Executive summary Lead-210 (210Pb) is a beta particle emitting radionuclide in the uranium decay series and has therefore been an important part of the monitoring and research program of the environmental radioactivity group of eriss. The eriss radioanalytical laboratories have routinely used gamma and alpha spectrometric methods for 210Pb determination, though current techniques have some limitations. For gamma spectrometry, analysis can be performed immediately after sample preparation, but, the detection limits are quite high, thus this technique cannot be used for determination of radionuclide activity in environmental samples with low activity concentrations. In contrast, detection limits are very low for alpha spectrometric measurement but 210Pb must be determined via its direct daughter polonium-210, requiring complex separation chemistry and long ingrowth periods in order to achieve the low detection limits. Liquid scintillation counting (LSC) provides an alternative technique for 210Pb determination that allows measurement via the beta decay. Although LSC methods have some limitations, the detection limits approach those of alpha spectrometry and are approximately two orders of magnitude lower than gamma spectrometry. When compared to the alpha spectrometric method for 210Pb determination, LSC can significantly reduce the time taken for separation chemistry and therefore lead to faster analysis. This report provides a detailed description of a radiochemical separation and radiation measurement technique developed in the eriss radioanalytical laboratories for the determination of 210Pb via LSC. Detection limits, uncertainty estimation and the applicability of the method to various sample matrices are presented and discussed.
LSC01 – Liquid Scintillation Counting for 210Pb 1. Purpose/ Scope This procedure is used for the counting of sources for prepared for LSC determination of 210Pb. This procedure follows from procedure RLR12, starting after the completion of step 11 in section 3.2 Analytical Procedure. 1.2 Considerations prior to starting The detection limits for 210Pb determination via LSC are dependent on the relative stage of ingrowth of the daughter products 210Bi and 210Po. Where rapid determination of 210Pb is required this method can be utilised immediately after chemical separation of Pb, with an adjusted count time. For samples where a lower detection limit is required a longer count time can be used (6 ks) and a minimum of 25 days should be allowed after separation before preparation of sources for LSC; the lowest detection limits can be achieved 33 days or more after separation of Pb. 2. Risks/Responsibilities Strong acids are used in the preparation of sources for 210Pb analysis via LSC, though these are thoroughly mixed with a scintillation cocktail when prepared for counting. The emulsion formed is very stable and poses a low risk to staff. 2.1 PPE Minimum PPE is required in all laboratories. No specific PPE is required when handling sources prepared for LSC once they are ready for counting – care should be taken and gloves used if necessary due to leakage or spillage of the scintillant cocktail. 2.2 Other considerations Any waste organic chemicals (ie – UGAB) generated must be stored in a designated container for the storage of this waste. The designated container must be registered in the most recent ‘Time sensitive chemical and organic waste register’, and a required disposal date recorded (by default this is 12 months unless otherwise specified) in this logbook. Disposal dates (again the default is 12 months) must also be recorded for samples prepared with UGAB, this is recorded in the “LSC 210Pb – Method batch log” logbook (form S6). 3. Procedure 3.1 Equipment/Apparatus preparation TriCarb 3100TR liquid scintillation counter (TriCarb) for use in the TriCarb 3.2 Counting Procedure Select a rack and a protocol flag to use for counting, attach the protocol flag to the rack (see the reference manual – PerkinElmer, 2004 – for details)