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.     

Design and construction of a facility for neutron activation analysis using the 14 MeV neutron generator at HMS Sultan

The potential for using a small, sealed tube, DT neutron generator for neutron activation analysis has been well documented but not well demonstrated, except for 14 MeV activation analysis. This paper describes the design, construction and characterization of a neutron irradiation facility incorporating a small sealed tube DT neutron generator producing 14 MeV neutrons with fluence rates of 2·10⁸ s⁻¹ in 4π (steady state) and 10¹¹ s⁻¹ in 4π (pulsed). Monte Carlo modeling using MCNP4c and McBend9 has been used to optimize the design of this facility, including the location of a thermal irradiation facility for conventional neutron activation analysis. A significant factor in designing the facility has been the requirement to conform with Ionising Radiation Regulations and the design has been optimized to keep potential radiation doses to less that 1 μSv/h at the external walls of the facility. Activation of gold foils has been used for flux characterization and the experimental results agree well with the modeling.     

Use of combined alpha-spectrometry and fission track analysis for the determination of240Pu/239Pu ratios in human tissue

Plutonium and other actinides were determined in human autopsy tissues of occupationally exposed workers who were registrants of the United States Transuranium and Uranium Registries (USTUR). In this study, Pu was purified and isolated from Am, U and Th, after drying and wet-ashing of the tissues, and the addition of²³⁸Pu as a radiotracer. After electrodeposition onto vanadium planchets the^239+240Pu activity was determined by alpha-spectrometry. A fission track method was developed to determine²³⁹Pu in the presence of²³⁸Pu and²⁴⁰Pu, using Lexan^TM polycarbonate detectors. Combining the two techniques allowed the determination of the²⁴⁰Pu/²³⁹Pu activity and atom ratios. Data from selected USTUR cases are presented.     

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.     

Radioactivity distribution of thorium in sediment core of the Sabah-Sarawak coast

Thorium activity concentrations were measured in seven marine sediment cores taken from Sabah-Sarawak coast on 2004 by using a gravity box corer. Collected sediments were generally homogenous mud which contained much more mixture of silt and clay compared to sand and relatively low content of organic carbon (i.e. less than 5% at all sampling stations). The results found that activity concentrations of ²³⁰Th, ²³²Th and ratios of ²³⁰Th/²³²Th were ranged from 6.5–20.4 Bq/kg dry wt., 6.8–27.8 Bq/kg dry wt. and 0.69–0.92, respectively. It seem that, ²³²Th activity concentrations are slightly higher than ²³⁰Th at all sampling stations and both radionuclides were generally high at Sabah compared to Sarawak coastal waters. ²³⁰Th activity concentrations in sediment core appear to be correlated with the activity concentrations of ²³²Th at some sampling station. These indicating both radionuclides were supplied from the same of environment and source origin of detrital from terrestrial and shallower water. The low ²³⁰Th/²³²Th activity ratio which is less than unity suggesting that ²³²Th was actively and rapidly regenerated compared to ²³⁰Th from ²³⁴U. It also can be attributed to less efficiently scavenge of ²³⁰Th onto particles prior deposited at the marine sediment bed.     

In vivo activation analysis of organ cadmium using the Tsing Hua Mobile Educational Reactor

This work describes a nuclear facility forin vivo prompt gamma activation analysis (IVPGAA) using a moderated neutron beam from a 0.1 W Tsing-Hua Mobile Educational Reactor (THMER). The IVPGAA measurement is a new technique for toxic cadmium determination in organs, which can efficiently be used in clinical diagnosis. The low-power nuclear reactor provides a total neutron flux of 3.3·10⁴ n·cm^–2·s^–1 on the surface of the central vertical neutron beam tube to which a liquid phantom is positioned. The capability of such partial-body irradiation facility is demonstrated. The detection limit of cadmium in the left kidney for a skin dose equivalent of 1.66 mSv (166 mrem) was 1.34 mg in a 500-s irradiation/counting period, and the sensitivity in the left kidney was 103 counts mg^–1·mSv^–1. The performance of IVPGAA system using the THMER nuclear facility therefore has the advantages of mobility and feasibility with high sensitivity under low neutron and gamma doses.     

Radiochemical neutron activation analysis of traces of vanadium in biological samples: A comparison of prior dry ashing with post-irradiation wet ashing

Summary A radiochemical neutron activation technique for nanogram and subnanogram levels of vanadium in biological samples was developed, based on dry ashing of the sample prior to irradiation, but without any chemical treatment. It was compared to and validated by parallel analyses of low-level reference materials and human blood using a previously developed totally post-irradiation technique based on rapid wet ashing of the irradiated sample. The irradiated ash was also wet ashed after neutron bombardment to ensure complete dissolution, equilibration with carrier and removal of ³⁸Cl. Results for reference materials, including Versieck's Second Generation Human Serum Reference Material and IAEA Animal Blood, were in excellent agreement by the two techniques, and with reference to literature values. Both methods give exceptionally good radiochemical decontamination of ⁵²V from other nuclides and have nearly quantitative yields. The dry ash mode is slightly quicker and easier to manipulate.     

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.     

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 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.     

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.     

Characterization and use of the new NIST rapid pneumatic tube irradiation facility

Recently a new rapid pneumatic tube facility was inserted into a long unused location in the NIST 20 MW nuclear reactor. This facility was designed and constructed specifically for rapid INAA using short lived activation products. Included is a computer controlled console which uses fast sensors to accurately measure the irradiation capsule flight time, and a loss-free counting system connected to a 32% efficient PHGe detector with a transistor reset preamplifier. Measurement of travel, times from end-of-irradiation to detector were 473±8 ms. Measurement of the thermal neutron fluence rate was 5.0·10¹³ n·cm⁻²·s⁻¹. The other three pneumatic tubes in the NIST reactor have transfer times of 3 to 15 seconds, and no timing capability more accurate than human response. This new facility substantially improves our ability to accurately determine activation products with half-lives from 1 to 100 seconds. Characterization information reported on this new irradiation facility includes absolute fluence measurements, fluence rate variations within the capsule and variations with time, and determination of analytical sensitivities for fluorine-20 selenium-77m, and silver-110g.     

Improving the analytical sensitivity of alpha-emitting impurities in grinding high purity silica

The source of contaminating high purity silica during grinding process and analytical sensitivity of alpha-emitting impurities have been investigated. To improve the analytical sensitivity for alpha-emitting impurities in grinding high purity silica, a new procedure was studied by employing an irradiation facility in the hydraulic transfer system (HTS) and the nitrogen flushing system. The source of silica contamination was found to be attributed mostly to alumina balls. The detection limit of U and Th in 3N-grade silica samples by neutron activity analysis (NAA) could be improved to a sub-ng/g level. The contamination rates of Th, U, Cl, Fe and Na from the alumina balls were calculated to be 95%, 15%, 15% and 3.5%, respectively.     

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.     

Characterization of the neutron flux gradients in typical irradiation channels of a TRIGA Mark II reactor

The neutron distribution in a defined volume (gradient) for different matrices (air, water, cellulose, biological material and silicon dioxide) in two typical irradiation channels (pneumatic tube (PT) and IC40-channel in the carousel facility) in the TRIGA Mark II reactor at the Joef Stefan Institute (IJS) was studied. Our experiment was based on inserting Fe wires (flux monitors) into the chosen matrices. The wires were cut into small pieces after irradiation and the induced activities of ⁵⁹Fe measured. The results showed that for the studied geometry the average spatial thermal neutron flux inhomogeneities (for five studied matrices) are about 2.3% in the PT-channel and about 2.9% in the IC40-channel.     

Vanadium determination in rat tissues and biological reference materials by neutron activation analysis

Vanadium was determined in adrenal gland, brain, ileum, kidney, liver, lung, muscle, myocard, skin, spleen, gonads, thyroid, and tibia of rats fed with normal diet and exposed to high vanadium doses in drinking water. Both radiochemical neutron activation analysis (RNAA) and instrumental neutron activation analysis (INAA) were employed. The RNAA procedure consisted in dry ashing samples prior to irradiation and vanadium separation from the irradiated samples by extraction with N-benzoyl-N-phenylhydroxylamine (BPHA) in toluene from 5 mol·l^–1 HCl. Vanadium accumulation as a function of a type of the tissue, exposure time, sex of rats, and administration of V(IV) or V(V) was studied. For quality assurance purposes, the biological (standard) reference materials NBS SRM 1571 Orchard Leaves, NBS SRM 1577a Bovine Liver, IAEA H-4 Animal muscle, and Bowen's Kale were analyzed.Presented at the 2nd Balkan Conference on Activation Analysis and Nuclear-Related Analytical Techniques, Bled, 4–6 October, 1989.     

Analysis of phosphate rock samples for vanadium using accelerator-based thermal neutrons

Extensive sedimentary phosphate deposits exist in the Sirhan-Turayf basin in northwestern Saudi Arabia containing significant amounts of uranium, thorium, vanadium and rare earth elements. The determination of the concentration and pattern of distribution of some of these elements is essential for economic aspects. This study reports the analysis of vanadium in selected phosphate rock samples from the basin using accelerator-based thermal neutrons activation analysis (TNAA). Samples were irradiated in a thermal neutron flux of 2.5·10⁶ n·cm⁻²·s⁻¹. The induced activity was measured with a HP-GMX detector coupled to a PC-based data acquisition and analysis system. The facility was calibrated using certified standards of vanadium. The minimum detection limit of vanadium was about 1 mg. Three independent measurements on each sample yielded comparable results indicating the reliability of the technique. The vanadium concentrations in the samples vary from 23 to 457 ppm. This revised version was published online in July 2006 with corrections to the Cover Date.     

Simultaneous determination of tantalum,niobium, thorium and uranium in placer columbite-tantalite deposits from the Akim Oda District of Ghana by epithermal instrumental neutron activation analysis

Summary Tantalum, niobium, thorium and uranium contents of 21 columbite-tantalite minerals from the Akim Oda District of Ghana have been determined using epithermal instrumental neutron activation analysis (ENAA) with a conventional counting system. As thermal neutron filters, boron carbide was encapsulated in the irradiation vials during irradiation. The samples were irradiated at one of the outer irradiation channels in the Ghana Research Reactor-1 facility, and counted directly. Various elements were identified and quantified using 172 keV of ^182mTa, 871 keV of ^94mNb, 86.5 keV of ²³³ Th and 74.7 keV of ²³⁹U. The precision and accuracy of the method were evaluated using standard reference materials and the precision was found to be within 5% with comparable accuracy. The composition of columbite-tantalite minerals ranged from 33.1% to 59.2% Ta₂O₅ and 19.2% to 44.6% Nb₂O₅, ThO₂ and U₃O₈ the levels were 0.024-0.045% and 0.014-0.048%, respectively.     

The leachability of some natural and man-made radionuclides from soil and sediments on acid attack

A radiotracer study was made of the leachability of some natural and man-made radionuclides from soils and sediments subjected to attack by various acid mixtures. Particular attention was paid to the nuclides²³⁸U,²³²Th, Pa (as²³³Pa) and Np (as²³⁹Np), since for these neutron activation can be used to study recoveries and/or to induce in situ radionuclides in samples. Thus conventional NAA allowed determination of total²³⁸U and²³²Th instrumentally, and also enabled analysis of leachates and residues by radiochemical or instrumental NAA. Activation of these nuclides produced samples endogenously labelled with²³³Pa and²³⁹Np whose behaviour on acid dissolution/leaching could be examined. Furthermore, comparison of neutron irradiated and non-irradiated samples allowed us to investigate the possibility of increased leachability induced by nuclear recoil reactions; however, this effect was negligible. We also investigated the acid leaching of americium adsorbed on the surface of sediment. In general, unsatisfactory leaching recoveries were found for²³³Pa,²³⁹Np and²³²Th for most materials, indicating the need for total dissolution procedures.