Evaluating practicability of an LSC method for routine monitoring gross alpha and beta activities in water samples in Taiwan

A procedure using liquid scintillation counting for the monitoring of gross alpha and beta activities in environmental water was implemented to improve the conventional procedure using GFPC adopted in Taiwan. The new procedure was acquired through calibration and validation, and then was applied to the monitoring of surface water in Taiwan. This procedure can improve 2-4 times of detection efficiencies and takes only 70-80% of analysis time with reliable accuracy. With these features, the newly developed procedure is favorable during emergency situations.     

Establishment of a method for the rapid measurement of gross alpha and gross beta activities in sea water

The risk associated with NORM and with their redistribution in the environment due to human activities requires rapid methods which can be applied to a wide range of samples. The present work studies the effect that salts present in sea water have in the gross alpha/beta determination using low-level liquid scintillation counting and a new method is established. This is done from a previously established method for such determinations in non-saline water. External quenching parameter, efficiency, spectra position and interference values are evaluated to look for differences in matrix behaviour. Some quality parameters of the new method are reported.     

Determination of photon self-absorption corrections for soil samples

A method is described for the determination of photon self-absorption in soil samples. Only the density needs to be determined to apply the method, which often produces agreement with measured correction factors for the density range 0.7–1.8 g/cm³ and energy range 46–1764 keV. A comparison of known activities with those obtained from direct-transmission measurements found severe discrepancies at energies <100 keV, while the described procedure produced good agreement. Density-corrected count rates for a sample can be determined from a single spectral measurement using formulas easily incorporated into existing computer programs.     

探讨串道现象对饮用水中的总放射性测量结果的影响

目的：研究串道现象对饮用水中的总放射性测量结果的影响程度。方法：采用不同标准物质、不同的厚度、不同面积进行总放射性测量试验。结果：用²⁴¹Am粉测试的串道因子数随质量增加无明显变化,铀粉则随质量增加而增加。仪器测试通道不同,串道因子数也不同。总α对总β的串道影响很明显,反过来串道因子数很小,可忽略。串道因子数受样品盘面积的影响不明显。结论：做饮用水中总放射性检测使用α-β同时测量模式时,总β放射性活度受α串道影响,可以通过粉末标准物质进行修正,减小对测量结果的影响。     

The implications of particle energy and acidic media on gross alpha and gross beta determination using liquid scintillation

The interaction of humans with radioactivity present in the environment from natural and artificial sources necessitates an evaluation of its risk on human health. Gross alpha and gross beta activities can provide a rapid evaluation of the radioactive content of a sample and can be simultaneously determined by using liquid scintillation counters. However, calibration of the liquid scintillation counter is required and is affected by many factors, such as particle energy and the acidity of the media. This study investigates what effect the particle energy used for calibration has on misclassification and how to account for this misclassification in routine measurements.The variability in measurement produced by the final pH, as well as any acids used in sample treatment, was also studied. These results showed that the most commonly used acid for these types of analyses, HNO₃, produced a high amount of misclassifications at very low pH. The results improved when HCl was used to adjust the sample to low pH.     

Spectrometric determination of low activities of gamma emitters in water samples.

Two variants of the ISO 10703:1997 procedure were developed: (i) the partial substitution of the reagents, thereby reducing the mass of the dry residue by a factor of 2; (ii) preparation of two gamma spectrometry sources by selectively precipitating the radioiodines onto one, and the rest of the radioactive cations onto the other. In variant (ii) the time required to prepare a 131I source from 20 l of water is 2.5 days, and the detection limit is 0.9 mBq/l.     

水中总α放射性测量的制源厚度及效率校准研究

目的 研究制源厚度及效率校准对水中总α放射性测量结果的影响。方法 分别采用²⁴¹Am和天然铀标准溶液加入水样制备样品源、²⁴¹Am标准物质粉末直接铺样制备标准源,研究质量厚度与探测效率的关系、质量厚度对样品源净计数率的影响、有效厚度的测定以及有效厚度法和厚源法对同一水样测量结果的影响。结果 在水样中分别加入²⁴¹Am和天然铀标准溶液测得水样的有效厚度均与经验值4 mg/cm²基本一致;水样的α计数率在质量厚度为2A~5A mg/cm²时近似处于线性增长趋势,>10A mg/cm²时计数率基本平稳不再增加（A为样品盘面积,cm²）;分别采用有效厚度法和厚源法测量同一水样的总α放射性,铺样在质量厚度为10A mg/cm²时,两种方法的结果最一致。结论 在分析水样总α放射性时,为减少因样品源质量厚度不同引起的总α计数率偏差、因标准源质量厚度不同所致的探测效率偏差,铺样厚度均选择为10A mg/cm²较为合理。     

Monte Carlo simulation of the self-absorption corrections for natural samples in gamma-ray spectrometry

Gamma-ray self-attenuation corrections in the energy range 60–2000 keV were evaluated by means of Monte Carlo calculations for environmental samples in a cylindrical measuring geometry. The dependence of the full-energy peak efficiency on the sample density was obtained for some particular photon energies and, as a result, the corresponding self-attenuation correction factors were obtained. The calculations were performed by assuming that natural materials have mass attenuation coefficients very similar to those of water in the energy range studied. Three different HpGe coaxial detectors were considered: an n-type detector with 44.3% relative efficiency and two p-type detectors of relative efficiencies 20.0% and 30.5%. Our calculations were in very good agreement with the self-attenuation correction factors obtained experimentally by other workers for environmental samples of different densities. This work demonstrates the reliability of Monte Carlo calculations for correcting photon self-attenuation in natural samples. The results also show that the corresponding correction factors are essentially unaffected by the specific coaxial detector used.     

A simplified technique to determine the self-absorption correction for sediment samples

A self-absorption correction factor is required for the determination of the radioactive isotope concentration in sediment samples at the low-energy region. A simplified technique for this correction factor for a well-type HP germanium detector is described. This correction is especially important when the ratio between the sample density and reference sample density (ρ_sa./ρ_re) is greater than approximately 1.4.     

Self-absorption correction for beta radioactivity measurements in water samples.

Two methods were used to apply self-absorption corrections for the determination of beta radioactivity in water samples (either for an identified radionuclide or for monitoring trends). One method was performed by estimating the absorption coefficient by assuming an exponential behaviour of absorption using external absorbers, while the other method was performed by preparing empirical curves using standards of different mass. In the first method, a relationship between absorption coefficient and maximum beta energy was also derived. In the second method, self-absorption curves for 90Sr/90Y standard were prepared with several sodium salts (carbonate, nitrate and sulphate salts) and, for 40K standard using a potassium salt. Both beta emitters are usually necessary to calibrate detectors for beta radioactivity measurements. This study showed that, for 90Sr/90Y, results using standards of different mass were more accurate than using external absorbers. Furthermore, it is highly recommended to melt sodium nitrate salts in order to perform a self-absorption curve for 90Sr/90Y because standards were stable in time and homogeneously distributed. For 40K, a self-absorption curve may be easily derived using paper absorbers of different thickness, instead of performing a time consuming self-absorption curve using a set of varying thickness of 40K standards. In order to test the two methods, the gross beta activity of several environmental water samples was analysed.     

Monte Carlo determination of water concentration effect on gamma-ray detection efficiency in soil samples

Water concentration effect on full energy peak efficiency in a soil sample taken from a soil profile in Erzurum (39°55′ N; 41°16′ E; 200 m above sea level), Turkey was determined using Monte Carlo simulation technique. The dependence of the full energy peak efficiency on the water concentration in the soil was obtained for some particular photon energies ranging from 60 keV to 2 MeV and, as a result, the corresponding correction factors were obtained. It was observed that the correction factor approaches unity with increasing energy and decreases with increasing water concentration.     

Separation techniques for low-level determination of actinides in soil samples

The separation methods for soil samples applied at PSI are based on extraction chromatography and ion exchange. After sample leaching, the actinides are pre-concentrated via precipitation using oxalic acid. Besides the classical separation methods applying the extraction chromatographic resins U/TEVA™ (for U, Th), TRU™ (Pu, Am), new methods were recently implemented to increase the radiochemical recovery of particularly trivalent Am and Cm. These methods do not require initial reduction of Pu(IV) to Pu(III) but stabilize Pu on the tetravalent oxidation state using a mixture of NaNO₂/H₂O₂ in strong acidic medium. The Pu-fraction is then fixed along with Th onto Dowex AG 1-X2 anion exchanger resin. Th is eluted via complexation with 10 M HCl, Pu via reduction with HI. The fractions of Am+Cm and U are loaded onto DGA™ resin. This resin shows extraordinary high distribution coefficients (k′-values) exceeding 10⁴ (for Am) in strong nitric acid medium. The separation between U and Am is obtained quantitatively by decreasing the HNO₃ concentration from 3 to 0.25 M (stripping of the U-fraction) while Am can be easily eluted thereafter using 0.25 M HCl as complexation compound.     

Influence of source composition and particle energy on the determination of gross alpha activity

The influence of different source compositions and α-particle energies on the detection efficiency of a gas-flow proportional counter was examined using experimental measurements and Monte Carlo simulations. Efficiency variation with alpha-particle energy was very marked, being less significant with the substrate composition. These results show that the determination of gross alpha activity in an unknown sample must be carried out very carefully in order to give a correct estimation of its activity.     

Procedures for the determination of Rn exhalation and effective Ra activity in soil samples

Two methods for measuring ²²²Rn exhalation and effective ²²⁶Ra in soil samples were studied. In the first determination, the method employed was based on the adsorption of radon onto activated charcoal and subsequent measurement of the activity of its daughters with an HPGe (high-purity germanium) detector. In the second, vials containing an aqueous suspension of the sample, mixed with an insoluble high efficiency mineral oil scintillation cocktail, were measured using a low-level liquid scintillation counter. Studies of optimum sampling time, efficiency in both procedures, variation of ²²⁶Ra efficiency with quenching, as well as the effect of sample amount and granulometry upon the quenching parameter, were carried out. The two methods were applied to the determination of ²²²Rn exhalation and effective ²²⁶Ra in environmental samples.     

Study of the representativity of uranium and thorium assays in soil and sediment samples by alpha spectrometry.

The activity concentrations of soil and sediment samples are determined by alpha spectrometry with a sample size usually < 1 g. Hence, a systematic and careful procedure must be followed in the sampling and pre-treatment of the sample in order to ensure that the aliquot analysed represents the activity concentration of the entire sample. Statistical analyses of the activity concentrations of uranium and thorium isotopes in soil and sediment samples were performed to study the activity variation due to the lack of the representativity of the sample. These studies showed that, although some uncertainty in the determination of the sample activity concentration may be due to the lack of representativity (about 10%), the procedure followed for the pre-treatment of the sample can be considered adequate, providing an acceptable level of representativity.     

A method for the determination of low-level organic-bound tritium activities in environmental samples.

The authors discuss some of the difficulties encountered when analyzing organically bound tritium (OBT) and describe a sensitive method for low-level OBT determination in biological samples. The methodology, which combines suitable sample treatment, a combustion apparatus for large-sized samples and low-background liquid scintillation counting, provides exclusively carbon-bound tritium measurement. Two key points of this methodology are described and illustrated. The first one is the stage of removal of the exchangeable organic tritium. The efficiency of this stage, carried out by mixing powdered dry samples with tritium-free water, is evaluated. The second key point is the set of precautions taken at any stage of the treatment to avoid contamination of the samples by ambient atmospheric moisture. The detection limit of the method is about 0.5 Bq kg(-1) of dry material. Low-level applications of this methodology are given with estimation of the OBT/HTO ratios.     

Determination of self-absorption corrections for gamma analysis of environmental samples: comparing gamma-absorption curves and spiked matrix-matched samples.

Accurate determination of the massic activity of gamma-emitting radionuclides in environmental samples, particularly sediments and soils, cannot be achieved without taking into account sample self-absorption. The extent of self-absorption in the sample will depend on a number of factors including sample composition, density, sample size and gamma-ray energy. The preferred method for correcting for this effect is to use spiked or natural matrix reference materials that match each sample type to be analysed. However, for laboratories that must measure a wide variety of sample matrices this method is too costly and time-consuming. Another technique commonly used is to make direct gamma-ray transmission measurements for each sample. This method, while more practical, still requires a minimum of three measurements to be made for each sample analysed. A more convenient method is to prepare sets of gamma-absorption curves. This approach involves making a series of direct transmission measurements for samples of varying densities but similar type. Sets of matching samples, both spiked and unspiked, were prepared and density correction factors determined using the direct transmission method and the spiked sample approach. It was found that, for soil and sediment samples, these two methods typically differed by 5-9% for higher energy gamma rays and by 12-15% for the 59.54 keV 241Am peak. Gamma-absorption curves were also derived and, for the admittedly limited dataset, 95% confidence intervals of +/-7% for the curve generated using the spiked samples method were obtained.     

A simple method for determination of natural and depleted uranium in surface soil samples

A simple and efficient method for determination of uranium content in surface soil samples contaminated with depleted uranium, by gamma ray spectrometry is presented. The content of natural uranium and depleted uranium, as well as the activity ratio ²³⁵U/²³⁸U of depleted uranium, were determined in contaminated surface soil samples by application of this method.