lead chromate . knowing the time of separation and the activity of the lead-210 solution , the ingrowth of the bismuth-210 can be calculated . the absolute activity of the reference standards can be calculated from the known activity of the lead-210 solution and the chemical yield , but this calculation is unnecessary provided the same lead carrier solution is used to prepare the reference standards and for the analyses . only the weights of the recovered lead chromate precipitates need be known because the concentration of the lead carrier solution cancels out of the algebraic equations . an effort was made to detect the presence of any radioactive impurities in the tracer by separating the lead-210 and the bismuth-210 by anion exchange . the b-counting of the lead-210 fraction began within a few minutes of completing the separation . the ingrowth of bismuth-210 was followed for ten days and showed no abnormalities . any impurity in the lead fraction must have been well below one percent . some separated lead-210 was used to make reference standards and as a tracer in recovery experiments . there was no significant difference between these results and those obtained using the original lead-210 solution supplied by the radiochemical centre which we concluded was radiochemically pure . 2.2 . the recovery of lead-210 tracer from solution . Rosenquist ( 4 ) showed that minute quantities of lead can be isolated from large volumes of solution by coprecipitating the lead with a strontium sulphate . lead and strontium form mixed crystals so that the more insoluble lead sulphate is almost completely recovered even if precipitation of the strontium sulphate is incomplete . using ten milligrams of lead carrier and six hundred milligrams of strontium per liter , more than 95 % of added lead tracer was recovered in each experiment . gravimetric recoveries were less in the presence of ethylenediamine tetra-acetic acid ( 10 ppm ) , Teepol ( 0.2 ml commercial Teepol per liter ) and Calgon ( 250 and 500 ppm ) , but always exceeded 70 % . radiochemical recovery of the tracer corrected for gravimetric recovery of the carrier averaged 97.5 &amp;plusmn; 0.5 % in all cases where these additives were present . excessive quantities of chloride also reduce the gravimetric recovery of lead . up to 0.1 N chloride ion ( 96 % recovery ) the effect is negligible but becomes increasingly important thereafter : 0.3 N Cl- ( 85 % recovery ) , 0.5 N Cl- ( 79 % recovery ) , 1.0 N Cl- ( 56 % recovery ) . no more than ten milligrams of lead was used in order to ensure good separation on the ion exchange column and to make it possible to keep the lead in solution in small volumes of dilute hydrochloric acid . absorption of the beta particles is also kept to a minimum but the accuracy and precision of weighing the precipitated sources suffers . all precipitates were weighed on a semi-micro balance which had been calibrated with a set of certificated weights from the national physical laboratory . complete chemical exchange between the radio-lead and the added lead carrier is necessary if the analytical results are to be correct . in the preliminary experiments , the tracer was added to a liter of effluent and immediately coprecipitated with strontium sulphate from hot solution . chemical exchange was complete under these conditions ; but when the tracer was added to alkaline effluent and allowed to stand for several days before the addition of lead carrier , the recovery of lead-210 was as much as five per cent low when corrected for gravimetric recovery of the carrier . complete exchange was obtained by acidifying the effluent with five milliliters of concentrated nitric acid and boiling for more than half an hour before completing the coprecipitation . boiling the effluent with more than 5 ml of acid resulted in gravimetric recoveries which were too low to be tolerated . to ensure complete chemical exchange , one hour at , or very near , the boiling point is recommended in the analytical method . the presence and growth of algae in the alkaline effluent does not prevent the recovery of radio-lead under the prescribed conditions although some radioactivity remains on the algae until the metathesis has been completed by heating the mixed sulphates with three separate portions of dilute ( 1.25 normal ) sodium carbonate solution . the strength of the carbonate solution was chosen after experiments with lead tracer alone which indicated that less lead was lost than at other concentrations . the mixed carbonates are dissolved in 2 N hydrochloric acid and fed to a column of anion exchange resin . the algae , if any was present in the effluent , is simply filtered out on top of the resin bed . 2.3 . separation of lead and bismuth by anion exchange . the anion exchange resin ( Amberlite ira-400 , 60-100 mesh , chloride form ) is prepared by drying the commercial product and grinding it to pass the correct sieves . a small manual coffee grinder is useful as the resin can not be ground with a mortar and pestle . the sieved resin is washed repeatedly with distilled water to remove fines and then with hydrochloric acid to convert the resin completely to the chloride form . the 60-100 mesh resin is again washed with water to remove the acid and finally dried in air . the drying may be speeded up by heating in a low temperature ( 40 &amp;deg; - 60 &amp;deg; C ) oven until damp dry . the final drying should be at room temperature with the resin protected against dust by a covering of filter paper . for each aliquot to be analysed , about 3.5 grams of air-dried resin is weighed out and slurried into a glass tube with 2 N hydrochloric acid . the glass tube is 11 cm long and 1 cm in internal diameter . one end of the tube is drawn down to a fine tip and a B14 conical glass socket is fitted to the other end as shown in figure 1 . the reservoir is a 50 ml cylindrical separating funnel with a capillary tap modified by the addition of a B14 cone to fit the glass column . the exact volume of the eluting agents must be found by experiment for each batch of resin using radioactive tracer ( lead-210 , bismuth-210 ) . a typical elution curve is shown in figure 3 . once these volumes have been established , the weight of resin used is also fixed . all available evidence indicates that the fractions containing lead and bismuth are free of cross contamination apart from the natural ingrowth of the daughters arising from the decay of lead . polonium-210 remains on the column and does not interfere . strontium does not form a chloro-complex and therefore passes through in the feed solution and the first wash . the resin is used for a single separation and then thrown away . in the first stage of the analysis , only the fraction containing lead-210 is collected . the lead is precipitated as the chromate , washed , slurried onto an aluminium counting tray , dried under an infra-red lamp , weighed , and set aside for five days while the bismuth-210 grows in . at the end of five days , bismuth-210 will have reached one-half of its equilibrium value and can be counted through an aluminium absorber sufficiently thick to stop the beta particles from lead-210 and the alpha particles from polonium-210 . earlier beta counting is permissible but the sensitivity of the method is reduced ( see fig 4 ) . the presence of other lead nuclides may be demonstrated by observing the ingrowth of the bismuth activity and comparing the shape of the normalised curve with the curve in fig 4 . using an arbitrary scale of activity proportional to the existing ordinate . during the first few hours the curve will be distorted if activity other than bismuth-210 is present . these bismuth nuclides may include : together with their lead parents . all but lead-212 will decay completely within six hours . the decay of lead-212 will distort the observed activity for four and a half days if it is present . thereafter the normalised curve should follow fig 4 exactly . if it is essential to confirm that the beta activity is indeed due to bismuth-210 , or if much higher decontamination factors are required , the lead chromate is washed quantitatively with 2 N hydrochloric acid into a centrifuge tube and dissolved in the presence of bismuth carrier . the solution is kept near the boiling point for fifteen minutes to assist chemical exchange . chromate , which would interfere with the ion exchange separation , is reduced by adding a few drops of hydrogen peroxide . the ion exchange separation is repeated and the bismuth is eluted with 1 N sulphuric acid . the bismuth fraction is diluted and phosphoric acid is added . the phosphate precipitation is repeated to remove contamination by sulphate or occluded sulphuric acid . if there is sufficient beta activity , the radiochemical purity of the Bi-210 may be checked by observing the decay curve . the removal of lead chromate from the aluminium counting tray together with the bismuth-210 prior to the second ion exchange separation has been checked by counting the trays . not more than 0.2 % of the bismuth-210 remains on the tray after the acid wash . this loss is acceptably small for an analytical step when no correction for carrier recovery is possible . the completeness of the chemical exchange between the bismuth-210 and the added carrier was also tested . two samples of precipitated lead chromate ( lead-210 , bismuth-210 ) were counted and dissolved in nitric acid in silica basins . the contents were evaporated to dryness with bismuth carrier and then taken up in 2 N hydrochloric acid and the ion exchange separation completed . the bismuth was recovered from the eluate as the phosphate . results did not differ from those obtained by the more convenient method of heating the dissolved chromates in 2 N hydrochloric acid for fifteen minutes . the more rigorous method of securing chemical exchange was unnecessary . 2.4 . decontamination from other nuclides . lead-210 when present in effluent is likely to be found only at very low concentrations . with a permitted maximum concentration of 10-12 curies &amp;sol; ml it is one of the three most stringently controlled b-emitting nuclides at the present time . therefore it is essential that the radiochemical procedure for the assay of lead-210 shall provide for a high degree of decontamination from major fission products and other nuclides which are likely to be present in amounts greatly exceeding lead-210 so that these shall not be mistakenly reported as lead-210 . table 1 shows the decontamination factors obtained experimentally for ten radionuclides , accompanied in two instances by radioactive daughters . the decontamination factor falls below 104 only for ruthenium-106 and zirconium-95 with their daughters in the first stage , i.e the lead chromate source containing the bismuth-210 daughter . when the second stage ( the bismuth phosphate source ) is completed , the decontamination factors are exceptionally high . 3 . results and discussion . a known quantity ( approx 1 x 10-2 mc ) of lead-210 was added to 1-litre aliquots of different batches of typical low-activity effluent . the aliquots were allowed to stand for about seven days ( except where noted to the contrary ) before lead carrier was added and the analytical procedure begun . 