S. R. Joshi

Nondestructive determination of lead-210 and radium-226 in sediments by direct photon analysis

A method is described for the non-destructive determination of210Pb and226Ra in sediments. The procedure is based on the direct counting of the 46.5-keV γ-ray of210Pb and the 351.9-keV γ-emission of214Pb. The self-absorption of the 46.5-KeV γ-ray is corrected using a technique involving direct gamma transmission measurements on sample and efficiency calibration standard. Several reference materials when assayed by the described method yielded results in general agreement with the certified values. The application of the method is illustrated through the analysis of the excess210Pb profile of a Lake Ontario sediment core.

AB initio derivation of formulations for210Pb dating of sediments

Basic formulations used in the sedimentary210Pb dating technique are derived from fundamental considerations. The formulations are applicable to models that assume either a constant rate of supply (CRS) of unsupported210Pb to the sediments or a constant initial concentration (CIC) of this210Pb in the sediments. A previously proposed expression linking age discrepancy to mass sedimentation rates afforded by the two models is found to be inadequate in explaining the observed discrepancy between the CIC and CRS ages of segments of an undisturbed sediment core. It is noted that this age discrepancy is attributable to the mathematical treatment of data in the two models. A new expression, relating age discrepancy to the fluxes of unsupported210Pb obtainable by the two models, is derived and found to give more accurate and consistent results.

Lanthanum fluoride coprecipitation technique for the preparation of actinides for alpha-particle spectrometry

A simple and rapid procedure is described by which actinides can be mounted in a form suitable for alpha-particle spectrometry without recourse to electrodeposition. The actinides are coprecipitated as fluorides with 100 μg of lanthanum carrier from an acidic solution. The precipitate is mounted on a 0.1 μ-pore membrane filter containing LaF3 substrate, also prepared from 100 μg of lanthanum. The resolution obtained using samples of pure radiotracers and of frations isolated from lake sediments is generally comparable to that afforded by the conventional electrodeposition technique. The coprecipitation yield is generally quantitative. The entire procedure can be completed in about 90 min.

The calculation of lead-210 dates for McKay Lake sediments

Lead-210 dates are assigned to a sediment core retrieved from McKay Lake, Ottawa, Canada. Sediment mixing is found to have little, but discernible, influence on the age/depth profile. Consideration of mixing yields lower estimates of the derived ages, in agreement with the prediction based on a mathematical model. The inferred dates support the previous assignments based on Ambrosia horizon and the known occurrence of a catastrophic event. The procedures used in the calculation of the age profiles are fully described.

Isolation and alpha-particle spectrometric determination of238Pu and239,240Pu in sediments

A simple method is described for the isolation and determination of plutonium isotopes in sediments. The method involves leaching of sample with nitric acid and subsequent separation of plutonium on an anion-exchange column. Major matrix elements and several potential radiochemical interferences are removed during 8M HNO3 sample loading on the column. Thorium is removed by thorough washing with 10M HCl. Plutonium (IV) is eluted with 4M HCl. Source for alpha-particle spectrometry is prepared by LaF3 coprecipitation technique at which stage a complete separation from uranium(VI) is also achieved. The entire analytical procedure is completed in about two days.

Partitioning and transport of lead-210 in the Ottawa river watershed

The partitioning of 210Pb between dissolved and the suspended sediment phases of the Ottawa River waters has been studied. The results indicate high affinity of the radionuclide for particulates (Kd, 1.9 × 105 mL g−1). Companion studies on the precursor 226Ra give an average (Kd value of 7.8 × 103 mL g−1). Nearly 77% of the total 210Pb (i.e., dissolved plus particulate) and 99% of the total 226Ra are transported through the system in the dissolved form. The suggestion is made that the low availability of particulates in the system promotes the export of 210Pb in the dissolved form. The conventional approach gives a mean residence time of about 2000 yr for the atmospherically-derived 210Pb in the watershed. However, if the flux of this 210Pb falling directly on the rivers surface is taken into account, a mean residence time of about 6200 yr is inferred for this radionuclide in the area soils. This aspect of the study is largely applicable to metals with geochemical behavior similar to that of 210Pb for the radionuclide itself has a mean life of only 32.2 yr.

The chemistry of uranium and related radionuclides in Lake Ontario waters

The chemistry of uranium and its eight associated daughters has been considered in Lake Ontario water along with that of selected transuranic elements. These transuranics arise mainly from fallout, and to a lesser extent from nuclear fuel reprocessing. Most exist as free ions or oxides of various kinds, uranium probably exists as a strong uranyl carbonate complex.

A study of the indium and germanium photopeaks in the background spectra of Ge spectrometers with a passive shield

Cosmic ray neutron interactions with indium, used as electrical contact within a Ge diode, the diode itself and the surrounding materials can give rise to a large number of photopeaks in the 50 to 1300 keV region of background spectra of Ge spectrometers with a passive shield. The nuclear processes and decays involved in the production of these photopeaks are discussed. These cosmic ray produced photopeaks are compared with those due to primordial radionuclides. Some useful information can be drawn from these measurements on the contribution of the cosmic rays on the background of Ge detectors with a passive shield.

Radionuclide concentrations in two sewage treatment plants on Western Lake Ontario, Canada

Radionuclide concentrations in digester sludge and effluent samples from Hamilton and Dundas sewage treatment plants, located at the western tip of Lake Ontario, have been determined by high-resolution γ-ray spectrometry. The radionuclides51Cr,75Se and131I, which are used in nuclear medicine procedures, were found in sludge samples. Very low concentrations of51Cr, entering Lake Ontario through the Hamilton plant effluent discharge, have little effect on lake water quality.

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.