Nuclear Instruments and Methods in Physics Research 223 (1984) 407-411 407 North-Holland, Amsterdam RADIUM ISOTOPE MEASUREMENTS USING GERMANIUM DETECTORS Willard S. MOORE D~7;artmenl qf Geoh)gv. University of South Carolina. Columbia. S.('. 29208, USA Recent advances in sampling and counting techniques have provided the means of measuring 226Ra. 22SRa and 224Ra at low activities in natural waters. Samples are preconcentrated in the field by adsorbing radium on a fibre coated with manganese oxides. Absolute activities and activity ratios are measured using germanium detectors supplemented in some cases by alpha scintillation measurements of 222Rn. This paper describes tests and results obtained using a Ge(Li) crystal and an intrinsic germanium crystal with a 1 cm diameter well. The well detector has an efficiency two to three times greater than the flat Ge(Li) system and thus has a considerably higher sensitivity. Results from ground waters and estuarine waters are presented which demonstrate the usefulness of the germanium detectors in studies of radium in natural waters. 1. Introduction Studies of radium isotopes in natural waters have a wide field of application including their use in deep ocean mixing studies [1], exchange rates of estuarine and coastal waters [2], calculation of metal scavenging rates in ground waters [3] assessment of the effects of phosphate mining on ground water quality [4], and development of predictive models for the radium con- tent of ground water [5]. This surge of interest in radium in water has been driven by improved techniques for sampling and mea- suring radium isotopes and an inc,'eased awareness of the usefulness of the different isotopes in a number of studies. Another factor has been the US Environmental Protection Agency's interim standard for radium in drinking water which required that all public water supplies in the US be analyzed for 226Ra and (if the 226Ra was above 3 pCi/1) 228Ra. Problems with the currently accepted analytical techniques for 22SRa spawned a number of new procedures. 2. Sampling and measurement techniques To provide a relatively high activity sample, we pre-concentrate radium in the field using a fibre coated with manganese oxides [6]. Packed in columns this so-called Mn-fibre will quantitatively extract radium from water at flow rates of tenths of litres per minute [7]. For very large volume extractions when only radium isotope ratios are to be measured, we expose the Mn- fibre to the water by towing it behind a boat or setting it in a flowing stream. A handful (-50 g) of the Mn-fibre may extract Ra from thousands of litres of water. In such cases we also collect a sample for the 0167-5087/84/$03.00 ~.,2Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) quantitative measurement of 22,5 Ra by the 222 Rn emana- tion technique [8] by slowly passing about 20 L through a small column of the Mn-fibre. The Mn-fibre is returned to the laboratory, leached with hydroxylamine hydrochloride to dissolve Mn and Ra, and the radium is co-precipitated with BaSO 4 which is packed in an appropriate counting vial. These proce- dures are discussed in detail elsewhere [9,10], We have recently installed an intrinsic germanium detector with a 1 cm diameter by 4 cm deep well which we call a WeGe (for well germanium) detector. This detector increased our counting sensitivity for radium isotopes by a factor of 2-3 over the Ge(Li) detector we have reported previously [9]. 3. Well germanium (WeGe) detector The detector is a 78 cm 3 coaxial intrinsic germanium crystal with a 1 cm diameter and 4 cm deep well produced by Princeton Gamma Tech of Princeton, N.J. The peak to Compton ratio for the ~3VCs peak is better than 90 : 1 for samples counted in the well. At 300 keV the fwhm is 1.4 keV and at 1000 keV it is 1.8 keV. Gamma-rays are measured by placing the sample in capped polypropylene test tubes which fit inside the well. Samples which may lose 222Rn are capped with a layer of epoxy cement, Most of our radium in water samples are measured as BaSO 4. We do not cap these with epoxy as we have not detected 222 Rn loss from this matrix. Several standards have been used to calibrate the WeGe. These include monazite and pitchblend ores, as well as 232Th and 226Ra solutions from the US Environ- mental Protection Agency Quality Assurance Division (EPA QAD), a phosphate ore from the US National 111. SPECTROMETRY