Laurence F. Miller

Uncertainties In Dose Coefficients From Ingestion Of 131i, 137cs, And 90sr

Abstract— Quantification of uncertainties in doses from intakes of radionuclides is important in risk assessments and epidemiologic studies of individuals exposed to radiation. In this study, the uncertainties in the doses per unit intake (i.e., dose coefficients) for ingestion of 131I, 137Cs, and 90Sr by healthy individuals have been determined. Age-dependent thyroid dose coefficients were derived for 131I. The analysis for 131I uses recent measurements of thyroid volume obtained by ultrasonography, which indicate a thyroid mass lower than that previously obtained using autopsy measurements. The coefficients for 137Cs are determined using the relationship between the biological half-lives and the amount of potassium in the human body. The most recent International Commission on Radiological Protection biokinetic model was employed to determine the uncertainties for 90Sr. For 137Cs and 90Sr, the dose coefficients represent exposure in adulthood and they were determined for all organs of radiological importance. The uncertainty in the estimated dose coefficients represent state of knowledge estimates for a reference individual, and they are described by lognormal distributions with a specified geometric mean (GM) and geometric standard deviation (GSD). The estimated geometric means vary only slightly from the dose coefficients reported by ICRP publications. The largest uncertainty is observed in the dose coefficients for bone surface (GSD = 2.6), and red bone marrow (GSD = 2.4) in the case of ingestion of 90Sr. For most other organs, the uncertainty in the 90Sr dose coefficients is characterized by a GSD of 1.8 (or less for some organs). For 131I, the uncertainty in the thyroid dose coefficients is well represented by a GSD of 1.7 for both sexes and all ages other than infants for whom a GSD of 1.8 is more appropriate. The lowest uncertainties are obtained for the dose coefficients from ingestion of 137Cs (GSD = 1.24 for males; 1.4 for females). A dominant source of uncertainty in the ingestion dose coefficients is the variation of the biokinetic parameters. For 131I, the largest contribution to the uncertainty comes from the variation in the thyroid mass, but the contribution of the biokinetic parameters is comparable. The biokinetic parameters with the largest contribution to the uncertainty are (a) the fractional uptake from blood to thyroid in the case of ingestion of 131I, (b) the absorbed fraction from the gastrointestinal tract (f1) in the case of 90Sr, and (c) the amount of potassium in the body for 137Cs. The contribution to the uncertainty of the absorbed fraction (which accounts for the fraction of energy deposited in the target organ) is the smallest contributor to the uncertainty in the dose coefficients for most organs. To reduce the uncertainty in the dose estimated for a real individual, one should determine the above-mentioned parameters for the specified individual rather than to rely on assumptions for a reference individual.

Thermal Neutron Scintillator Detectors Based on Poly (2-Vinylnaphthalene) Composite Films

A series of novel 6Li-loaded plastic scintillation films have been designed and fabricated to detect thermal neutrons. Organolithium salts containing enriched 6Li were synthesized and interspersed in a series of matrices comprising a polymer doped with an antenna fluor. Thermal neutron capture by 6Li produces charged particles with kinetic energy which is sufficient to ionize and excite the polymeric matrix. This energy is collected by the antenna fluor, which produces a photonic response detectable by a photomultiplier tube. Design and optimization of these scintillation films is discussed herein. A current problem in the design and fabrication of polymeric composite materials is phase separation. The matrix is a low-dielectric aromatic polymer; hence, ionic molecules in the composite tend to phase-separate from the matrix and produce agglomerates which decrease the quantum efficiency by scattering and/or quenching the photonic response to thermal neutrons. Based on the experimental results, the importance of synthesizing polymers with high quantum yield and efficiency in energy migration and transport for making effective composite neutron scintillators is emphasized.

Microcantilever charged-particle flux detector

A micromechanical sensor for charged-particle flux detection utilizing the extreme force sensitivity of microcantilevers is demonstrated with alpha particles. The latter create a charge buildup on an electrically isolated collection plate, a metallic sphere. The cantilever detector measures the electric field emanating from the charge collection sphere using variations in mechanical parameters. Results are compared with responses due to fixed voltages on the sphere. Parameters investigated include cantilever deflection due to total charge and frequency and damping rate variation due to electrostatic force gradient. The minimum detectable particle fluence using this technique was calculated to be around 1000 particles in air, or about 3.2×10−4 pC.

Neutron detector based on lithiated sol-gel glass

Abstract A neutron detector technology is demonstrated based on 6 Li/ 10 B doped sol–gel glass. The detector is a sol–gel glass film coated silicon surface barrier detector (SBD). The ionized charged particles from ( n , α) reactions in the sol–gel film enter the SBD and are counted. Data showing that gamma-ray pulse amplitudes interfere with identifying charged particles that exit the film layer with energies below the gamma-ray energy is presented. Experiments were performed showing the effect of 137 Cs and 60 Co gamma rays on the SBD detector. The reaction product energies of the triton and alpha particles from 6 Li are significantly greater than the energies of the Compton electrons from high-energy gamma rays, allowing the measurement of neutrons in a high gamma background. The sol–gel radiation detection technology may be applicable to the characterization of transuranic waste, spent nuclear fuel and to the monitoring of stored plutonium.

Thin Film Polymer Composite Scintillators for Thermal Neutron Detection

Thin film polystyrene composite scintillators containing and organic fluors have been fabricated and tested as thermal neutron detectors. Varying fluorescence emission intensities for different compositions are interpreted in terms of the Beer-Lambert law and indicate that the sensitivity of fluorescent sensors can be improved by incorporating transparent particles with refractive index different than that of the polymer matrix. Compositions and thicknesses were varied to optimize the fluorescence and thermal neutron response and to reduce gamma-ray sensitivity. Neutron detection efficiency and neutron/gamma-ray discrimination are reported herein as functions of composition and thickness. Gamma-ray sensitivity is affected largely by changing thickness and unaffected by the amount of in the film. The best neutron/gamma-ray discrimination characteristics are obtained for film thicknesses in the range 25–150 μm.

Gamma radiolysis studies of uranyl fluoride

Summary The safe handling and storage of radioactive materials require an understanding of the effects of radiolysis on those materials. Radiolysis may result in the production of gases (e.g., corrosives) or pressures that are deleterious to storage containers. A study has been performed to address these concerns as they relate to the radiolysis of residual fluoride compounds in uranium oxides. Samples of UO2F2·xH2O and U3O8 (with ∼1.4 wt.% fluorine content) were irradiated in a 60Co source and in spent nuclear fuel (SNF) elements from the High Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory. Container pressures were monitored throughout the irradiations, and gas and solid samples were analyzed after the irradiations. The irradiation of UO2F2·xH2O produced O2 – with G(O2)-values ranging from 0.007 to 0.03 molecules of O2 produced per 100 eV. Neither F2 nor HF was produced by the irradiations. Chemical analyses of solid samples showed that some of the uranium was reduced from U(VI) to U(IV). A saturation damage limit for the UO2F2·xH2O was demonstrated by using the HFIR SNF elements, and the limit was found to be 7–9% at ∼108 rad/h). It is shown that the covalently bonded oxygen is more susceptible to radiation damage than is the ionically bonded fluorine. Irradiation of U3O8 (with ∼1.4 wt.% fluorine content) resulted in neither gas production nor a pressure increase. These experiments led to the conclusion that during long-term storage U3O8 is safe from overpressurization and the production of corrosives caused by gamma radiolysis of residual fluorides.

Neutron detection based upon a lithiated sol–gel glass

The addition of dopants in the wet chemistry formation of glass in sol-gel technology is used for many purposes. Among the uses are the fabrication of films sensitive to pH, temperature, the presence of dissolved chemicals in solutions and many others. This paper describes the use of sol-gel technology for the doping of B-10, Li-6 and U-235 into glass films applied to silicon surface barrier (SSB) detectors. Using a commercial preamp/amplifier and an IEEE-488 NIM counter, the response was recorded of the neutron profile in a neutron activation source. A portable personal computer recorded the thermal neutron map as the detector was lowered in 1.27-cm steps down a Numec thermal neutron activation source. The lithiated glass film on the SSB gave about four times the count rate of the boron-10 doped glass film. The U-235 doped glass film gave a much lower count rate, but maintained a constant output signal due to the presence of the alpha activity from the U-235 and the trace of U-234 present in the sample.

Development of a Modern Pressurized Water Reactor Simulator: Instrumentation, Design and Data Acquisition

A method is proposed for redesigning an existing pressurized water reactor simulator with modern control and data acquisition electronics along with a custom designed GUI. This system will be used in providing instruction to undergraduate and graduate Nuclear Engineers on the operation of a Pressurized Water Reactor and for simulation of nuclear criticality events. This system will allow users from across the world to access the simulator in order to run modeling and optimization experiments for assisting in the future development of reactor technology and the development of this simulator project.

Secondary Electron Energy Deposition in Thin Polymeric Films for Neutron-Photon Discrimination

Thin polymeric films are evaluated in this research as a potential replacement technology for radiation portal monitors where specific attention is given to the physical basis for neutron-photon discrimination. It is shown that the difference in the energy deposition mechanics from charged particle reaction products and from the Compton scattered electrons allows for the effective discrimination between neutrons and gammas. One goal of this study was to establish optimal thickness for polymeric films that maximize the neutron interactions and simultaneously minimize the measured interaction of photons. Polymeric films ranging from 15 μm to 600 μm containing 6LiF were fabricated and tested for their capability to satisfy criteria established by the Monte Carlo simulations with the GEANT4 code and data from measurements confirm the technical basis for our proposed understanding of neutron-photon discrimination characteristics for thin films.Department of Homeland Security. Results from Monte Carlo simulations with the GEANT4 code and data from measurements confirm the technical basis for our proposed understanding of neutron-photon discrimination characteristics for thin films.

Characterization of neutron and photon sources from a 10.5 MeV proton beam on [18O] enriched water

The production of F-18 from a 10.5 MeV proton beam on oxygen-18 results in significant yields of neutrons and photons. In order to optimize personnel shielding that satisfies regulatory requirements, it is essential that both the intensity of both neutrons and of photons be determined as a function of energy and angle, which was accomplished by combining results from measurements and from calculations. Energy dependence for neutrons was estimated as a function by unfolding Bonner ball measurements, a hyper-pure germanium detector was used to obtain measurements of the photon spectra, and a well established computer program was used to obtain the calculated values. The radiation intensity was determined from calibrated survey meters for neutrons and for photons. The energy and angular dependence obtained from measurements and calculations agree within the uncertainty of the measurements, but calculated results, scaled by measurements, were used for input to radiation shield design studies. The neutron yiel...