Isotopic ratios of240Pu/239Pu in environmental samples can provide improtant data for identifying and confirming contamination sources. One low cost
alternative for performing this analysis uses a combination of high resolution alpha-spectroscopy and spectrum analysis software.
This method uses unmodified, commercially available equipment and software. Plutonium alpha-spectra were gathered with an
alpha-spectrometer system set up for low-level actinide determination. Experimental spectra were imported into a commercially
available data analysis program, and fit with mathematical descriptions of alpha-peaks using the Marquardt-Levenberg algorithm.
The spectra was then deconvoluted into its239Pu and240Pu components and the activity ratio was calculated.
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.
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.
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%.
Authors:S. Glover, R. Filby, S. Clark, and S. Grytdal
Alpha-spectrometric measurements using Si detectors is the standard method for the determination of alpha emitting actinide
elements. This method requires the preparation of sources for analysis which do not degrade the energy spectrum of the emitted
alpha particles via sample self-absorption. A variety of methods for the electrodeposition of actinides have been reported
in the literature, many of which require long deposition times and lack reproducibility. A sulfate based method has been evaluated
for the preparation of these sources using chemometric analysis to optimize the method and evaluate several variables and
their interactions with the goal to achieve high yield source preparation in 1 hour or less. Typical resolution for this method
is 30 keV or less with recoveries approaching unity.
Authors:S. Glover, H. Qu, S. LaMont, C. Grimm, and R. Filby
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.
Authors:H. Qu, D. Stuit, S. Glover, S. Love, and R. Filby
A method for the preconcentration of Am and Pu from human tissue solutions (liver, lung, bone etc) using the Actinide-CU Resin
(ElChroM Industries) has been developed for their alpha-spectrometric determination. With near 100% recoveries were obtained
by preconcentration, subsequent decomposition methods for eluent were developed. Good agreement for Pu and Am determination
with the USTUR anion-exchange/solvent extraction method was demonstrated using previously analyzed human tissue solutions
and NIST SRMs. The advantages of the preconcentration method applied to human tissue analysis are simplicity of operation,
shorter analysis time compared to anion exchange/solvent extraction methods, and capacity to analyze large tissue samples
(up to 15 g bone ash per analysis and 500 g soft tissue).
Authors:S. Love, R. Filby, S. Glover, R. Kathren, and D. Stuit
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 the239+240Pu activity was determined by alpha-spectrometry. A fission track method was developed to determine239Pu in the presence of238Pu and240Pu, using LexanTM polycarbonate detectors. Combining the two techniques allowed the determination of the240Pu/239Pu activity and atom ratios. Data from selected USTUR cases are presented.
Authors:S. LaMont, A. Maddison, R. Filby, and S. Glover
The dissolution rates for 238U, 230Th, and 231Pa from 8 contaminated soils in simulated lung fluid were determined. The soil samples were provided by the US Army Corps of Engineers and were collected from various areas at their St. Louis, Missouri FUSRAP sites. Each soil was subjected to a 100 day in vitro dissolution experiment, during which the amount of each radionuclide that had dissolved was periodically measured. At the conclusion of the experiment, a plot of the relative amount of radionuclide dissolved vs. time was constructed for each radionuclide in each soil. The dissolution rates for each radionuclide were then determined by fitting multiple first-order exponential functions to each plot. The results of these experiments were then used assign in vitro dissolution rate classifications to each radionuclide in each soil according to ICRP 30 guidelines.
Authors:S. LaMont, R. Gehrke, S. Glover, and R. Filby
There is a significant discrepancy in the reported values for the emission probability of the 186 keV gamma-ray resulting from the alpha decay of 226 Ra to 186 keV excited state of 222 Rn. Published values fall in the range of 3.28 to 3.59 gamma-rays per 100 alpha-decays. An interesting observation is that the lower value, 3.28, is based on measuring the 186 keV gamma-ray intensity relative to the 226 Ra alpha-branch to the 186 keV level. The higher values, which are close to 3.59, are based on measuring the gamma-ray intensity from mass standards of 226 Ra that are traceable to the mass standards prepared by HÓNIGSCHMID in the early 1930's. This discrepancy was resolved in this work by carefully measuring the 226 Ra alpha-branch intensities, then applying the theoretical E2 multipolarity internal conversion coefficient of 0.692±0.007 to calculate the 186 keV gamma-ray emission probability. The measured value for the alpha branch to the 186 keV excited state was (6.16±0.03)%, which gives a 186 keV gamma-ray emission probability of (3.64±0.04)%. This value is in excellent agreement with the most recently reported 186 keV gamma-ray emission probabilities determined using 226 Ra mass standards.