Comparison of two ultra-sensitive methods for the determination of 232Thby recovery corrected pre-concentration radiochemicalneutron activation analysis

The determination of isotopic thorium by alpha spectrometric methods is a routine practice for bioassay and environmental measurement programs. Alpha-spectrometry has excellent detection limits (by mass) for all isotopes of thorium except ²³²Th due to its extremely long half-life. This paper discusses improvements in the detection limit and sensitivity over previously reported methods of pre-concentration neutron activation analysis (PCNAA) for the recovery corrected, isotopic determination of thorium in various matrices. Following irradiation, the samples weredissolved, ²³¹Pa added as a tracer, and Pa isolated by two different methods and compared (extraction chromatography and anion exchange chromatography) followed by alpha spectrometry for recovery correction. Ion exchange chromatography was found to be superior for this application at this time, principally for reliability. The detection limit for ²³²Th of 3.5 · 10^-7 Bq is almost three orders of magnitude lower than foralpha spectrometry using the PCRNAA method and one order of magnitude below previously reported PCNAA methods.     

Determination of232Th in human tissues by pre-concentration neutron activation analysis with yield determination using227Th

The accurate and precise determination of²³²Th in biological samples is very important for the development of biokinetic models for thorium and for improving our knowledge on its distribution in human tissues. Radiochemical neutron activation analysis has long been one of the most sensitive methods for the determination of²³²Th. However, these determinations suffer in reliability because recovery information following the separation is not typically available. This information is particularly important for difficult matrices such as human bone where recoveries may be significantly less than unity. Also, the separation of difficult matrices following neutron activation may involve relatively high personal dose from the co-activated matrix. A novel approach for the determination of radiochemical yield has been developed which employs the use of a readily available, gamma-emitting isotope of thorium,²²⁷Th.²²⁷Th, obtained by radiochemical separation from²²⁷Ac, is added to each, dissolved sample prior to separation and the chemical yield determined by gamma-ray spectrometry following the separation. This pre-concentration step is then followed by neutron activation and the²³²Th determined via²³³Pa using gamma-ray spectrometry. Detection limits were approximately an order of magnitude lower than obtained by alpha-spectrometry.     

A pre-concentration neutron activation analysis method for the determination of 232Th utilizing a dry-tube irradiation facility

A revized method for determining ²³²Th using a pre-concentration neutron activation analysis (PCNAA) procedure was developed to accommodate irradiation in a dry irradiation tube environment. ²³²Th extracted by ion-exchange from a sample was electrodeposited onto 5/8″ diameter vanadium planchets, which are arranged in a stack and irradiated in the dry tube central irradiation facility (CIF) of the reactor. The higher neutron fluence of this facility improved sensitivity by approximately 37%, however, the higher temperatures required modifications to the irradiation procedure. Because the heat in the CIF would melt the plastic spacers used in the original method, a tube of high-purity quartz was used to contain samples, and high purity quartz spacers were used to separate the vanadium planchets during the irradiation. Test irradiations have determined that no significant transfer occurred from the disks to the silica disks and no significant variation in the neutron flux was observed. Finally, a thin film barrier was tested for its ability to reduce recoil contamination from ²²⁹Th onto the detector during alpha spectroscopy. The film was shown to reduce contamination to levels indistinguishable from normal background.     

Epithermal neutron activation analysis and detection limit calculation for trace amounts of thorium at nanogram level,in Israeli geological samples

Trace amounts of thorium in Israeli geological samples were determined by epithermal neutron activation analysis followed by high resolution gamma-ray spectrometry. Epithermal neutron irradiation has the advantage of enhancing the production of²³³Th via the²³²Th _→ ^{n,γ 233}Th reaction over that of interfering nuclides which have a lower I_o/σ_o ratio. The delay time between the end of irradiation and the start of measurements was shortened to 1–2 days. Under the experimental conditions described, the detection limit of Th was 3.3±0.7 ng. The method is nondestructive, accurate and highly sensitive.     

Evaluation of metal and radionuclide data from neutron activation and acid-digestion-based spectrometry analyses of background soils: Significance in environmental restoration

A faster, more cost-effective, and higher-quality data acquisition for natural background-level metals and radionuclides in soils is needed for remedial investigations of contaminated sites. The advantages and disadvantages of neutron activation analysis (NAA) compared with those of acid-digestion-based spectrometry (ADS) methods were evaluated using Al, Sb, As, Cr, Co, Fe, Mg, Mn, Hg, K, Ag,²³²Th,²³⁵U,²³⁸U, V, and Zn data. The ADS methods used for this project were inductively coupled plasma (ICP), ICP-mass spectrometry (ICP-MS), and alpha spectrometry. Scatter plots showed that the NAA results for As, Co, Fe, Mn,²³²Th, and²³⁸U are reasonably correlated with the results from the other analytical methods. Compared to NAA, however, the ADS methods underestimated Al, Cr, Mg, K, V, and Zn. Because of the high detection limits of ADS methods, the scatter plots of Sb, Hg, and Ag did not show a definite relationship. The NAA results were highly correlated with the alpha spectrometry results for²³²Th and²³⁸U but poorly correlated for²³⁵U. The NAA, including the delayed neutron counting, was a far superior technique for quantifying background levels of radionuclides (²³²Th,²³⁵U, and²³⁸U) and metals (Al, Cr, Mg, K, V, and Zn) in soils.     

Determination of228Th,230Th,and232Th in environmental samples from uranium mining and milling operations

A method is described for the determination of²²⁸Th,²³⁰Th, and²³²Th in environmental samples from uranium mining and milling operations. The analytical procedure is based on the direct determination of²²⁸Th in the sample by high resolution γ-spectrometry followed by extraction and purification of the thorium fraction using high molecular weight amines and an anion-exchange technique, respectively, prior to α-spectrometry to determine isotopic ratios. The lowest level of detection for each thorium isotope is 0.01 pCi/g for solid samples and 20 pCi/l for aqueous samples. Replicate analyses of a typical mine waste stream gave a standard deviation of ±3% for²²⁸Th. Standard deviations of the²³⁰Th and²³²Th increased to ±11% apparently due to traces of²¹⁰Po interfering in the α-spectrometry.     

Simultaneous determination of trace uranium and thorium by radiochemical neutron activation analysis

A method for the simultaneous, radiochemical neutron activation analysis of uranium and thorium at trace levels in biological materials is described, based on a technique known as LICSIR, in which a double neutron irradiation is employed. In the first, long irradiation²³³Pa (27.0 d) is induced by neutron capture on²³²Th and then the sample is cooled for several weeks. A second short irradiation to induce²³⁹U (23.5 m) is followed by a rapid sequential radiochemical separation by solvent extraction of²³⁹U with TBP and²³³Pa with TOPO. Chemical yields of²³⁹U and²³³Pa were measured for each sample aliquot using added²³⁵U and²³¹Pa tracers from the -spectra of the separated fractions. The technique was validated by quality control analyses.     

Determination of Traces of Uranium and Thorium in Environmental Matrices by Neutron Activation Analysis

Radioactive elements like ²³²Th and ²³⁸U along with their daughter products, form part of all environmental matrices and are getting transferred to living beings by different pathways, leading to a continuous radiation exposure and need to be monitored. This paper presents an analytical methodology, highlighting the need to separate interfering beta- and gamma-emitters from the analytes, when neutron activation analysis is employed for the determination of traces of uranium and thorium in soil and plant materials. The method has been applied to the soil and plant materials from selected regions of India, along with standard reference materials to verify the validity of the proposed separation scheme. The overall reproducibility of the procedure was 2–10%. The concentration values of uranium and thorium so obtained, have been used to calculate transfer factors from soil to various parts of wheat plant.     

Estimation of thorium intake due to consumption of vegetables by inhabitants of high background radiation area by INAA

A study was conducted to estimate the thorium concentration in locally grown vegetables in high background radiation area (HBRA) of southern coastal regions of India. Locally grown vegetables were collected from HBRA of southern coastal regions of India. Thorium concentration was quantified using instrumental neutron activation analysis. The samples were irradiated at CIRUS reactor and counted using a 40% relative efficiency HPGe detector coupled to MCA. The annual intake of thorium was evaluated using the consumption data provided by National Nutrition Monitoring Board. The daily intake of ²³²Th from the four food categories (green leafy vegetables, others vegetables, roots and tubers, and fruits) ranged between 0.27 and 5.352?mBq?d^?1. The annual internal dose due to ingestion of thorium from these food categories was 46.8?×?10^?8 for female and 58.6?×?10^?8?Sv?y^?1 for male.     

Ion-exchange separation of uranium,thorium and plutonium isotopes from environmental samples

Radioisotopes of uranium, thorium and plutonium in water, soil and fertilizer samples, have been chemically separated and determined by alpha-spectrometry method. Radiochemical procedure involving ion-exchange, enabled to determine these isotopes in very low concentrations (under 50 Bq/g).²³²U,²²⁹Th and²³⁸Pu were used as a tracers for radiochemical yield recoveries (up to 90%). Thin layer sources have been obtained by electrodeposition.     

Measurements of238U,234U and230Th in impure carbonates for age determination

The carbonate cements of conglomeritic deposits of late Pleistocene age have been leached with 0.2N hydrochloric acid and analyzed radiochemically. The leachate and the residue fractions were separately measured for²³⁸U,²³⁴U and²³⁰Th, using isotope-dilution and alpha-spectrometric techniques. The data are used to estimate the isotopic activities of uranium and thorium in the carbonate phase. These activities give age information for the carbonate cementation. Ages in the range of 185–320·10³ years were obtained for the samples studied.     

Natural radioactivity in bioassay by alpha-spectrometry measurements

Personnel of nuclear facilities are checked regularly for internal contamination by bioassay measurements. Although these persons are generally not involved in any incident, natural radioactivity from U, Th and Ra can be found in their urine or faeces. Uranium total activity in urine has been found with a range of 0.051 to 3.0 mBq/24 h and in faeces from 14.5 to 380 mBq/d. ²³⁴U/²³⁸U ratio for urine is 1.48 but this ratio varies from 0.47 to 19. By comparison, the ²³⁴U/²³⁸U ratio found in urine from workers in volved with natural uranium or 4.5% enriched uranium is 1.0 and around 4.0 respectively. ²³⁰Th, ²²⁸Th and sometimes ²³²Th have also been detected. The total thorium activity varies from 0.137 to 5.6 mBq/24 h in urine and from 9 to 183 mBq/d in faeces. ²²⁸Th has generally been found in excess of ²³²Th. All these measurements were performed by alpha-spectrometry. The few ²²⁶Ra results have been measured using the Lucas or emanation method.     

Improved sample preparation method for environmental plutonium analysis by ICP-SFMS and alpha-spectrometry

A rapid and simple sample preparation method for plutonium determination in environmental samples by inductively coupled plasma sector field mass spectrometry (ICP-SFMS) and alpha-spectrometry is described. The developed procedure involves a selective CaF₂ co-precipitation for preconcentration followed by extraction chromatographic separation. The proposed method effectively eliminates the possible interferences in mass spectrometric analysis and also removes interfering radionuclides that may disturb alpha-spectrometric measurement. For ²³⁹Pu, ²⁴⁰Pu and ²⁴¹Pu limits of detection of 9.0 fg·g⁻¹ (0.021 mBq), 1.7 fg·g⁻¹ (0.014 mBq) and 3.1 fg·g⁻¹ (11.9 mBq) were achieved by ICP-SFMS, respectively, and 0.02 mBq by alpha-spectrometry. Results of certified reference materials agreed well with the recommended values.     

Measuring activity of 235, 238U, 232Th and 40K in geological materials using neutron activation analysis

The radioactivity of the ^{235, 238}U and ²³²Th isotope decay chains for geological samples can usually be assumed to be in equilibrium due to their age. Similarly, one can assume that the isotopic mass proportions are equal to natural isotopic abundance. Current methods used to ascertain activity in these decay chains involve alpha particle spectrometry, ICP-MS or passive gamma-ray spectrometry, all of which can be laborious and time consuming. In this research, we have used thermal and epithermal neutron activation analysis (NAA) of small sample sizes of various geological materials in order to ascertain these activities. By using NAA, we aim to obviate cumbersome sample preparation, the need for large samples and extended counting time. In addition to the decay chains of uranium and thorium, ⁴⁰K was also determined using epithermal neutron activation analysis to determine total potassium content and then subtracting out its isotopic contribution.     

Determination of uranium in thorium matrixes by epithermal neutron activation analysis

Uranium in thorium matrixes or in minerals and ores containing thorium is determined by epithermal neutron activation analysis (ENAA). In some minerals and ores, such as monazite sands, the analysis can be carried out by purely instrumental means with no chemical separation of uranium or thorium from the irradiated matrix. For thorium compound matrixes with very low uranium contents, a rapid radiochemical separation method, based on the retention of uranyl ion on anion-exchange resins, is first carried out, before counting the gamma-ray peaks for²³⁹U in multichannel analysers coupled to NaI(Tl) scintillators or to Ge(Li) detectors.     

The microenvironment of iron in (Ba,Ca)(Fe,Co)O3−δ catalyst system: A Mössbauer study

Undisturbed, non-fertilized woodland soil (“loamy sandy soil” type) from 1 m below surface was dry and wet sieved. Sieving fractions of <10–1000 μm were analyzed for total alpha-activity. Thorium and uranium contents were determined by alpha-spectrometry after radiochemical separation. Soluble and insoluble parts of thorium and uranium were determined in the sieved fractions indicating that the isotope distribution in soil correlates with the particle size distribution: The smaller the size fraction the higher the isotope content. Isotope ratios of²²⁸Th/²³²Th, and²³⁴U/²³⁸U are discussed.     

Thorium determination in a liver autopsy sample via direct γ-spectrometry and neutron activation analysis

Thorium was determined in a liver autopsy sample from a person treated with Thorotrast ca. 40 y earlier. The decay products²²⁸Ac,²²⁴Ra,²¹²Pb,²¹²Bi, and²⁰⁸Tl from the²³²Th series were identified by direct -spectrometry. Instrumental neutron activation analysis yielded a value of ca 22 g thorium per kg dry liver material. The total radiation dose to the whole liver was estimated in the order of 16 Gy.     

Determination of concentrations of U and Th in their mixed oxides by INAA methods

Uranium and thorium mixed oxides are being prepared using natural U and Th for studies on fuels for Advanced Heavy Water Reactors, wherein composition of U and Th is specific and requires strict control in terms their contents and homogeneity. Chemical quality control necessitates accurate and precise compositional characterization of the fuel material by a suitable analytical method. Among various analytical methods for U and Th, instrumental neutron activation analysis (INAA) is one of the best methods for their simultaneous determination without chemical dissolution and separation. INAA methods using reactor neutrons namely thermal NAA and epithermal NAA were standardized for the determination of U and Th in their mixed oxides. Standards, synthetic samples and U–Th mixed oxide samples, prepared in cellulose matrix, were irradiated at pneumatic carrier facility of Dhruva reactor as well as at self serve facility of CIRUS reactor under cadmium cover (0.5 mm). Radioactive assay was carried out using a 40% relative efficiency HPGe detector. Both activation and daughter products of ²³⁸U (²³⁹U and ²³⁹Np) and ²³²Th (²³³Th and ²³³Pa) were used for their concentration determination. The method was validated by analyzing synthetic samples of 6–48%U–Th mixed oxides. The standardized method was used for the concentration determination of U and Th in 4–30%U–Th mixed oxide samples. Results of U and Th concentrations including associated uncertainties obtained from the INAA methods are presented in this paper.     

Distribution of thorium in soils surrounding the rare-earth tailings reservoir in Baotou,China

Thorium distribution was investigated in the soils surrounding the rare-earth (RE) tailings reservoir near the Baotou grassland of Inner Mongolia, northern China. Totally 77 soil samples were collected from 8 different directions in the periphery of the RE tailings reservoir, and then were determined for ²³²Th. The ²³²Th activity degree ranges from 9.1 to 307.1 Bq kg^?1 with an average value of 42.4 Bq kg^?1. In some samples, the degree is higher than that of global average, showing that these soils were polluted by thorium. There is a high linear correlation coefficient between the thorium diffusion coefficient parameter and the wind intensity parameter which indicates that the distribution of ²³²Th is mainly correlated with wind speed and direction. The geo-accumulation index method was used to evaluate the level of thorium pollution, and the Kriging method was applied to estimate the land area at each level. By calculation, result shows that the area at each pollution level is 2.10 km² with medium-strong pollution, 38.29 km² with medium pollution, and 47.19 km² with slight pollution. The remaining 738.63 km² of land investigated is clear from thorium pollution.     

Extraction Preconcentration of Uranium and Thorium Traces in the Analysis of Bottom Sediments by Inductively Coupled Plasma Mass Spectrometry

A procedure was developed for determining trace amounts of uranium and thorium isotopes in bottom sediments from Lake Baikal. This procedure involves sample decomposition, the coextraction of uranium and thorium with trioctylphosphine oxide, the quantitative back extraction after diluting the extract with caprylic acid, and the ICP MS analysis of the back extract. The procedure was verified by analyzing a BIL-1 Lake Baikal bottom silt standard reference material using the developed procedure and independent methods. The detection limits of abundant uranium and thorium isotopes are restricted by blank measures and equal to 1 × 10^–7 mass %. The detection limits for²³⁴U and ²³⁰Th are 4 × 10^–10 and 6 × 10^–10 mass %, respectively.