Bernd Kahn

Long-Term Selective Retention of Natural Cs and Rb by Highly Weathered Coastal Plain Soils

Naturally occurring Cs and Rb are distinctly more abundant relative to K in the highly weathered upland soils of the Savannah River Site, South Carolina, than in average rock of Earths upper continental crust (UCC), by factors of 10 and 4, respectively. Naturally occurring Cs has been selectively retained during soil evolution, and Rb to a lesser extent, while K has been leached away. In acid extracts of the soils, the Cs/K ratio is about 50 times and the Rb/K ratio about 15 times the corresponding ratios for the UCC, indicating that relatively large amounts of natural Cs and Rb have been sequestered in soil microenvironments that are highly selective for these elements relative to K. Cation exchange favoring Cs and Rb ions, and subsequent fixation of the ions, at sites in interlayer wedge zones within hydroxy-interlayered vermiculite particles may account for the observations. The amounts of stable Cs retained and the inferred duration of the soil evolution, many thousands of years, provide new insights regarding long-term stewardship of radiocesium in waste repositories and contaminated environments. Study of natural Cs in soil adds a long-term perspective on Cs transport in soils not available from studies of radiocesium.

Analysis of 228Ra and 226Ra in Public Water Supplies by a Gamma-ray Spectrometer

Methods for measuring 228Ra were reviewed to select a brief and simple screening procedure under NIPDW Regulations for public water supplies. A two-step method was considered to concentrate Ra by evaporation or co-precipitation and to count it with a gas ionization detector, a liquid-scintillation detector, or a Ge detector with multichannel analyzer. Gross beta particle counting appears to be feasible for screening to meet the 0.04 Bq L-1 detection limit. One can utilize the same sample volume as currently collected and measure radiation with commonly available equipment in reasonable time. The required sample volumes were estimated on the basis of known counting efficiencies and background count rates. Gamma-ray spectral analysis is the recommended option, however, because 226Ra and 228Ra can be determined directly and simultaneously. Several aspects of the method were examined to assure that the concentration procedure is nearly quantitative and that the detection limit can be reached with a 3.8-L sample in a 6000-s counting period. The method was tested with Ra tracer solutions and EPA intercomparison samples over the range of 0.04 to 1 Bq L-1. It was found appropriate for 228Ra and 226Ra analyses.

Evaluation of Monte Carlo simulation of photon counting efficiency for germanium detectors.

The reliability of calculating the full-energy peak counting efficiency of germanium detectors by Monte Carlo simulation was evaluated by comparing MCNP-4 code results with measurements over a range of conditions. The measurements were performed for two detectors of 20% and 110% nominal efficiencies with a point source at two distances and with four different volume sources, including a reentrant beaker, placed on the end caps. The radionuclides were National Institute of Standards and Technology standard sources that emit photons between energies of 42.8 and 1,596.4 keV. Each detector was modeled in detail with respect to the dimensions of the detection volume and attenuation material in the can, dead-layer, and crystal holder foil. The measurements of the 42.8-keV photon were used to check dimensions and to change slightly the value of the dead-layer thickness so that the simulation agreed with measurements at this energy. After this adjustment, the average ratio of simulation results to measurements for 13 photon energies above 42.8 keV was between 0.97 and 1.03 for all sets of point and volume source comparisons. Ratios at individual energies were between 0.92 and 1.06 for the point source and between 0.94 and 1.09 for volume sources. The observed differences were consistent with the estimated standard deviations of simulation and of measurement, which ranged from 1 to 6% and typically were 2 to 3% except for larger uncertainties at low energies. On the basis of this comparison, simulation with the MCNP-4 code is equivalent to measurement with photon standards if the detector and source configuration can be modeled within a tenth of a millimeter.

Freundlich and dual Langmuir isotherm models for predicting 137Cs binding on Savannah River Site soils.

Distribution of 137Cs and stable cesium between aqueous solution and near-surface soil samples from five locations at the Savannah River Site was measured in order to develop a predictive model for 137Cs uptake by the soils. Sorption of 137Cs in these soils appears to be mostly by hydroxy-interlayered vermiculite. Batch sorption studies with 4 d for equilibration were conducted at three cesium concentrations and at two backing electrolyte (NaNO3) concentrations. The soil-solution mixtures were pH-adjusted to evaluate the effects of pH on cesium sorption. Sorbed cesium was related to the equilibrium aqueous cesium concentrations by a Freundlich isotherm model. Model fits on logarithmic scales have a common slope of 0.60 ± 0.03 for acidic mixtures and 0.69 ± 0.04 for neutralized mixtures but have unique intercepts that are influenced by backing electrolyte concentration and pH. An ion-exchange model is proposed that pertains to all five soils and relates the Freundlich isotherms to the cation exchange capacity of soil and the aqueous concentrations of cesium, sodium, and a third ionic species that was hydrogen in the acidic mixtures and potassium in the neutralized mixtures. Model fits are consistent with Kd values in the entire range of 5–2,300 L kg−1 determined for the five soil types. As an alternate model, dual Langmuir isotherms were fitted to the data. The results suggest cesium sorption by (1) relatively few interlayer-wedge sites, highly selective for cesium, and (2) much more abundant but less selective sites on internal and external planar surfaces.

Transparent oxyhalide glass and glass ceramics for gamma-ray detection

Nuclear radiation detection is continuously becoming more important for todays society. Conventional scintillator based gamma-ray detectors use single crystal materials such as NaI:Tl, LaBr3:Ce, which provide excellent radiation detection properties, but suffer from their environment-related fluctuation, high cost and size limitation. The incorporation of nanophosphors or quantum dots (QD) into a transparent host matrix has been studied recently as a cost-saving alternative that may solve the scalability and stability problems while still providing considerable optical performance. In this work, a new glass based detecting material with promising gamma-ray detection performance is reported. Transparent alumino-silicate glasses containing cerium-doped gadolinium halide nanocrystals were prepared by a melt-quench method followed by annealing to form nanocrystal precipitates. Samples were cast and polished for optical and radiation characterization. The preliminary results indicated a similar gamma-ray detection efficiency compared to a conventional NaI:Tl detector and a gamma-ray peak resolution of ~27% at 662 KeV from some of these samples. By controlling elemental composition and ratio of the in-situ precipitated nanoparticles, radiation detection performance is expected to be improved.

Radionuclides in Peconic River fish, mussels, and sediments.

For regulatory oversight and quality control of Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) actions, fish, mussels, and sediments were analyzed from the Peconic River system on Long Island, NY, downstream of the Brookhaven National Laboratory, as well as from control locations. The analyses were for photon-emitting radionuclides (notably60Co and137Cs), uranium, plutonium, and americium. Sediments were cored in 4 sections to 0.37 m depth, whole fish were analyzed, and mussels were separated into flesh and shells. Radioisotopes of the cited elements were detected in sediment, some of the fish contained137Cs,241Am, and uranium, and mussel flesh contained137Cs and uranium. All of the60Co,233U, and enriched uranium, and some of the137Cs and241Am, can most likely be attributed to Brookhaven National Laboratory. The other radionuclides (and some of the137Cs and241Am) are believed to have either fallout or nature as their origin. The New York State Department of Health (NYSDOH) evaluated the radiological data in terms of adverse health implications due to consumption of fish with the levels of reported radioactivity. The NYSDOH determined that the added radiation doses likely to result from eating this fish are a small fraction of the radiation dose that normally results from radionuclides present in the body from natural sources.

Composition optimization of scintillating rare-earth nanocrystals in oxide glass-ceramics for radiation spectroscopy.

Glass-ceramic nanocomposites comprising GdBr₃/CeBr₃ loaded sodium-aluminosilicate glasses in which scintillating crystallites are precipitated in situ from a host glass matrix were studied. This materials system shows promise as an alternative to single-crystal scintillators, with potential to be fabricated into a wide variety of sizes, shapes, and compositions. Batch compositions containing 15-18 mol. % GdBr₃ and 3-4 mol. % CeBr₃ were prepared and analyzed for photoluminescent light yield. Light yield peaked with rare-earth content of 15 mol. % GdBr₃ and 4 mol. % CeBr₃. Preliminary ceramization studies on this composition found that the precipitated phase more closely matched a Gd₂O₃-CeO₂ mixture rather than the GdBr₃(Ce) that was targeted.

Correcting tritium concentrations in water vapor monitored with silica gel

A correction is needed to calculate the activity concentration of airborne tritium oxide when dried silica gel is used as the collector. A tracer study using tritiated water with silica gel showed that the concentration of tritium in desorbed water is lower than that in adsorbed water by a fraction that increases with the amount of adsorbed water. The hypothesis was tested that adsorbed tritiated water is diluted by isotopic exchange with both non-tritiated water and hydroxyl groups within the silica gel matrix. The extent of dilution was measured from 4% to 14% adsorbed water, which is typical of moisture on field collectors for monitoring airborne tritium oxide. For this range of percent adsorbed water, the inferred percent exchangeable water in the silica gel under study was 6.3 +/- 0.1%. This value compares to the silica gel weight loss of 5.3% after heating to 1,050 degrees C. An explanation of the difference between 6.3% and 5.3% is proposed. The contribution of the HTO/H2O vapor pressure isotope effect was considered in calculating isotopic exchange. A curve is presented for correcting the measured tritium concentration in the distillate from the silica gel as a function of the amount of adsorbed water. The tritium tracer procedure is recommended for determining the percent exchangeable water in other silica gels to correct tritium measurements of water vapor collected by them.

Nanocomposites for radiation sensing

The use of light emitting nanoparticles in polymer and glass matrices was studied for the detection of radiation. These nanocomposite scintillators were produced by various approaches including quantum dot/polymer, fluoride nanophosphor/epoxy and halide nanophosphor containing glass-ceramic composites. The synthesis and characterization of these nanoparticles as well as their incorporation into composites is discussed. Further, the application of these composites for radiation detection and spectroscopy is described.

GdBr3: CE in a glass wafer as a nuclear radiation monitor.

AbstractA glass wafer that contains cerium-activated gadolinium-based scintillator has been tested as a nuclear radiation monitor. The detector is prepared by mixing powdered gadolinium and cerium (3+) bromides with alumina, silica, and lithium fluoride, melting the mixture at 1,400°C, and then quenching and annealing the glass. The resulting clear glass matrix emits stimulated blue light that can be collected by a conventional photomultiplier tube. Spectral analysis of radionuclides with this detector shows the energy peaks for alpha particles, the energy continuum for beta particles, the Compton continuum and full-energy peaks for gamma rays, and an energy continuum with specific reaction-product peaks for neutrons. Energy resolution for the 5.5-MeV alpha particle and 0.662-MeV gamma-ray peaks is about 20%. This resolution, although threefold poorer than for single-crystal NaI(Tl) scintillators, contributes to radionuclide identification and quantification. Application of this detector to radiation monitoring is proposed, as well as approaches for improving light collection and energy resolution that will facilitate radionuclide identification and monitoring, especially for alpha particles, beta particles, and low-energy gamma rays.