Determination of isotopic thorium in biological samples by combined alpha-spectrometry and neutron 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 except²³²Th due to its extremely long half-life. This paper reports a pre-concentration neutron activation analysis (PCNAA) method for²³²Th that may be performed following alpha-spectrometry if a suitable source preparation material is utilized. Human tissues and other samples were spiked with²²⁹Th and the thorium was isolated from the sample using ion exchange chromatography. The thorium was then electrodeposited from a sulfate-based medium onto a vanadium planchet, counted by alpha-spectrometry, and then analyzed for²³²Th by neutron activation analysis. The radiochemical yield was determined from the alpha-spectrometric method. Detection limits for²³²Th by this PCNAA method are approximately 50 times lower than achieved by alphaspectrometry.     

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.     

Determination of uranium isotopes in irradiated thorium dioxide by alpha spectrometry using WinALPHA for deconvolution of complex spectra

Isotopic composition of uranium obtained from irradiated thorium dioxide was determined using alpha spectrometry by employing WinALPHA for the deconvolution of the alpha spectra recorded using electrodeposited sources. The results obtained were found to agree within 1% with those determined by thermal ionization mass spectrometry. The deconvolution methodology is important since it is possible to account for the in-growth of ²²⁸Th, which interferes in the determination of ²³²U by alpha spectrometry. The present methodology has the potential to determine isotopic composition of uranium in the irradiated thorium based nuclear fuels, employing alpha spectrometry.     

A new method for the determination of radium-228, thorium-228, and radium-224 in groundwaters via thoron (radon-220)

Current techniques for determining low levels of dissolved thorium involve chemical separations, generally by coprecipitation with a carrier cation, purification by ion exchange procedures, electroplating and, finally, alpha counting by alpha spectrometry. Similarly, measurements of low²²⁸Ra and²²⁴Ra activities requires concentration, by coprecipitation with barium sulfate, followed by gamma counting. An improved method for determining radium and thorium from the²³²Th decay series has been developed which measures the activity of²²⁰Rn as an assay of its parents. Although some ingrowth corrections and minor separation procedures for Th are required, the results to date show that the dynamic counting of²²⁰Rn via de-emanation and alpha counting by the alpha-scintillation method is a preferable approach for determining these radium and thorium isotopes accurately and efficiently. The method for lower limit detection depends on the emanation rate, which depends on purge-gas flow rate and sample volume analyzed. Using 50-cc and 1000-cc bubblers, and maximum effective purge gas flow rate, a lower limit of detection of 0.4 and 0.06 pCi/L²²⁰Rn can be obtained, respectively.     

Surface’s weights of electrodeposited thorium samples determined by alpha spectrometry

The aim of this work was the preparation of samples with thorium on the steel discs by electrodeposition for determination of natural thorium by alpha spectrometry and for following analysis by secondary ion mass spectrometry. The samples with ²³²Th isotope were prepared by electrodeposition from solution Th(NO₃)₄·12H₂O on steel discs in electrodeposition cell with use of solutions of Na₂SO₄, NaHSO₄, KOH and ammonia oxalate by electric current of 0.75 A. Weights of electrodeposited thorium samples were calculated on the basis of intensities of peak of ²³²Th isotope in the alpha spectra. The layer thickness was calculated for following analysis of surface layers of thorium using secondary ion mass spectrometry.     

Determination of alpha-emitting isotopes of uranium and thorium in vegetables and excreta

A radiochemical procedure for the determination of alpha-emitting isotopes of uranium and thorium in vegetables and excreta has been optimized, involving sample dissolution, separation by ionic exchange resin, electrodeposition and alpha-spectroscopy. Uranium and thorium isotopes were determined separately to prevent interference of ²²⁸Th from ²³²U tracer with ²²⁸Th from natural series of 232Th. This procedure was applied to faeces from people living in the Poços de Caldas plateau, a high natural radioactivity region of Brazil, and vegetables from the Laboratory of Environmental Monitoring (EML/DOE). Results show a chemical recovery of 80–95% for uranium and 46–72% for thorium.     

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.     

Secondary ion mass spectrometry and alpha-spectrometry of electrodeposited thorium films

The main aim of this work was the preparation of samples with thorium content on the steel discs by electrodeposition for determination of natural thorium isotope by alpha spectrometry and secondary ion mass spectrometry and finding out their possible linear correlation between these methods. The analysis of the composition of surface was other aim of study. Discs were measured by alpha spectrometer. After that, alpha spectrometry discs were analyzed by TOF-SIMS IV, which is installed in the International Laser Centre in Bratislava. The integral and normalized intensities of isotope of ²³²Th and intensities of ions of ThO⁺, ThOH⁺, ThO₂H⁺, Th₂O₄H⁺, ThO₂ ^?, ThO₃H^?, ThH₃O₃ ^? a ThN₂O₅H^? were measured. The linear correlation is between surface’s weights of Th and intensities of ions of Th⁺ from identified in SIMS spectra. We found out the chemical binding between thorium and oxygen and hydrogen on the surface of samples by SIMS method. Obtained intensities of ions ²³²ThO⁺, ²³²ThOH⁺, ²³²ThO₂H⁺ prove the presence of oxidized forms of thorium in the upper layers of surface. The oxidized ions predominate in univalent form of thorium up to deep about 3,000 nm.     

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.     

Neutron activation analysis of thorium after separation of233Th by isotopic ion exchange

The procedure for thorium determination in ammonium diuranate (ADU) and rocks, by neutron activation analysis after chemical separation of²³³Th, is presented. The separation of²³³Th from the interfering radioisotopes is based on the retention of²³³Th by a resin saturated with thorium (isotopic exchange) and on the elution of the interfering radioisotopes by a dilute solution of thorium in 0.5M HCl (ion exchange). The determination limit of thorium in rocks and ADU was found to be 0.56 and 9.3 g, respectively, when a 20% relative standard deviation was assumed as acceptable. The highest value obtained for the determination limit of thorium in uranium compounds, on account of the²³⁴Th activity present, is discussed.From a thesis submitted by C. S. Munita to the Instituto de Pesquisas Energéticas e Nucleares (CNEN/SP) University of São Paulo, in partial fulfillment for a Doctor of Science's Degree. Work supported by the Brazilian Atomic Energy Commission.     

Massenspektrometrische Bestimmung von ppm-Mengen Thorium in Uranerzen durch Isotopenverdünnungsanalyse

Thorium is chemically separated from uranium ores in a non-quantitative way. In one sample the isotopic ratio ²³⁰Th/²³²Th is determined in a mass spectrometer. In another sample ionium is added as indicator for the isotopic dilution; the thorium of this particular sample is also analysed in a mass spectrometer. The content of ²³⁰Th and ²³²Th is calculated from the measurements of the isotopic ratios and the amount of the ionium indicator. The results are compared with the results obtained by other methods.     

Effect of humic acid,pH and ionic strength on the sorption of Eu(III) on red earth and its solid component

A highly sensitive separation procedure has been developed to investigate uranium and thorium activities and their isotopic ratios in environmental water samples in Tokushima, Japan. Uranium and thorium isotopes in environmental water samples were simultaneously isolated from interfering elements with extraction chromatography using an Eichrom UTEVA™ resin column. After the chemical separation, activities of U and Th isotopes coprecipitated with samarium fluoride (SmF₃) were measured by α-spectrometry. It has been confirmed that uranium isotopes are isolated successfully from thorium decay chains by analyzing a test aqueous solution as a simulation of an environmental water sample. The separation procedure has been first applicable to the determination of U and Th activities and their isotopic ratios in a drinking well water named “Kurashimizu” in Tokushima City, Japan. The specific activities of ²³⁸U and ²³²Th in “Kurashimizu” were deduced to be within the upper limits of <0.31 and <0.19 mBq/l, respectively.     

The determination of natural uranium in human tissues by recovery corrected kinetic phosphorescence analysis

Two typical methods used for the determination of uranium in human autopsy tissues are kinetic phosphorescence analysis (KPA) and alpha-spectrometry, both of which have significant limitations and advantages. KPA is limited because of the amount of sample used (1–10 ml for sample digestion followed by one ml KPA aliquots), no isotopic information is provided, phosphorescence degradation by salts in solution, and even more importantly, it does not provide chemical recovery information. For samples with sub ng uranium concentrations per g of inorganic material, preconcentration is necessary, which may require chemical recovery (other than simple evaporation). While alpha-spectrometry has very good radiometric detection limits for ²³⁸U, the very long half-life of ²³⁸U (4.468·10⁹ y) restricts its mass detection limit (27 ng). KPA, on the other hand, has a detection limit three orders of magnitude lower (0.02 ng) for natural uranium. A recovery corrected method for the determination of natural uranium in human tissues was developed combining preconcentration of human tissues dissolved in 6M HCl by anion exchange with alpha-spectrometry and kinetic phosphorescence analysis, utilizing ²³²U as a tracer. Solution aliquots containing up to 6 g of bone ash were pre-concentrated for KPA measurement thereby allowing the use of up to 25% of the original sample solution weight for analysis by KPA. The radiochemical yield of ²³²U was determined by alpha-spectrometry and the uranium content was determined by KPA. The mean radiochemical yields obtained for human tissue samples range from 65% to 106% with a mean of 85%±8%.     

Low level measurements of thorium and neptunium in environmental samples using the (γ, f) reaction

A new highly sensitive track method for the determination of thorium (²³²Th) and neptunium (²³⁷Np) has been developed. The technique includes the radiochemical separation procedure of the isotopes followed by the irradiation of the resultant samples on the MT-25 microtron. The detection limit is équivalent up to 3·10^–13 g of²³²Th and 7·10^–14 of²³⁷Np. The method was used to determine²³²Th and²³⁷Np isotopes in water samples.     

Investigation of radioactive lead,uranium, and thorium in environmental waters by extraction chromatography resins

A time-saving and sensitive method for monitoring low concentration (activities) of ²¹⁰Pb, ²³²Th, and ²³⁰Th and ²³⁸U, ²³⁴U, and ²³⁵U in water samples has been developed. Through the combination of co-precipitation and extraction chromatography by 3M RAD disks and UTEVA (Eichrom) columns effective radiochemical separation of the analytes was carried out. Thorium and uranium activities were determined by alpha spectrometry and lead activity by LSC, respectively. The minimal detectable activities obtained were 0.6?Bq?m^?3 for uranium, 0.29?Bq?m^?3 for thorium, and 2.5?Bq?m^?3 for ²¹⁰Pb. More than 150 different waters were analysed for uranium content and only 30 for lead and thorium. The investigations are still in progress.     

Flow injection extraction chromatography with ICP-MS for thorium and uranium determination in human body fluids

A method based on flow injection extraction chromatography coupled to on-line ICP-MS (FI-EXC-ICPMS) has been developed and validated for simultaneous thorium and uranium determination in human body fluids. By using extraction chromatography, the limits of detection achieved for ²³²Th, ²³⁵U and ²³⁸U were 0.06 ng^.l^-1, 0.0014 ng^.l^-1 and 0.05 ng^.l^-1, respectively. The accuracy of the FI-EXC-ICPMS method was 102.4% and 101.5% with overall precision (RSD_max at 95% CI) of 5.3% and 4.9% for ²³²Th and ²³⁸U, respectively. The ²³⁵U/²³⁸U atom ratio is measured with 1.8% precision. The technique was employed for simultaneous thorium and uranium analyses in human urine and blood samples after microwave digestion. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

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.     

Interpretation of thorium bioassay data

This study is a comparison between bioassay data of thorium-exposed workers from two different facilities. The first of these facilities is a monazite sand extraction plant. Isotopic equilibrium between²³²Th and²²⁸Th was not observed in excreta samples of these workers. The second facility is a gas mantle factory. An isotopic equilibrium between²³²Th and²²⁸Th was observed in excreta samples. Whole body counter measurements have indicated a very low intake of thorium through inhalation. As the concentration of thorium in feces was very high we concluded that the main pathway of entrance of the nuclide was ingestion, mainly via contamination through dirty hands.The comparison between the bioassay results of workers from the two facilities shows that the lack of Th isotopic equilibrium observed in the excretion from the workers at the monazite sand plant possibly occurred due to an additional Th intake by ingestion of contaminated fresh food. This is presumably because²²⁸Ra is more efficiently taken up from the soil by plants, in comparison to²²⁸Th or²³²Th, and subsequently,²²⁸Th grows in from its immediate parent,²²⁸Ra.